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Beneficiation of titania by sulfuric acid pressure leaching of Panzhihua ilmenite
Hydrometallurgy 150 (2014) 92–98
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Technical note
Beneficiation of titania by sulfuric acid pressure leaching of Panzhihua ilmenite Linie Jia, Bin Liang, Li Lü, Shaojun Yuan, Lijuan Zheng, Xiaomei Wang, Chun Li ⁎ College of Chemical Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 19 September 2014 Accepted 27 September 2014 Available online 5 October 2014 Keywords: Beneficiation of TiO2 Ilmenite H2SO4 Pressure leaching
a b s t r a c t The beneficiation of titania (TiO2) by sulfuric acid (H2SO4) pressure leaching of Panzhihua ilmenite was investigated. The reaction temperature, H2SO4 concentration, and concentration of ferrous ions (Fe2+) had significant effects on the enrichment of TiO2. With increasing reaction temperatures, the dissolution of iron from ilmenite was enhanced, while the titanium loss was reduced. Increasing the concentration of Fe2+ had an adverse effect on the beneficiation of TiO2. In contrast, the dissolution of iron from ilmenite was accelerated by increasing concentrations of H2SO4, up to 40 wt.% H2SO4. SEM analyses of the leach residues under different leaching conditions indicated that severe agglomeration occurred among the primary hydrolysate particles at high concentrations of H2SO4 or with the addition of ferrous sulfate (FeSO4). Furthermore, a compact layer was formed on the surface of unreacted ilmenite particles, thus retarding the ilmenite leaching. The agglomeration might have resulted from the adsorption of H2SO4 on the primary particle surfaces, as revealed by energy-dispersive X-ray spectroscopy (EDX) and thermogravimetric analysis (TGA). The optimal conditions for the beneficiation process were as follows: H2SO4 concentration 40 wt.%, acid/ore mass ratio 2:1, reaction temperature 150 °C, and reaction time 3 h. Thus, a Ti-rich material with a TiO2 content of ~85 wt.% was obtained. Moreover, the results demonstrated the technical feasibility of upgrading Panzhihua ilmenite in 40 wt.% sulfuric acid obtained by concentrating the diluted acid waste discharged from the sulfate TiO2 process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Titania (TiO2) is the most important white pigment and is widely used in coatings, plastics, paper, printing ink, chemical fibers, and the cosmetics industry (McNulty, 2007). There are two different commercial processes for the production of TiO2, i.e., the chloride process and sulfate process. The current annual production capacity of titanium oxide pigment worldwide is approximately 6.5 million tonnes. More than 60% of TiO2 is produced by the chloride process, while the rest is produced by the sulfate process (Li and Liang, 2007). The chloride process requires Ti-enriched feedstocks with high TiO2 contents, including natural rutile, synthetic rutile, and titanium slag. Generally, synthetic rutile and titanium slag are derived from ilmenite. The sulfate process can directly employ titaniferous ores with low titanium contents, such as ilmenite, as feedstocks. However, more than 30% of the sulfate TiO2 production factories recently shifted to using Ti-enriched feedstocks to reduce FeSO4 and hydrolytic spent acid discharge (Sahu et al., 2006). Ilmenite and natural rutile are two primary natural titaniferous ores. However, natural rutile reserves constitute less than one-tenth of those of ilmenite (Itoh et al., 2006). Therefore, the development of various processes to upgrade the TiO2 from ilmenite is of great importance. ⁎ Corresponding author. Tel.: +86 28 85408056; fax: +86 2885461108. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.hydromet.2014.09.016 0304-386X/© 2014 Elsevier B.V. All rights reserved.
Several processes including the smelting process, the reduction and corrosion process (also called the Becher process), and the acid leaching process have been developed for the beneficiation of TiO2 from ilmenite. Among them, the smelting process is an energy-consuming process that can only remove iron impurities (Natziger and Elger, 1987). The product derived from the smelting process is called titanium slag and usually has a TiO2 content of N85%. On the other hand, the products obtained by the other processes are called synthetic rutile and typically have a TiO2 content of N92%. The Becher process is an inexpensive and environmentally friendly process. However, it is only suitable to upgrade beach sand ilmenite with low calcium and magnesium contents (Bracanin et al., 1980; Cassidy et al., 1986; Farrow et al., 1987; Hoecker, 1994; Reaveley, 1980). The acid leaching processes include HCl and H2SO4 leaching. Normally, a high-temperature pretreatment of ilmenite, either by carbothermic reduction or by sequential oxidation and reduction, is an indispensable step prior to the leaching treatment. The beneficiation of TiO2 by HCl leaching (the so-called Benelite process) has been widely studied. In this process, ferric irons (Fe3+) are carbothermically reduced to ferrous iron (Fe2+) and are subsequently dissolved in dilute hydrochloric acid (Sinha, 1978; Walpole, 1997). Lasheen (2005) and Mahmoud et al. (2004) both studied the effect of metallic iron reductants on the HCl leaching of ilmenite and found that the leaching of iron and the hydrolysis of titanium ions were substantially enhanced. To moderate the
L. Jia et al. / Hydrometallurgy 150 (2014) 92–98
leaching conditions, Akhgar et al. (2010) and our group (Li et al., 2008b) investigated the effect of mechanical activation on the HCl leaching of ilmenite. We ascertained that mechanical activation facilitated the leaching of iron from the ilmenite matrix. The TiO2 content of the resultant synthetic rutile was more than 91 wt.%. Industrial usage of the HCl leaching process is limited because HCl corrodes the production equipment, despite the advantages of the process including high efficiency, easy regeneration, and recyclability of HCl. The H2SO4 leaching process is less efficient in removing impurities from ilmenite than the HCl leaching process. However, the H2SO4 waste discharged from the sulfate TiO2 process can be utilized. Moreover, H2SO4 corrodes the production equipment to a lesser degree than HCl. An example of a typical H2SO4 leaching process is the Kataoka process (Kataoka and Yamada, 1973) in which Fe3+ ions in ilmenite are carbothermically reduced to the Fe2+ form at 900–1000 °C, followed by pressure leaching using ~20 wt.% H2SO4 at 120–130 °C. To circumvent the energy-consuming reduction step at high temperatures, a mechanical activation pretreatment of Panzhihua ilmenite was utilized as an alternative (Li and Liang, 2007). An iron extraction over 90% was achieved with 5–20 wt.% H2SO4 under atmospheric pressure to prepare high-grade synthetic rutile. However, the conditions of the mechanical activation included ball milling times of at least 4 h under an oxygen-free atmosphere, resulting in an overall uneconomic process. The TiO2 output in China reached 1.89 million tons in 2012 (Deng, 2013), of which more than 95% was manufactured using the sulfate process (Deng et al., 2003). Approximately 12 million tons of hydrolytic waste H2SO4 with a concentration of ~20 wt.% was discharged annually (Tang, 2000). Consequently, it is a great challenge to utilize the waste H2SO4 in the TiO2 industry in China. Panzhihua deposits are one of the world's largest rock-type ilmenite reserves, and they account for approximately 93% of the titanium reserve in China (Liang et al., 2010). Compared to beach sand ilmenite, rock ilmenite has a better acid solubility due to the absence of rutile and pseudorutile (Fe2Ti3O9), which stem from the weathering of ilmenite (FeTiO3). In this study, the beneficiation of TiO2 from Panzhihua ilmenite through sulfuric acid pressure leaching was investigated. Specifically, the effect of reaction temperature, H2SO4 concentration, acid/ore ratio, leaching time, and the concentration of Fe2+ on the pressure leaching process were systematically examined. The resulting Ti-rich materials were characterized, and the reaction mechanism was discussed. 2. Experimental 2.1. Materials An ilmenite concentrate was provided by the Titanium Company of Pangang Group Corp. (Panzhihua, China). The chemical composition of the ilmenite is shown in Table 1. X-ray diffraction (XRD) analysis of the ore powder confirmed that the major mineral constituent of the ore was hexagonally structured FeTiO3. The − 45 μm fraction of the ore was utilized. All chemicals, including sulfuric acid and ferrous sulfate, were of analytical reagent grade and were used as received without further purification. 2.2. Leaching experiments Ilmenite was leached in a 700-mL lead-lined stainless steel pressurized reactor. It was stirred with a magnetic agitator and heated using an electric furnace. Its temperature was well controlled by an intelligent temperature control device. In each experiment, 100–200 mL of H2SO4 Table 1 Chemical composition of the ilmenite used in this study (wt.%). TiO2
FeO
Fe2O3
SiO2
MgO
Al2O3
CaO
V2O5
MnO2
46.8
37.6
5.1
4.9
1.5
1.6
1.4
0.1
0.9
93
