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Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile
Hydrometallurgy 73 (2004) 99 – 109 www.elsevier.com/locate/hydromet
Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile M.H.H. Mahmoud *, A.A.I. Afifi, I.A. Ibrahim Central Metallurgical R&D Institute, P.O. Box 87, Helwan, Cairo 14421, Egypt Received 28 June 2003; received in revised form 13 August 2003; accepted 14 August 2003
Abstract The reactivity of ilmenite ore during leaching with hydrochloric acid can be greatly enhanced by reduction in solution using metallic iron. Addition of a particular amount of iron powder after a certain time of reaction will reduce all the dissolved Fe3 + to Fe2 + and reduce a portion of the dissolved Ti4 + to Ti3 +. As the leaching continues, any further dissolved Fe3 + will be simultaneously reduced by the action of the formed Ti3 + to produce Fe2 + and Ti4 +. As a result, dissolution of the iron and titanium contents from the ore will be improved. Meanwhile, the dissolved Ti4 + will be hydrolysed and precipitated leaving mainly ferrous chloride in the solution. The hydrolysis is accompanied by fast dissolution of the unreacted ore. A reductive leaching process is proposed taking advantages of the recent advances in HCl regeneration from spent leach liquor. The different leaching parameters were optimised to obtain as little iron as possible in the titanium dioxide product. Synthetic rutile of about 90% TiO2 and 0.8% Fe2O3 was obtained from an Egyptian medium-grade ilmenite ore of grain size 100% – 75 Am using 20% HCl at boiling temperature (110 jC), with addition of 0.11 g iron powder/g ore for 5 h. The product has a low content of colouring elements (total MnO2, Cr2O3 and V2O5 = 0.12%) and chlorine consuming elements (total CaO, MgO and Al2O3 = 0.08%) but contain high silica (SiO2 = 5.8%). The process is applicable for higher grade ilmenite ores which will produce higher quality synthetic rutile. D 2004 Elsevier B.V. All rights reserved. Keywords: Ilmenite ore; Acid leaching; Reduction in solution; Hydrochloric acid; Synthetic rutile
1. Introduction Titanium dioxide is an important intermediate in the manufacture of paints, pigments, welding-rod coatings, ceramics, papers, and in other areas of chemical industry (Diebold, 2003). White titanium dioxide pigments have been produced by two pro-
* Corresponding author. Tel.: +20-2-501-0642; fax: +20-2-5010639. E-mail address: [email protected] (M.H.H. Mahmoud). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2003.08.001
cesses, namely the sulfate process, and the dry chlorination process. The sulfate process which utilises ilmenite (FeTiO3) as a raw material is well known and widely applied but it is lengthy, costly and the byproduct ferrous sulfate is less marketable (Afifi, 1994; Abdel-Aal et al., 2000). The dry chlorination process which utilises rutile (TiO2) as a raw material presently enjoys more favourable economics and generates less waste materials (Mackey, 1994). Shortage of natural rutile has encouraged research efforts to convert ilmenite into synthetic rutile for the chlorination process.
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There are several processes for the production of synthetic rutile from ilmenite, most of which fall into one of five classifications: (1) Smelting processes, where the iron part of the ilmenite is reduced and melted to separate the iron from titanium (Natziger and Elger, 1987). The titania slag is then leached with the sulfuric acid or with hydrochloric acid at elevated temperatures. (2) Reduction of ilmenite to convert ferric iron partially to the ferrous form or completely to the metallic iron form followed by acid leaching (Balderson and MacDonald, 1999; Kahn, 1984). (3) Reduction of the iron content of the ilmenite to the metallic iron followed by corrosion with oxygen and ammonium chloride (Becher, 1963; Farrow and Ritchie, 1987). (4) Oxidation and reduction of ilmenite followed by hydrochloric acid leaching, MURSO process (Sinha, 1973, 1979). (5) Roasting and magnetic separation followed by hydrochloric acid leaching, ERMS process (Walpole, 1997). All these processes depend mainly on reductive and/or oxidative thermal pretreatment of ilmenite which is an extensive energy consuming stage. Reductive pretreatment aims at converting the ferric iron in ilmenite into the ferrous state which is more soluble in hydrochloric acid. It was stated elsewhere that the dissolution of ilmenite in hydrochloric acid could be enhanced by reduction in solution using metals such as iron, zinc and tin (El-Khaliny, 1967). However, this concept was not practiced to get benefit for dissolution of ilmenite and preparation of synthetic rutile. In the present work, we propose a process for the production of synthetic rutile based on hydrochloric acid leaching of ilmenite ore at ambient pressure in the presence of metallic iron as a reducing agent. The iron metal can be added as a pure powder or as scrap. This trend for synthetic rutile production is considered to be advantageous compared with the other mentioned processes in view of the avoidance of high temperature pretreatment or complicated pressure leaching. The acid consumption could be higher in the proposed process but recent advances in acid regeneration technology from the spent leaching liquor
have made acid leaching processes more attractive (Walpole, 1995; Newman and Balderson, 1993). Also, the by-product iron oxide may have commercial uses and add value to the process. Large ilmenite deposits are present in the South Eastern desert of Egypt. Among these are reserves in Abu Ghalaga ilmenite region, estimated to be about 50 million tons. The present work aims at studying the preparation of synthetic rutile by reductive leaching of Abu Ghalaga ilmenite ore in hydrochloric acid. The study outlines the expected reaction mechanism, optimisation of parameters affecting the process and evaluation of the product.