and a given amount of ilmenite were added to the reactor. For the experiment of effect of concentration of Fe2+ ions, a certain amount of FeSO4 was introduced at the same time. The reactor was then sealed and heated to the desired temperature at 2 °C/min with stirring at 300 rpm. Subsequently, the reactor was kept at this temperature for a given period of time with a temperature fluctuation of ±2 °C. Following the pressure leaching, the autoclave was cooled to approximately 80 °C. The reactor was opened, and the slurry was filtered and washed. The filtrate was diluted to a set volume to determine the concentration of Ti and Fe ions. To avoid the possible hydrolysis of titanium ions, washing and dilution were conducted with 10 wt.% H2SO4. The leach residues were washed thoroughly with copious amounts of distilled water and dried at 100 °C for 2 h prior to characterization. A titanium-rich material was obtained by calcination of the leach residues at 1000 °C for 1 h. 2.3. Analysis and characterization The concentrations of titanium and iron ions in the filtrate were determined by redox titrations of ammonium ferric sulfate (NH4Fe(SO4)2) and potassium dichromate (K2Cr2O7), respectively. For the determination of titanium and iron contents in the leach residues, the residues were melted with sodium dioxide (Na2O2) at 700 °C and then leached with dilute hydrochloric acid. The resulting solution was then analyzed by the aforementioned redox titration method. XRD was performed using a DX-2007 X-ray diffraction spectrometer (Danton, China) operating with a Cu Kα radiation source filtered with a graphite monochromator at a frequency of λ = 1.54 nm. The continuous scanning mode with a 0.03-s interval and 0.05-s set time was used to collect the XRD patterns. The voltage and anode current were 40 kV and 30 mA, respectively. The surface morphologies of the ilmenite samples were observed before and after leaching using a Hitachi S-4800 scanning electron microscope (SEM) at an accelerating voltage of 5 kV. To reduce agglomeration, the samples were ultrasonically dispersed in water for 15 min prior to the analysis. An X-ray fluorescence (XRF) spectrometer (Swiss ARL Advant'XP +405) was used to analyze the chemical composition of the Ti-rich materials. The relative elemental abundance of the leach residues was determined with a Hitachi S-450 SEM equipped with an energy-dispersive X-ray spectrometer (EDX, Thermo Electron V4105). Thermal analyses were performed on a thermogravimetric analyzer (TG) (Seiko, Japan, EXSTAR6000) with a heating rate of 10 K/min and nitrogen flow rate of 30 mL/min. 3. Results and discussion 3.1. H2SO4 pressure leaching of Panzhihua ilmenite 3.1.1. Effect of reaction temperature The effect of reaction temperature on the dissolution of Ti and Fe from Panzhihua ilmenite was studied under the following experimental conditions: H2SO4 concentration of 20 wt.%, acid/ore ratio of 2:1 (w/w) (100% H2SO4/ore mass ratio), and leaching time of 3 h. The results are shown in Fig. 1. The temperature significantly affected the leaching of both iron and titanium from ilmenite. The extraction of Fe gradually increased with leaching temperatures. The extraction curve of Fe leveled off with temperatures above 140 °C, and a maximum Fe extraction of ~76% was achieved. However, the extraction of titanium exhibited an opposite trend and underwent a monotonic decrease with increasing temperatures. The extraction of Ti was less than 1% at 150 °C. It has been widely accepted that there is no selectivity in the acid leaching of ilmenite, as Ti and Fe dissolve at their stoichiometric ratios (Chen, 1997; Sinha, 1978). The Ti/Fe molar ratio of Panzhihua ilmenite is close to 1:1. However, the Ti/Fe ratio in the leachate obtained in this study was much less than 1, indicating that the hydrolysis of the dissolved
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40 wt.%. Moreover, the Fe extraction reached as high as 94% at 40 wt.% H2SO4. However, a further increase in the H2SO4 concentration to 50 wt.% led to an evident decrease in the Fe extraction, which is discussed later. On the other hand, the Ti loss underwent a marginal increase with increasing H2SO4 concentrations from 10 wt.% to 30 wt.%. Once the H2SO4 concentration exceeded 40 wt.%, the Ti loss increased rapidly and reached as high as 35% in 50 wt.% H2SO4, indicating that increasing H2SO4 concentrations can inhibit the hydrolysis of dissolved Ti ions. The relative Ti and Fe contents in the Ti-rich materials obtained under different H2SO4 concentrations were determined, and the results are listed in Table 2. The optimal enrichment effect was achieved at 40 wt.% H2SO4. The resulting Ti-rich material contained 85.2 wt.% TiO2 and 5.4 wt.% Fe2O3. The results were consistent with those shown in Fig. 2. Thus, the optimal H2SO4 concentration for the H2SO4 pressure leaching of ilmenite was determined to be 40 wt.%.
Fig. 1. Effect of the reaction temperature on the ilmenite leaching. Experiment conditions: H2SO4 concentration of 20 wt.%, acid/ore mass ratio of 2:1, and reaction time of 3 h.
Ti ions occurred simultaneously with the leaching process. A similar phenomenon was also observed in previous studies (Li and Liang, 2007; Li et al., 2008a). Therefore, the Ti extraction shown in Fig. 1 is virtually the apparent extraction, which represents the loss fraction of Ti during the beneficiation of ilmenite. In the present study, 150 °C corresponded to a maximum pressure of ≤0.5 MPa in the reactor. Because stricter requirements would be imposed on industrial reactors upon a further increase of the reaction temperature, the optimum reaction temperature was determined to be 150 °C. 3.1.2. Effect of H2SO4 concentration The concentration of waste H2SO4 discharged from the hydrolysis step in the sulfate process was approximately 20 wt.%. The energy consumption for concentrating of the 20 wt.% waste H2SO4, e.g., to ~50 wt.%, is not high. Industrial production has proven that the amount of steam required to obtain 1 t of 50 wt.% H2SO4 is approximately 1.5–1.8 t. However, it is not economically feasible to further concentrate 50 wt.% waste H2SO4, as the steam consumption for additional concentration increases dramatically due to the high viscosity and low water vapor partial pressure for the concentrated H2SO4 solutions. Fig. 2 shows the effect of different H2SO4 concentrations on ilmenite leaching under a reaction temperature of 150 °C, an acid/ore ratio of 2:1, and a reaction time of 3 h. The total extraction of Fe increased nearly linearly with increasing H2SO4 concentrations between 10 wt.% and
3.1.3. Effect of acid/ore ratio The effect of different acid/ore ratios on the ilmenite leaching under a H2SO4 concentration of 40 wt.% and reaction temperature of 150 °C was investigated. The acid/ore ratio (w/w) ranging from 1:1 to 2.4:1 corresponds to a liquid/solid ratio (v/w) ranging from 2:1 to 4.9:1. The results are shown in Fig. 3. The extraction of Fe gradually increased from ~73% to ~94% with increasing acid/ore ratios from 1:1 (w/w) to 2:1 (w/ w). Above 2:1, the Fe extraction remained almost unchanged. On the other hand, the Ti loss underwent a marginal change with increased acid/ore ratios and only varied between ~3% to ~5%. Therefore, the optimal acid/ore ratio of the ilmenite leaching was concluded to be 2:1 (w/w). Notably, the leach residues obtained under high acid/ore ratios were easy to filter and wash, probably due to the dissolution–precipitation mechanism (Yanagisawa and Ovenstone, 1999) in which the minute (nano-sized) TiO2 hydrolysate particles precipitated in the initial stage were dissolved due to their high surface energy in concentrated acid solutions with high acid/ore ratios. The dissolved Ti ions then recrystallized and precipitated on the surface of the previously formed large hydrolysate particles. The process caused the hydrolysate particles to grow and become more uniform, thus resulting in the improvement of the filtration. 3.1.4. Effect of reaction time Fig. 4 shows the effect of the reaction time on the ilmenite leaching under a H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and a reaction temperature of 150 °C. The extraction of Fe gradually increased with longer reaction times until a time of 3 h. Thereafter, the extraction of Fe remained almost unchanged at approximately 94%. On the other hand, the Ti loss decreased with longer reaction times. This was probably due to the hydrolysis rate of dissolved titanium ions being higher than the leaching rate of ilmenite. With leaching times over 3 h, the Ti loss only varied from ~3% to ~4%. Therefore, for the H2SO4 leaching of ilmenite, the optimal reaction time was suggested to be 3 h. 3.1.5. Effect of concentration of Fe2+ ions One of the objectives of the present study was to utilize the waste H2SO4 acid discharged from the sulfate TiO2 process to obtain Ti-rich materials. Waste acid typically contains a certain amount of ferrous sulfate (FeSO4). The concentration of Fe2 + ions in the waste acid varies with the type of titaniferous feedstock and the production process. Table 2 Composition of the Ti-rich materials obtained at different H2SO4 concentrations. H2SO4 concentration, wt.%
Fig. 2. Effect of H2SO4 concentration on the ilmenite leaching. Experiment conditions: acid/ore ratio of 2:1, reaction temperature of 150 °C, and reaction time of 3 h.
TiO2 in Ti-rich material, wt.% Fe2O3 in Ti-rich material, wt.%
10
20
30
40
50
70.0 22.7
73.8 18.4
79.4 12.2
85.2 5.4
67.9 21.1
L. Jia et al. / Hydrometallurgy 150 (2014) 92–98
Fig. 3. Effect of acid/ore ratio on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, reaction temperature of 150 °C, and reaction time of 3 h.
The concentration of Fe2+ for Panzhihua ilmenite can reach up to 40 g/L. It is therefore of great importance to understand the effect of the existing Fe2+ on the beneficiation of TiO2. The effects of different concentrations of Fe2+ from 15 to 90 g/L on the ilmenite leaching process under a temperature of 150 °C, H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and a leaching time of 3 h were investigated. The results are shown in Fig. 5. The extraction of Fe was up to ~94% in the absence of FeSO4. On the contrary, it decreased gradually with increasing concentrations of Fe2+. The extraction of Fe decreased dramatically to ~76% at the Fe2+ concentration of 75 g/L. The leaching curve of Fe leveled off upon further increase in the Fe2+ ion concentration. On the other hand, the Ti loss increased with increasing concentrations of Fe2+ ions and reached ~20% at the Fe2+ concentration of 75 g/L. With higher concentrations of Fe2+, the Ti loss remained almost unchanged. The decrease in the extraction of Fe was probably due to an increase in the hydration water of iron ions with the increasing FeSO4 dosages, which led to a decrease in the free water, thus increasing the effective H2SO4 concentration in the solution. As mentioned, the extraction of Fe decreased with H2SO4 concentrations higher than 40 wt.% (Fig. 2). As such, the increase in the effective H2SO4 concentration with increasing concentrations of Fe2+ resulted in the decrease in the extraction of Fe. In addition, the increase in the effective H2SO4 concentration also inhibited the hydrolysis of titanium, thus resulting in the increased Ti loss. Overall, the optimum conditions for the beneficiation of TiO2 by H2 SO 4 pressure leaching of Panzhihua ilmenite were as follows:
95
Fig. 5. Effect of the concentration of Fe2+ ions on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, reaction temperature of 150 °C, and reaction time of 3 h.
H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1 (w/w), reaction temperature of 150 °C, and reaction time of 3 h. The presence of FeSO4 in the H2SO4 was unfavorable for the enrichment of Ti in the ilmenite leaching process. Therefore, lower FeSO4 concentrations led to a superior beneficiation of TiO2. Fortunately, industrial production has proven that approximately 90% of the sulfate is crystallized due to the salting-out effect when waste acid is concentrated from 20 wt.% to 50 wt.%. Thus, the adverse effect of FeSO 4 can be minimized. The Ti-rich material obtained under the optimum beneficiation conditions was subjected to chemical and XRF analyses. The results are summarized in Table 3. The main impurities were iron and silica. Further purification was necessary to improve its purity. Iron can be effectively removed by hydrochloric acid in which iron has a better solubility than in sulfuric acid. Silica can be easily removed by a base (Wu et al., 2011; Wu et al., 2013). In addition, the sum of the calcium and magnesium impurities, to which the chloride process is very sensitive, was only 0.61 wt.%, which meets the widely accepted requirement of ≤1.5 wt.% in industry (Li et al., 2008a). 3.2. Characterization of leach residues To further understand the H2SO4 pressure leaching of Panzhihua ilmenite, the leach residues obtained under different leaching conditions were characterized. Fig. 6 shows the XRD patterns of ilmenite and its leach residues obtained under the optimum beneficiation conditions, both before and after calcination. The diffraction peaks of the residues were relatively weak and broad, indicative of a lower crystallinity than in the ilmenite. A phase analysis showed that all of the peaks matched well with the positions and intensity distribution of the standard XRD pattern of anatase TiO2 (PDF01-021-1272). The crystalline size of the TiO2 hydrolysate, calculated using the Scherrer formula, was ~ 12 nm. The anatase TiO2 was almost completely transformed into the rutile phase after calcination at 1000 °C for 1 h, and no silica- or iron-containing crystal phase was observed. Table 3 Chemical compositions of the Ti-rich material prepared under the optimized conditions (wt.%).
Fig. 4. Effect of leaching time on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and reaction temperature of 150 °C.
TiOC2
Fe2OC3
SiOX2
MgOX
CaOX
Al2OX3
MnOX2
85.59
5.10
8.57
0.4
0.21
0.085
0.037
C: chemical analysis; X: X-ray fluorescence analysis.
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L. Jia et al. / Hydrometallurgy 150 (2014) 92–98 Table 4 The relative sulfur and iron contents in the leach residues obtained under different leaching conditions by EDX analysis.
S/wt.% Fe/wt.%
Fig. 6. XRD patterns of the ilmenite, leach residue and Ti-rich materials. 1—ilmenite; 2—leach residue; 3—Ti-rich materials.
The SEM images of ilmenite and its leach residues obtained under different beneficiation conditions are shown in Fig. 7. The ilmenite particles seemed to be irregular and compact (Fig. 7A). All leach residues were the aggregate of spherical particles with a size of 200–300 nm (Fig. 7B–D). The agglomeration states of these spherical particles, defined as the primary particles in this study, were clearly observed and varied with the leaching conditions. The aggregate obtained with 40 wt.% H2SO4 appeared to be relatively loose (Fig. 7B), whereas agglomeration was more severe (Fig. 7C) with a H2SO4 concentration of 50 wt.%. Presumably, the aggregate would form a compact layer on
40 wt.% H2SO4
50 wt.% H2SO4
40 wt.% H2SO4+ 75 g/L Fe2+
2.35 1.74
4.37 7.55
2.41 2.31
the surface of the unreacted ilmenite to hinder further leaching reactions, thus resulting in a lower extraction of Fe in 50 wt.% H2SO4 than in 40 wt.% H2SO4. This increased agglomeration among the primary particles was also observed with the addition of FeSO4 in 40 wt.% H2SO4 (Fig. 7D). Similarly, this led to a decrease in the leaching of ilmenite. In addition, small amounts of minute particles (only dozens of nanometers in size) were observed near the primary particles (Fig. 7B–D) and were suggested to be amorphous silica according to the chemical and XRD analysis (Table 3). To explore the explanations behind the agglomeration, EDX was used to determine the primary elements in the residues after a thorough washing with acid and distilled water. The results are shown in Table 4. Unexpectedly, in addition to Ti, O, and Fe, sulfur also appeared in the residues, and the relative contents of S and Fe in the residues obtained with 50 wt.% H2SO4 and 40 wt.% H2SO4 + 75 g/L FeSO4 were remarkably higher than that in 40 wt.% H2SO4. Obviously, Fe is associated with unreacted ilmenite, while S is either in the form of sulfate or sulfuric acid. However, no insoluble sulfate was detected by XRD, indicating that the sulfur was mainly in the form of sulfuric acid. HsSO4 might be adsorbed within the residues. A similar situation was found in the hydrolysates (metatitanic acid) obtained during hydrolysis in the industrial sulfate process in which approximately 0.1 mole H2SO4 was adsorbed in one mole of metatitanic acid (Tang, 2000). Sekhar Sathyamoorthy et al. reported that the wash water remained acidic even after thorough washing of the hydrolysate from the titanyl sulfate solution (Sathyamoorthy et al., 2001).
Fig. 7. SEM images of the ilmenite and leach residues obtained under different leaching conditions. A—ilmenite, B—leach residue (40 wt.%H2SO4, acid/ore ratio 2:1, 150 °C, 3 h), C—leach residue (50 wt.%H2SO4, acid/ore ratio 2:1,150 °C, 3 h), D—leach residue (40 wt.%H2SO4, 75 g/L Fe2+, acid/ore ratio 2:1, 150 °C, 3 h).
L. Jia et al. / Hydrometallurgy 150 (2014) 92–98
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Table 5 The H2O and H2SO4 contents in the leach residues and their molar ratios with TiO2 in the metatitanic acid. Leaching condition
H2O/wt.%T
H2SO4/wt.%T
TiO2/wt.%C
n(TiO2):n(H2O):n(H2SO4)(molar ratio)
40 wt.% H2SO4 50 wt.% H2SO4 40 wt.% H2SO4 + 75 g/L Fe2+
3.54 4.67 3.77
3.58 4.25 3.74
74.64 42.93 46.05
1:0.209:0.039 1:0.484:0.081 1:0.364:0.066
T: thermogravimetric analysis. C: chemical analysis. The TiO2 content was obtained by deduction of the TiO2 in the form of unreacted ilmenite.