2. Experimental 2.1. Materials and procedure One ton of ilmenite ore from Abu Ghalaga region, Red Sea, was thoroughly mixed and a representative sample of about 15 kg was crushed and ground to 100% –75 Am and used for conducting this work. The particle size analysis of the used ilmenite ore is given in Table 1. Acid leaching of ilmenite ore was carried out using a 250 cm3 three necked glass reactor provided with a reflux condenser and a mechanical agitator with Teflon-coated stirring rod. A stirring speed of 400 rpm was applied to keep the slurry suspended during the leaching experiment. The desired volume of hydrochloric acid of the required concentration was poured in the reactor and heated using a thermostatically controlled glycerol/water bath. After reaching the desired temperature, a 20 g ilmenite ore sample was added and after a certain time a weighed amount of iron powder was added. In the preliminary investigations, samples of 1 cm3 were withdrawn from the leach liquor to determine the dissolved contents of Fe2 +, Fe3 +, Ti4 +, Ti3 + and the residual HCl concentration. After the required reacTable 1 The particle size distribution of ilmenite ore Size, Am
Cumulative percentage passed
75 63 53 45
100.0 81.8 60.7 33.8
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tion period, the slurry was filtered off and the solid titanium concentrate was thoroughly washed with 3% HCl to minimize the concentration of iron in the liquor that is entrained within the porous solid. The washed titanium concentrate was dried in air at 110 jC and calcined at 900 jC to completely dehydrate the sample. 2.2. Materials characterization Phase identification of the ilmenite ore and the titanium concentrate were carried out using X-ray diffraction. Analysis of the Abu Ghalaga ilmenite ore showed that the strongest lines correspond to the typical ilmenite structure (card #29-733) with lines of lower but still appreciable intensities indicating the presence of free hematite (card #13-534 and 24-27). Photomicrographs of the Abu Ghalaga ilmenite ore are shown in Fig. 1. It can be seen that ilmenite grains enclose exsolution bodies of hematite. It also reveals that minute hematite lamellae are segregated forming elongated bodies of hematite within the ilmenite host (plate A). Small grains of hematite are exsolved and segregated at the borders of ilmenite grains (plate B). Thus, the ore is mainly formed from ferri-ilmenite together with small quantities of titano-hematite. For the chemical analysis of the ilmenite ore, a weighed sample of finely ground ore was fused with potassium pyrosulfate, dissolved in 1:1 H2SO4 and filtered. The metallic contents were analysed in the filtrate and the SiO2 content was analysed in the insoluble residue by evaporation with HF. Titanium was determined spectrophotometrically by the hydro-
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Table 2 The chemical analysis of the ilmenite ore Component
Weight, %
Component
Weight, %
TiO2 FeO Fe2O3 Fe Total Al2O3 MgO CaO MnO
41.10 24.40 28.60 39.01 0.63 0.64 0.15 0.36
Cr2O3 V2O5 SO3 CO2 P2O5 SiO2 Moisture
0.36 0.40 0.11 0.65 0.02 2.43 0.15
gen peroxide method at wavelength 410 nm (Vogel, 1978). Titanium (III) was determined by titration against FeCl3 using KSCN as indicator (Furman, 1975). Total iron and ferrous iron were determined spectrophotometrically using the phenanthroline method at wavelength 515 nm (Vogel, 1978). Ferric iron was calculated by difference between total and ferrous iron contents. For the chemical analysis of the titanium concentrate, a weighed sample of the calcined concentrate was dissolved in ammonium sulfate and concentrated sulfuric acid. The content of titanium in the filtrate was determined spectrophotometrically; iron and other metallic impurities were determined by atomic absorption spectrometer (Perkin Elmer 3100). The SiO2 content was determined by the HF method. The chemical composition of the ore sample is given in Table 2. The ore contains 41.1% TiO2, 24.4% FeO and 28.6% Fe2O3. This ore is considered as medium grade since it contains a relatively small amount of titanium. The ore also contains 2.4% SiO2, 0.02 P2O5 and minor elements such as Mn, V, Cr and Mg. Most of the impurities in the ilmenite ore
Fig. 1. Photomicrographs of Abu Ghalaga ilmenite ore.