To quantitatively determine the amount of sulfur, the leach residues were subjected to thermogravimetric analysis under a nitrogen atmosphere, and the results are shown in Fig. 8. All TG curves are divided into three stages. The initial rapid weight loss occurred in the temperature range from ambient temperature to ~ 200 °C. This was attributed to the evaporation of physically adsorbed water. The second stage was from 200 °C to ~ 400 °C and was ascribed to the evaporation of chemisorbed water. The third stage occurred from 550 °C to 750 °C and was associated with H2SO4 desorption. A similar TG curve was previously obtained for a porous rutile TiO2 prepared by H2SO4 leaching of mechanically activated Panzhihua ilmenite (Li et al., 2008a). Based on the results shown in Fig. 8 and Table 3, the adsorbed water and H2SO4 and the TiO2 in the form of metatitanic acid in the leach residues were calculated. The results are summarized in Table 5. With increasing H2SO4 concentrations or with the addition of FeSO4, both the water and H2SO4 in the metatitanic acid increased significantly. The point of zero charge (PZC) of anatase TiO2 is approximately 5.2. In a solution with a pH less than the PZC, the oxide surface would adsorb more protons than hydroxyls. Thus, under those conditions, TiO2 carries net positive charges. The lower the pH value is, the more net positive charges the oxide surface has (Finnegan et al., 2007). In the present study, because the effective H2SO4 concentration increased with increasing H2SO4 concentrations or the addition of FeSO4, the surface of the hydrolysate (hydrated TiO2) can combine more protons (probably through hydrogen bonds), while the protons attract more anionic sulfates by electrostatic forces. A bivalent sulfate ion may combine with two protons from the surfaces of two primary particles to maintain electronic neutrality. Consequently, an adsorbed electric triple layer between two neighboring primary particles is formed through the sulfate bridge bonds. The existence of numerous adsorbed electric triple layers leads to the aforementioned agglomeration of primary particles. The formation of the adsorbed electric triple layer is schematically illustrated in Fig. 9.
In the present study, the leaching behaviors of Ti and Fe from Panzhihua ilmenite were significantly affected by the concentration of H2SO4. A similar phenomenon was also observed previously (Li and Liang, 2007), where mechanically activated Panzhihua ilmenite was dissolved with dilute H2SO4 concentrations of 5–20 wt.% at 100 °C and a liquid/solid ratio of 100:1 (v/w). The hydrolysate obtained in 20 wt.% H2SO4 was the most compact and formed a compact layer on the surface of the unreacted ilmenite particles. Therefore, the maximum extraction of Fe occurred with 10 wt.% H2SO4 rather than with 20 wt.% H2SO4. The reason might also be related to the increased adsorption of H2SO4 in the hydrolysate with 20 wt.% H2SO4. In the present study, the agglomeration among the primary particles increased with increasing amounts of FeSO4, thus further retarding the leaching of ilmenite. A similar phenomenon was previously observed upon leaching of mechanically activated Panzhihua ilmenite with dilute hydrochloric acid (Li et al., 2008b) in which the TiO2 hydrolysates became very compact and the filtration of the reacting slurry dramatically declined upon addition of FeCl3 in the HCl leaching solution, which was probably caused by increased adsorption of HCl on the hydrolysates. 4. Conclusions The beneficiation of TiO2 by H2SO4 pressure leaching of Panzhihua ilmenite was investigated. The effects of reaction temperature, H2SO4 concentration, and the concentration of Fe2+ ions on ilmenite leaching were systematically investigated. The primary results are as follows: (1) The reaction temperature, H2SO4 concentration, and the concentration of Fe2+ had significant effects on the enrichment of TiO2. With increasing reaction temperatures, the dissolution of iron from ilmenite was enhanced, while the titanium loss was reduced. An increase in the concentration of Fe2 + showed an adverse effect on the beneficiation of TiO2. In contrast, the dissolution of iron from ilmenite was accelerated with increasing H2SO4 concentrations until the acid concentration exceeded 40 wt.% H2SO4. (2) High concentrations of H2SO4 or the addition of FeSO4 resulted in severe agglomeration among the primary hydrolysate particles,
H
primary particle
+
primary particle 2-
SO4
primary particle Fig. 8. Thermogravimetric curve of the leach residues obtained under different leaching conditions. 1—(40 wt.%H 2 SO 4 , acid/ore ratio 2:1, 150 °C, 3 h), 2—(50 wt.%H 2 SO 4 , acid/ore ratio 2:1, 150 °C, 3 h), 3—(40 wt.%H2 SO4 , 75 g/L Fe2 +, acid/ore ratio: 2:1, 150 °C, 3 h).
primary particle
Fig. 9. The formation of an agglomerate through electronic triple layers.
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which formed a compact layer on the surface of the unreacted ilmenite particles, thus retarding the ilmenite leaching. The agglomeration might have resulted from the adsorption of H2SO4 on the primary particle surfaces. High concentrations of H2SO4 or the addition of FeSO4 would increase the adsorption, thus aggravating the agglomeration. (3) The optimal conditions of the beneficiation process were as follows: H2SO4 concentration of 40 wt.%, acid/ore mass ratio of 2:1, reaction temperature 150 °C, and reaction time of 3 h. Under these conditions, a Ti-rich material with a TiO2 content of ~85 wt.% was obtained. In conclusion, it is feasible in technology to upgrade the Panzhihua ilmenite in 40 wt.% sulfuric acid, obtained by concentrating the waste acid discharged from the sulfate TiO2 process. Acknowledgments The authors are grateful for the financial support of the key National Natural Science Foundation of Chinakey (No. 21236004). References Akhgar, B.N., Pazouki, M., Ranjbar, M., Hosseinnia, A., Keyanpour-Rad, M., 2010. Preparation of nanosized synthetic rutile from ilmenite concentrate. Miner. Eng. 23 (7), 587–589. Bracanin, B.F., Clements, R.J., Davey, J.M., 1980. Direct reduction—the Western titanium process for the production of synthetic rutile, ferutil and sponge iron AusIMM Pro. 275, pp. 33–42, (September). Cassidy, P.W., Clements, R.J., Ellis, B.A., Rolfe, P.R., 1986. The AMC Narngulu synthetic rutile plant, a world source of ilmenite, rutile, monazite and zircon. Conference Proceedings. AusIMM, Perth, W.A, pp. 123–128. Chen, Y., 1997. Low-temperature oxidation of ilmenite (FeTiO3) induced by high energy ball milling at room temperature. J. Alloys Compd. 257 (1), 156–160. Deng, J., 2013. Prediction of the Chinese TiO2 industry operation and development in 2012. China Coat. 28 (3), 25–27. Deng, J., Wu, L., Qiao, L., et al., 2003. TiO2 Application Manual. Chemical Industrial Process, Beijing, pp. 15–18. Farrow, J.B., Ritchie, I.M., Mangono, P., 1987. The reaction between reduced ilmenite and oxygen in ammonium chloride solution. Hydrometallurgy 18, 21–38. Finnegan, M.P., Zhang, H., Banfield, J.F., 2007. Phase stability and transformation in titania nanoparticles in aqueous solutions dominated by surface energy. J. Phys. Chem. C 111 (5), 1962–1968.
Hoecker W., 1994. Process for the production of synthetic rutile. European Patent EP0612854. Itoh, S., Sato, S., Ono, J., Okada, H., Nagasaka, T., 2006. Feasibility study of the new rutile extraction process from natural ilmenite ore based on the oxidation reaction. Metall. Mater. Trans. B 37 (6), 979–985. Kataoka, S., Yamada, S., 1973. Acid leaching upgrades ilmenite to synthetic rutile. Chem. Eng. 80 (7), 92–93. Lasheen, T.A.I., 2005. Chemical benefication of Rosetta ilmenite by direct reduction leaching. Hydrometallurgy 76, 123–129. Li, C., Liang, B., 2007. Dissolution of mechanically activated Panzhihua ilmenites in dilute solutions of sulphuric acid. Hydrometallurgy 89, 1–10. Li, C., Liang, B., Xu, J.Q., Wang, X.Q., 2008a. Preparation of porous rutile titania from ilmenite by mechanical activation and subsequent sulfuric acid leaching. Microporous Mesoporous Mater. 115 (3), 293–300. Li, C., Liang, B., Wang, H., 2008b. Preparation of synthetic rutile by hydrochloric acid leaching of mechanically activated Panzhihua ilmenite. Hydrometallurgy 91 (1), 121–129. Liang, X., Wu, P., Lü, L., et al., 2010. Effect of oxidation way on HCl leaching of Panzhihua ilmenite. Chin. J. Process. Eng. 10 (5), 234–238. Mahmoud, M.H.H., Afifi, A.A.I., Ibrahim, I.A., 2004. Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile. Hydrometallurgy 73 (1), 99–109. McNulty, G.S., 2007. Production of titanium dioxide. In Proc. Conf. Seville, Spain (March). Natziger, R.H., Elger, G.W., 1987. Preparation of titanium feedstock from Minnesota ilmenite by smelting and sulfation leaching. US Bureau of Mines, Report Invest No. 9065. Reaveley, Y.B., 1980. Synthetic rutile production at Associated Minerals Consolidated LTD, Capol, W. A. In: Woodcock, J.T. (Ed.), Mining and Metallurgical Practices in Australasia. AusIMM, pp. 782–784. Sahu, K.K., Alex, T.C., Mishra, D., Agrawal, A., 2006. An overview on the production of pigment grade titania from titania-rich slag. Waste Manag. Res. 24 (1), 74–79. Sathyamoorthy, S., Moggridge, G.D., Hounslow, M.J., 2001. Particle formation during anatase precipitation of seeded titanyl sulfate solution. Cryst. Growth Des. 1 (2), 123–129. Sinha, H.N., 1978. Fluidized-bed leaching of ilmenite. In 11th Commonwealth Mining and Metallurgical Congress (4 p.). Tang, Z., 2000. Production and Environmental Protection of TiO2 Pigment. Chemical Industrial Press, Beijing, pp. 64–65. Walpole, E.A., 1997. The Austpac ERMS and EARS processes. A Cost Effective Route to High Grade Synthetic Rutile and Pigment Grade TiO2. Heavy Minerals. SAIMM, Johannesburg, pp. 169–174. Wu, F., Li, X., Wang, Z., Wu, L., Guo, H., Xiong, X., Wang, X., 2011. Hydrogen peroxide leaching of hydrolyzed titania residue prepared from mechanically activated Panzhihua ilmenite leached by hydrochloric acid. Int. J. Miner. Process. 98 (1), 106–112. Wu, F., Li, X., Wang, Z., Xu, C., He, H., Qi, A., Guo, H., 2013. Preparation of high-value TiO2 nanowires by leaching of hydrolyzed titania residue from natural ilmenite. Hydrometallurgy 140, 82–88. Yanagisawa, K., Ovenstone, J., 1999. Crystallization of anatase from amorphous titania using the hydrothermal technique: effects of starting material and temperature. J. Phys. Chem. B 103 (37), 7781–7787.
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Technical note
Beneficiation of titania by sulfuric acid pressure leaching of Panzhihua ilmenite Linie Jia, Bin Liang, Li Lü, Shaojun Yuan, Lijuan Zheng, Xiaomei Wang, Chun Li ⁎ College of Chemical Engineering, Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 19 September 2014 Accepted 27 September 2014 Available online 5 October 2014 Keywords: Beneficiation of TiO2 Ilmenite H2SO4 Pressure leaching
a b s t r a c t The beneficiation of titania (TiO2) by sulfuric acid (H2SO4) pressure leaching of Panzhihua ilmenite was investigated. The reaction temperature, H2SO4 concentration, and concentration of ferrous ions (Fe2+) had significant effects on the enrichment of TiO2. With increasing reaction temperatures, the dissolution of iron from ilmenite was enhanced, while the titanium loss was reduced. Increasing the concentration of Fe2+ had an adverse effect on the beneficiation of TiO2. In contrast, the dissolution of iron from ilmenite was accelerated by increasing concentrations of H2SO4, up to 40 wt.% H2SO4. SEM analyses of the leach residues under different leaching conditions indicated that severe agglomeration occurred among the primary hydrolysate particles at high concentrations of H2SO4 or with the addition of ferrous sulfate (FeSO4). Furthermore, a compact layer was formed on the surface of unreacted ilmenite particles, thus retarding the ilmenite leaching. The agglomeration might have resulted from the adsorption of H2SO4 on the primary particle surfaces, as revealed by energy-dispersive X-ray spectroscopy (EDX) and thermogravimetric analysis (TGA). The optimal conditions for the beneficiation process were as follows: H2SO4 concentration 40 wt.%, acid/ore mass ratio 2:1, reaction temperature 150 °C, and reaction time 3 h. Thus, a Ti-rich material with a TiO2 content of ~85 wt.% was obtained. Moreover, the results demonstrated the technical feasibility of upgrading Panzhihua ilmenite in 40 wt.% sulfuric acid obtained by concentrating the diluted acid waste discharged from the sulfate TiO2 process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Titania (TiO2) is the most important white pigment and is widely used in coatings, plastics, paper, printing ink, chemical fibers, and the cosmetics industry (McNulty, 2007). There are two different commercial processes for the production of TiO2, i.e., the chloride process and sulfate process. The current annual production capacity of titanium oxide pigment worldwide is approximately 6.5 million tonnes. More than 60% of TiO2 is produced by the chloride process, while the rest is produced by the sulfate process (Li and Liang, 2007). The chloride process requires Ti-enriched feedstocks with high TiO2 contents, including natural rutile, synthetic rutile, and titanium slag. Generally, synthetic rutile and titanium slag are derived from ilmenite. The sulfate process can directly employ titaniferous ores with low titanium contents, such as ilmenite, as feedstocks. However, more than 30% of the sulfate TiO2 production factories recently shifted to using Ti-enriched feedstocks to reduce FeSO4 and hydrolytic spent acid discharge (Sahu et al., 2006). Ilmenite and natural rutile are two primary natural titaniferous ores. However, natural rutile reserves constitute less than one-tenth of those of ilmenite (Itoh et al., 2006). Therefore, the development of various processes to upgrade the TiO2 from ilmenite is of great importance. ⁎ Corresponding author. Tel.: +86 28 85408056; fax: +86 2885461108. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.hydromet.2014.09.016 0304-386X/© 2014 Elsevier B.V. All rights reserved.
Several processes including the smelting process, the reduction and corrosion process (also called the Becher process), and the acid leaching process have been developed for the beneficiation of TiO2 from ilmenite. Among them, the smelting process is an energy-consuming process that can only remove iron impurities (Natziger and Elger, 1987). The product derived from the smelting process is called titanium slag and usually has a TiO2 content of N85%. On the other hand, the products obtained by the other processes are called synthetic rutile and typically have a TiO2 content of N92%. The Becher process is an inexpensive and environmentally friendly process. However, it is only suitable to upgrade beach sand ilmenite with low calcium and magnesium contents (Bracanin et al., 1980; Cassidy et al., 1986; Farrow et al., 1987; Hoecker, 1994; Reaveley, 1980). The acid leaching processes include HCl and H2SO4 leaching. Normally, a high-temperature pretreatment of ilmenite, either by carbothermic reduction or by sequential oxidation and reduction, is an indispensable step prior to the leaching treatment. The beneficiation of TiO2 by HCl leaching (the so-called Benelite process) has been widely studied. In this process, ferric irons (Fe3+) are carbothermically reduced to ferrous iron (Fe2+) and are subsequently dissolved in dilute hydrochloric acid (Sinha, 1978; Walpole, 1997). Lasheen (2005) and Mahmoud et al. (2004) both studied the effect of metallic iron reductants on the HCl leaching of ilmenite and found that the leaching of iron and the hydrolysis of titanium ions were substantially enhanced. To moderate the
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leaching conditions, Akhgar et al. (2010) and our group (Li et al., 2008b) investigated the effect of mechanical activation on the HCl leaching of ilmenite. We ascertained that mechanical activation facilitated the leaching of iron from the ilmenite matrix. The TiO2 content of the resultant synthetic rutile was more than 91 wt.%. Industrial usage of the HCl leaching process is limited because HCl corrodes the production equipment, despite the advantages of the process including high efficiency, easy regeneration, and recyclability of HCl. The H2SO4 leaching process is less efficient in removing impurities from ilmenite than the HCl leaching process. However, the H2SO4 waste discharged from the sulfate TiO2 process can be utilized. Moreover, H2SO4 corrodes the production equipment to a lesser degree than HCl. An example of a typical H2SO4 leaching process is the Kataoka process (Kataoka and Yamada, 1973) in which Fe3+ ions in ilmenite are carbothermically reduced to the Fe2+ form at 900–1000 °C, followed by pressure leaching using ~20 wt.% H2SO4 at 120–130 °C. To circumvent the energy-consuming reduction step at high temperatures, a mechanical activation pretreatment of Panzhihua ilmenite was utilized as an alternative (Li and Liang, 2007). An iron extraction over 90% was achieved with 5–20 wt.% H2SO4 under atmospheric pressure to prepare high-grade synthetic rutile. However, the conditions of the mechanical activation included ball milling times of at least 4 h under an oxygen-free atmosphere, resulting in an overall uneconomic process. The TiO2 output in China reached 1.89 million tons in 2012 (Deng, 2013), of which more than 95% was manufactured using the sulfate process (Deng et al., 2003). Approximately 12 million tons of hydrolytic waste H2SO4 with a concentration of ~20 wt.% was discharged annually (Tang, 2000). Consequently, it is a great challenge to utilize the waste H2SO4 in the TiO2 industry in China. Panzhihua deposits are one of the world's largest rock-type ilmenite reserves, and they account for approximately 93% of the titanium reserve in China (Liang et al., 2010). Compared to beach sand ilmenite, rock ilmenite has a better acid solubility due to the absence of rutile and pseudorutile (Fe2Ti3O9), which stem from the weathering of ilmenite (FeTiO3). In this study, the beneficiation of TiO2 from Panzhihua ilmenite through sulfuric acid pressure leaching was investigated. Specifically, the effect of reaction temperature, H2SO4 concentration, acid/ore ratio, leaching time, and the concentration of Fe2+ on the pressure leaching process were systematically examined. The resulting Ti-rich materials were characterized, and the reaction mechanism was discussed. 2. Experimental 2.1. Materials An ilmenite concentrate was provided by the Titanium Company of Pangang Group Corp. (Panzhihua, China). The chemical composition of the ilmenite is shown in Table 1. X-ray diffraction (XRD) analysis of the ore powder confirmed that the major mineral constituent of the ore was hexagonally structured FeTiO3. The − 45 μm fraction of the ore was utilized. All chemicals, including sulfuric acid and ferrous sulfate, were of analytical reagent grade and were used as received without further purification. 2.2. Leaching experiments Ilmenite was leached in a 700-mL lead-lined stainless steel pressurized reactor. It was stirred with a magnetic agitator and heated using an electric furnace. Its temperature was well controlled by an intelligent temperature control device. In each experiment, 100–200 mL of H2SO4 Table 1 Chemical composition of the ilmenite used in this study (wt.%). TiO2
FeO
Fe2O3
SiO2
MgO
Al2O3
CaO
V2O5
MnO2
46.8
37.6
5.1
4.9
1.5
1.6
1.4
0.1
0.9
93
and a given amount of ilmenite were added to the reactor. For the experiment of effect of concentration of Fe2+ ions, a certain amount of FeSO4 was introduced at the same time. The reactor was then sealed and heated to the desired temperature at 2 °C/min with stirring at 300 rpm. Subsequently, the reactor was kept at this temperature for a given period of time with a temperature fluctuation of ±2 °C. Following the pressure leaching, the autoclave was cooled to approximately 80 °C. The reactor was opened, and the slurry was filtered and washed. The filtrate was diluted to a set volume to determine the concentration of Ti and Fe ions. To avoid the possible hydrolysis of titanium ions, washing and dilution were conducted with 10 wt.% H2SO4. The leach residues were washed thoroughly with copious amounts of distilled water and dried at 100 °C for 2 h prior to characterization. A titanium-rich material was obtained by calcination of the leach residues at 1000 °C for 1 h. 2.3. Analysis and characterization The concentrations of titanium and iron ions in the filtrate were determined by redox titrations of ammonium ferric sulfate (NH4Fe(SO4)2) and potassium dichromate (K2Cr2O7), respectively. For the determination of titanium and iron contents in the leach residues, the residues were melted with sodium dioxide (Na2O2) at 700 °C and then leached with dilute hydrochloric acid. The resulting solution was then analyzed by the aforementioned redox titration method. XRD was performed using a DX-2007 X-ray diffraction spectrometer (Danton, China) operating with a Cu Kα radiation source filtered with a graphite monochromator at a frequency of λ = 1.54 nm. The continuous scanning mode with a 0.03-s interval and 0.05-s set time was used to collect the XRD patterns. The voltage and anode current were 40 kV and 30 mA, respectively. The surface morphologies of the ilmenite samples were observed before and after leaching using a Hitachi S-4800 scanning electron microscope (SEM) at an accelerating voltage of 5 kV. To reduce agglomeration, the samples were ultrasonically dispersed in water for 15 min prior to the analysis. An X-ray fluorescence (XRF) spectrometer (Swiss ARL Advant'XP +405) was used to analyze the chemical composition of the Ti-rich materials. The relative elemental abundance of the leach residues was determined with a Hitachi S-450 SEM equipped with an energy-dispersive X-ray spectrometer (EDX, Thermo Electron V4105). Thermal analyses were performed on a thermogravimetric analyzer (TG) (Seiko, Japan, EXSTAR6000) with a heating rate of 10 K/min and nitrogen flow rate of 30 mL/min. 3. Results and discussion 3.1. H2SO4 pressure leaching of Panzhihua ilmenite 3.1.1. Effect of reaction temperature The effect of reaction temperature on the dissolution of Ti and Fe from Panzhihua ilmenite was studied under the following experimental conditions: H2SO4 concentration of 20 wt.%, acid/ore ratio of 2:1 (w/w) (100% H2SO4/ore mass ratio), and leaching time of 3 h. The results are shown in Fig. 1. The temperature significantly affected the leaching of both iron and titanium from ilmenite. The extraction of Fe gradually increased with leaching temperatures. The extraction curve of Fe leveled off with temperatures above 140 °C, and a maximum Fe extraction of ~76% was achieved. However, the extraction of titanium exhibited an opposite trend and underwent a monotonic decrease with increasing temperatures. The extraction of Ti was less than 1% at 150 °C. It has been widely accepted that there is no selectivity in the acid leaching of ilmenite, as Ti and Fe dissolve at their stoichiometric ratios (Chen, 1997; Sinha, 1978). The Ti/Fe molar ratio of Panzhihua ilmenite is close to 1:1. However, the Ti/Fe ratio in the leachate obtained in this study was much less than 1, indicating that the hydrolysis of the dissolved
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40 wt.%. Moreover, the Fe extraction reached as high as 94% at 40 wt.% H2SO4. However, a further increase in the H2SO4 concentration to 50 wt.% led to an evident decrease in the Fe extraction, which is discussed later. On the other hand, the Ti loss underwent a marginal increase with increasing H2SO4 concentrations from 10 wt.% to 30 wt.%. Once the H2SO4 concentration exceeded 40 wt.%, the Ti loss increased rapidly and reached as high as 35% in 50 wt.% H2SO4, indicating that increasing H2SO4 concentrations can inhibit the hydrolysis of dissolved Ti ions. The relative Ti and Fe contents in the Ti-rich materials obtained under different H2SO4 concentrations were determined, and the results are listed in Table 2. The optimal enrichment effect was achieved at 40 wt.% H2SO4. The resulting Ti-rich material contained 85.2 wt.% TiO2 and 5.4 wt.% Fe2O3. The results were consistent with those shown in Fig. 2. Thus, the optimal H2SO4 concentration for the H2SO4 pressure leaching of ilmenite was determined to be 40 wt.%.
Fig. 1. Effect of the reaction temperature on the ilmenite leaching. Experiment conditions: H2SO4 concentration of 20 wt.%, acid/ore mass ratio of 2:1, and reaction time of 3 h.
Ti ions occurred simultaneously with the leaching process. A similar phenomenon was also observed in previous studies (Li and Liang, 2007; Li et al., 2008a). Therefore, the Ti extraction shown in Fig. 1 is virtually the apparent extraction, which represents the loss fraction of Ti during the beneficiation of ilmenite. In the present study, 150 °C corresponded to a maximum pressure of ≤0.5 MPa in the reactor. Because stricter requirements would be imposed on industrial reactors upon a further increase of the reaction temperature, the optimum reaction temperature was determined to be 150 °C. 3.1.2. Effect of H2SO4 concentration The concentration of waste H2SO4 discharged from the hydrolysis step in the sulfate process was approximately 20 wt.%. The energy consumption for concentrating of the 20 wt.% waste H2SO4, e.g., to ~50 wt.%, is not high. Industrial production has proven that the amount of steam required to obtain 1 t of 50 wt.% H2SO4 is approximately 1.5–1.8 t. However, it is not economically feasible to further concentrate 50 wt.% waste H2SO4, as the steam consumption for additional concentration increases dramatically due to the high viscosity and low water vapor partial pressure for the concentrated H2SO4 solutions. Fig. 2 shows the effect of different H2SO4 concentrations on ilmenite leaching under a reaction temperature of 150 °C, an acid/ore ratio of 2:1, and a reaction time of 3 h. The total extraction of Fe increased nearly linearly with increasing H2SO4 concentrations between 10 wt.% and
3.1.3. Effect of acid/ore ratio The effect of different acid/ore ratios on the ilmenite leaching under a H2SO4 concentration of 40 wt.% and reaction temperature of 150 °C was investigated. The acid/ore ratio (w/w) ranging from 1:1 to 2.4:1 corresponds to a liquid/solid ratio (v/w) ranging from 2:1 to 4.9:1. The results are shown in Fig. 3. The extraction of Fe gradually increased from ~73% to ~94% with increasing acid/ore ratios from 1:1 (w/w) to 2:1 (w/ w). Above 2:1, the Fe extraction remained almost unchanged. On the other hand, the Ti loss underwent a marginal change with increased acid/ore ratios and only varied between ~3% to ~5%. Therefore, the optimal acid/ore ratio of the ilmenite leaching was concluded to be 2:1 (w/w). Notably, the leach residues obtained under high acid/ore ratios were easy to filter and wash, probably due to the dissolution–precipitation mechanism (Yanagisawa and Ovenstone, 1999) in which the minute (nano-sized) TiO2 hydrolysate particles precipitated in the initial stage were dissolved due to their high surface energy in concentrated acid solutions with high acid/ore ratios. The dissolved Ti ions then recrystallized and precipitated on the surface of the previously formed large hydrolysate particles. The process caused the hydrolysate particles to grow and become more uniform, thus resulting in the improvement of the filtration. 3.1.4. Effect of reaction time Fig. 4 shows the effect of the reaction time on the ilmenite leaching under a H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and a reaction temperature of 150 °C. The extraction of Fe gradually increased with longer reaction times until a time of 3 h. Thereafter, the extraction of Fe remained almost unchanged at approximately 94%. On the other hand, the Ti loss decreased with longer reaction times. This was probably due to the hydrolysis rate of dissolved titanium ions being higher than the leaching rate of ilmenite. With leaching times over 3 h, the Ti loss only varied from ~3% to ~4%. Therefore, for the H2SO4 leaching of ilmenite, the optimal reaction time was suggested to be 3 h. 3.1.5. Effect of concentration of Fe2+ ions One of the objectives of the present study was to utilize the waste H2SO4 acid discharged from the sulfate TiO2 process to obtain Ti-rich materials. Waste acid typically contains a certain amount of ferrous sulfate (FeSO4). The concentration of Fe2 + ions in the waste acid varies with the type of titaniferous feedstock and the production process. Table 2 Composition of the Ti-rich materials obtained at different H2SO4 concentrations. H2SO4 concentration, wt.%
Fig. 2. Effect of H2SO4 concentration on the ilmenite leaching. Experiment conditions: acid/ore ratio of 2:1, reaction temperature of 150 °C, and reaction time of 3 h.
TiO2 in Ti-rich material, wt.% Fe2O3 in Ti-rich material, wt.%
10
20
30
40
50
70.0 22.7
73.8 18.4
79.4 12.2
85.2 5.4
67.9 21.1
L. Jia et al. / Hydrometallurgy 150 (2014) 92–98
Fig. 3. Effect of acid/ore ratio on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, reaction temperature of 150 °C, and reaction time of 3 h.
The concentration of Fe2+ for Panzhihua ilmenite can reach up to 40 g/L. It is therefore of great importance to understand the effect of the existing Fe2+ on the beneficiation of TiO2. The effects of different concentrations of Fe2+ from 15 to 90 g/L on the ilmenite leaching process under a temperature of 150 °C, H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and a leaching time of 3 h were investigated. The results are shown in Fig. 5. The extraction of Fe was up to ~94% in the absence of FeSO4. On the contrary, it decreased gradually with increasing concentrations of Fe2+. The extraction of Fe decreased dramatically to ~76% at the Fe2+ concentration of 75 g/L. The leaching curve of Fe leveled off upon further increase in the Fe2+ ion concentration. On the other hand, the Ti loss increased with increasing concentrations of Fe2+ ions and reached ~20% at the Fe2+ concentration of 75 g/L. With higher concentrations of Fe2+, the Ti loss remained almost unchanged. The decrease in the extraction of Fe was probably due to an increase in the hydration water of iron ions with the increasing FeSO4 dosages, which led to a decrease in the free water, thus increasing the effective H2SO4 concentration in the solution. As mentioned, the extraction of Fe decreased with H2SO4 concentrations higher than 40 wt.% (Fig. 2). As such, the increase in the effective H2SO4 concentration with increasing concentrations of Fe2+ resulted in the decrease in the extraction of Fe. In addition, the increase in the effective H2SO4 concentration also inhibited the hydrolysis of titanium, thus resulting in the increased Ti loss. Overall, the optimum conditions for the beneficiation of TiO2 by H2 SO 4 pressure leaching of Panzhihua ilmenite were as follows:
95
Fig. 5. Effect of the concentration of Fe2+ ions on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, reaction temperature of 150 °C, and reaction time of 3 h.
H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1 (w/w), reaction temperature of 150 °C, and reaction time of 3 h. The presence of FeSO4 in the H2SO4 was unfavorable for the enrichment of Ti in the ilmenite leaching process. Therefore, lower FeSO4 concentrations led to a superior beneficiation of TiO2. Fortunately, industrial production has proven that approximately 90% of the sulfate is crystallized due to the salting-out effect when waste acid is concentrated from 20 wt.% to 50 wt.%. Thus, the adverse effect of FeSO 4 can be minimized. The Ti-rich material obtained under the optimum beneficiation conditions was subjected to chemical and XRF analyses. The results are summarized in Table 3. The main impurities were iron and silica. Further purification was necessary to improve its purity. Iron can be effectively removed by hydrochloric acid in which iron has a better solubility than in sulfuric acid. Silica can be easily removed by a base (Wu et al., 2011; Wu et al., 2013). In addition, the sum of the calcium and magnesium impurities, to which the chloride process is very sensitive, was only 0.61 wt.%, which meets the widely accepted requirement of ≤1.5 wt.% in industry (Li et al., 2008a). 3.2. Characterization of leach residues To further understand the H2SO4 pressure leaching of Panzhihua ilmenite, the leach residues obtained under different leaching conditions were characterized. Fig. 6 shows the XRD patterns of ilmenite and its leach residues obtained under the optimum beneficiation conditions, both before and after calcination. The diffraction peaks of the residues were relatively weak and broad, indicative of a lower crystallinity than in the ilmenite. A phase analysis showed that all of the peaks matched well with the positions and intensity distribution of the standard XRD pattern of anatase TiO2 (PDF01-021-1272). The crystalline size of the TiO2 hydrolysate, calculated using the Scherrer formula, was ~ 12 nm. The anatase TiO2 was almost completely transformed into the rutile phase after calcination at 1000 °C for 1 h, and no silica- or iron-containing crystal phase was observed. Table 3 Chemical compositions of the Ti-rich material prepared under the optimized conditions (wt.%).
Fig. 4. Effect of leaching time on the ilmenite leaching. Experimental conditions: H2SO4 concentration of 40 wt.%, acid/ore ratio of 2:1, and reaction temperature of 150 °C.
TiOC2
Fe2OC3
SiOX2
MgOX
CaOX
Al2OX3
MnOX2
85.59
5.10
8.57
0.4
0.21
0.085
0.037
C: chemical analysis; X: X-ray fluorescence analysis.
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L. Jia et al. / Hydrometallurgy 150 (2014) 92–98 Table 4 The relative sulfur and iron contents in the leach residues obtained under different leaching conditions by EDX analysis.
S/wt.% Fe/wt.%
Fig. 6. XRD patterns of the ilmenite, leach residue and Ti-rich materials. 1—ilmenite; 2—leach residue; 3—Ti-rich materials.
The SEM images of ilmenite and its leach residues obtained under different beneficiation conditions are shown in Fig. 7. The ilmenite particles seemed to be irregular and compact (Fig. 7A). All leach residues were the aggregate of spherical particles with a size of 200–300 nm (Fig. 7B–D). The agglomeration states of these spherical particles, defined as the primary particles in this study, were clearly observed and varied with the leaching conditions. The aggregate obtained with 40 wt.% H2SO4 appeared to be relatively loose (Fig. 7B), whereas agglomeration was more severe (Fig. 7C) with a H2SO4 concentration of 50 wt.%. Presumably, the aggregate would form a compact layer on
40 wt.% H2SO4
50 wt.% H2SO4
40 wt.% H2SO4+ 75 g/L Fe2+
2.35 1.74
4.37 7.55
2.41 2.31
the surface of the unreacted ilmenite to hinder further leaching reactions, thus resulting in a lower extraction of Fe in 50 wt.% H2SO4 than in 40 wt.% H2SO4. This increased agglomeration among the primary particles was also observed with the addition of FeSO4 in 40 wt.% H2SO4 (Fig. 7D). Similarly, this led to a decrease in the leaching of ilmenite. In addition, small amounts of minute particles (only dozens of nanometers in size) were observed near the primary particles (Fig. 7B–D) and were suggested to be amorphous silica according to the chemical and XRD analysis (Table 3). To explore the explanations behind the agglomeration, EDX was used to determine the primary elements in the residues after a thorough washing with acid and distilled water. The results are shown in Table 4. Unexpectedly, in addition to Ti, O, and Fe, sulfur also appeared in the residues, and the relative contents of S and Fe in the residues obtained with 50 wt.% H2SO4 and 40 wt.% H2SO4 + 75 g/L FeSO4 were remarkably higher than that in 40 wt.% H2SO4. Obviously, Fe is associated with unreacted ilmenite, while S is either in the form of sulfate or sulfuric acid. However, no insoluble sulfate was detected by XRD, indicating that the sulfur was mainly in the form of sulfuric acid. HsSO4 might be adsorbed within the residues. A similar situation was found in the hydrolysates (metatitanic acid) obtained during hydrolysis in the industrial sulfate process in which approximately 0.1 mole H2SO4 was adsorbed in one mole of metatitanic acid (Tang, 2000). Sekhar Sathyamoorthy et al. reported that the wash water remained acidic even after thorough washing of the hydrolysate from the titanyl sulfate solution (Sathyamoorthy et al., 2001).
Fig. 7. SEM images of the ilmenite and leach residues obtained under different leaching conditions. A—ilmenite, B—leach residue (40 wt.%H2SO4, acid/ore ratio 2:1, 150 °C, 3 h), C—leach residue (50 wt.%H2SO4, acid/ore ratio 2:1,150 °C, 3 h), D—leach residue (40 wt.%H2SO4, 75 g/L Fe2+, acid/ore ratio 2:1, 150 °C, 3 h).
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Table 5 The H2O and H2SO4 contents in the leach residues and their molar ratios with TiO2 in the metatitanic acid. Leaching condition
H2O/wt.%T
H2SO4/wt.%T
TiO2/wt.%C
n(TiO2):n(H2O):n(H2SO4)(molar ratio)
40 wt.% H2SO4 50 wt.% H2SO4 40 wt.% H2SO4 + 75 g/L Fe2+
3.54 4.67 3.77
3.58 4.25 3.74
74.64 42.93 46.05
1:0.209:0.039 1:0.484:0.081 1:0.364:0.066
T: thermogravimetric analysis. C: chemical analysis. The TiO2 content was obtained by deduction of the TiO2 in the form of unreacted ilmenite.
To quantitatively determine the amount of sulfur, the leach residues were subjected to thermogravimetric analysis under a nitrogen atmosphere, and the results are shown in Fig. 8. All TG curves are divided into three stages. The initial rapid weight loss occurred in the temperature range from ambient temperature to ~ 200 °C. This was attributed to the evaporation of physically adsorbed water. The second stage was from 200 °C to ~ 400 °C and was ascribed to the evaporation of chemisorbed water. The third stage occurred from 550 °C to 750 °C and was associated with H2SO4 desorption. A similar TG curve was previously obtained for a porous rutile TiO2 prepared by H2SO4 leaching of mechanically activated Panzhihua ilmenite (Li et al., 2008a). Based on the results shown in Fig. 8 and Table 3, the adsorbed water and H2SO4 and the TiO2 in the form of metatitanic acid in the leach residues were calculated. The results are summarized in Table 5. With increasing H2SO4 concentrations or with the addition of FeSO4, both the water and H2SO4 in the metatitanic acid increased significantly. The point of zero charge (PZC) of anatase TiO2 is approximately 5.2. In a solution with a pH less than the PZC, the oxide surface would adsorb more protons than hydroxyls. Thus, under those conditions, TiO2 carries net positive charges. The lower the pH value is, the more net positive charges the oxide surface has (Finnegan et al., 2007). In the present study, because the effective H2SO4 concentration increased with increasing H2SO4 concentrations or the addition of FeSO4, the surface of the hydrolysate (hydrated TiO2) can combine more protons (probably through hydrogen bonds), while the protons attract more anionic sulfates by electrostatic forces. A bivalent sulfate ion may combine with two protons from the surfaces of two primary particles to maintain electronic neutrality. Consequently, an adsorbed electric triple layer between two neighboring primary particles is formed through the sulfate bridge bonds. The existence of numerous adsorbed electric triple layers leads to the aforementioned agglomeration of primary particles. The formation of the adsorbed electric triple layer is schematically illustrated in Fig. 9.
In the present study, the leaching behaviors of Ti and Fe from Panzhihua ilmenite were significantly affected by the concentration of H2SO4. A similar phenomenon was also observed previously (Li and Liang, 2007), where mechanically activated Panzhihua ilmenite was dissolved with dilute H2SO4 concentrations of 5–20 wt.% at 100 °C and a liquid/solid ratio of 100:1 (v/w). The hydrolysate obtained in 20 wt.% H2SO4 was the most compact and formed a compact layer on the surface of the unreacted ilmenite particles. Therefore, the maximum extraction of Fe occurred with 10 wt.% H2SO4 rather than with 20 wt.% H2SO4. The reason might also be related to the increased adsorption of H2SO4 in the hydrolysate with 20 wt.% H2SO4. In the present study, the agglomeration among the primary particles increased with increasing amounts of FeSO4, thus further retarding the leaching of ilmenite. A similar phenomenon was previously observed upon leaching of mechanically activated Panzhihua ilmenite with dilute hydrochloric acid (Li et al., 2008b) in which the TiO2 hydrolysates became very compact and the filtration of the reacting slurry dramatically declined upon addition of FeCl3 in the HCl leaching solution, which was probably caused by increased adsorption of HCl on the hydrolysates. 4. Conclusions The beneficiation of TiO2 by H2SO4 pressure leaching of Panzhihua ilmenite was investigated. The effects of reaction temperature, H2SO4 concentration, and the concentration of Fe2+ ions on ilmenite leaching were systematically investigated. The primary results are as follows: (1) The reaction temperature, H2SO4 concentration, and the concentration of Fe2+ had significant effects on the enrichment of TiO2. With increasing reaction temperatures, the dissolution of iron from ilmenite was enhanced, while the titanium loss was reduced. An increase in the concentration of Fe2 + showed an adverse effect on the beneficiation of TiO2. In contrast, the dissolution of iron from ilmenite was accelerated with increasing H2SO4 concentrations until the acid concentration exceeded 40 wt.% H2SO4. (2) High concentrations of H2SO4 or the addition of FeSO4 resulted in severe agglomeration among the primary hydrolysate particles,
H
primary particle
+
primary particle 2-
SO4
primary particle Fig. 8. Thermogravimetric curve of the leach residues obtained under different leaching conditions. 1—(40 wt.%H 2 SO 4 , acid/ore ratio 2:1, 150 °C, 3 h), 2—(50 wt.%H 2 SO 4 , acid/ore ratio 2:1, 150 °C, 3 h), 3—(40 wt.%H2 SO4 , 75 g/L Fe2 +, acid/ore ratio: 2:1, 150 °C, 3 h).
primary particle
Fig. 9. The formation of an agglomerate through electronic triple layers.
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which formed a compact layer on the surface of the unreacted ilmenite particles, thus retarding the ilmenite leaching. The agglomeration might have resulted from the adsorption of H2SO4 on the primary particle surfaces. High concentrations of H2SO4 or the addition of FeSO4 would increase the adsorption, thus aggravating the agglomeration. (3) The optimal conditions of the beneficiation process were as follows: H2SO4 concentration of 40 wt.%, acid/ore mass ratio of 2:1, reaction temperature 150 °C, and reaction time of 3 h. Under these conditions, a Ti-rich material with a TiO2 content of ~85 wt.% was obtained. In conclusion, it is feasible in technology to upgrade the Panzhihua ilmenite in 40 wt.% sulfuric acid, obtained by concentrating the waste acid discharged from the sulfate TiO2 process. Acknowledgments The authors are grateful for the financial support of the key National Natural Science Foundation of Chinakey (No. 21236004). References Akhgar, B.N., Pazouki, M., Ranjbar, M., Hosseinnia, A., Keyanpour-Rad, M., 2010. Preparation of nanosized synthetic rutile from ilmenite concentrate. Miner. Eng. 23 (7), 587–589. Bracanin, B.F., Clements, R.J., Davey, J.M., 1980. Direct reduction—the Western titanium process for the production of synthetic rutile, ferutil and sponge iron AusIMM Pro. 275, pp. 33–42, (September). Cassidy, P.W., Clements, R.J., Ellis, B.A., Rolfe, P.R., 1986. The AMC Narngulu synthetic rutile plant, a world source of ilmenite, rutile, monazite and zircon. Conference Proceedings. AusIMM, Perth, W.A, pp. 123–128. Chen, Y., 1997. Low-temperature oxidation of ilmenite (FeTiO3) induced by high energy ball milling at room temperature. J. Alloys Compd. 257 (1), 156–160. Deng, J., 2013. Prediction of the Chinese TiO2 industry operation and development in 2012. China Coat. 28 (3), 25–27. Deng, J., Wu, L., Qiao, L., et al., 2003. TiO2 Application Manual. Chemical Industrial Process, Beijing, pp. 15–18. Farrow, J.B., Ritchie, I.M., Mangono, P., 1987. The reaction between reduced ilmenite and oxygen in ammonium chloride solution. Hydrometallurgy 18, 21–38. Finnegan, M.P., Zhang, H., Banfield, J.F., 2007. Phase stability and transformation in titania nanoparticles in aqueous solutions dominated by surface energy. J. Phys. Chem. C 111 (5), 1962–1968.
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