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are expected to be dissolved by acid leaching while SiO2 content will be enriched in the final product. Silica content in the ore can be minimized by gravity concentration which will yield more acceptable product, but this work utilises as received ore. The size distribution of the produced synthetic rutile was determined using a laser sizing (Analysete 22, FRITCH). Surface areas were measured using high-speed gas sorption analyzer (NOVA 2200, version 7.11 Quantachrome).
3. Results and discussion 3.1. Preliminary investigation and proposed mechanism Several studies have been performed to illustrate the mechanism of ilmenite leaching in hydrochloric acid (van Dyk et al., 2002; Lanyon et al., 1999; Jackson and Wadsworth, 1976; Sinha, 1984). Most of these studies include reduction or oxidation pretreatment. Here we will investigate the direct leaching of ilmenite without and with addition of iron powder as a reducing agent. 3.1.1. Leaching without reducing agent Direct leaching of ilmenite ore was performed at a typical ilmenite/20% HCl ratio of 1:7.3 g/g at the boiling temperature, 110 jC. The extent of extraction of total Fe and Ti was measured at different time intervals based on the content of the dissolved metals. As shown in Fig. 2, the extraction increased with time reaching values of about 18% and 13% for total Fe and Ti after 30 min, respectively. The extraction of total Fe slowly increased at times longer than 30 min and reached only about 29% after 5 h. However, the values of Ti extraction slightly decreased with times longer than 30 min and reached about 10% after 5 h. This decrease may be due to transformation of a portion of the dissolved titanium into the white insoluble hydrated titanium dioxide. This was obvious by comparing the black colour of the ore and the dark grey colour of the residue. It is clear from these results that the ore is only partially leached; this may be due to the limited dissolution of hematite in hydrochloric acid. This will adversely affect the leaching process since the hema-
Fig. 2. The leaching of ilmenite ore with HCl. HCl concn.: 20%, temp.: 110 jC, ilmenite/acid ratio: 1/7.3 g/g.
tite is finely disseminated in the ilmenite (as shown in the microscopic investigations in Fig. 1). 3.1.2. Leaching with Fe powder as a reducing agent It is known that the hematite dissolution in hydrochloric acid can be enhanced in reducing medium (Lu and Muir, 1988). Thus, reduction in solution is suggested here to promote the leaching of ilmenite ore in hydrochloric acid. Iron metal is considered the most suitable reductant for this purpose since no foreign ions will be introduced to the reaction medium and the ferrous chloride formed can be separated from the solution and used for regeneration of HCl. A leaching experiment was carried out at the same conditions as the previous test (ilmenite/20% HCl ratio of 1:7.3 and 110 jC) but with the addition of Fe powder, as 0.075 g Fe/g ore, after 20 min of reaction. Plots of extracted percentages of total Fe, total Ti and Ti3 + are shown in Fig. 3. The added Fe powder was subtracted when the total Fe extraction is calculated. In this case, Ti3 + started to appear in the solution once the Fe powder was added and suddenly increased with time. Then, the Ti3 + content gradually decreased and was absent after about 90 min. It is clear that the extraction of total Fe and Ti in Fig. 3 are markedly changed by adding Fe powder, compared to the extractions without Fe powder addition, Fig. 2. The total extracted Ti rapidly increased after addition of the Fe powder, reached a peak value of about 28% after 40 to 60 min and then started to decrease to a
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Fig. 3. The Leaching of ilmenite ore with HCl in presence of iron powder. HCl concn.: 20%, ilmenite/acid ratio: 1/7.3 g/g, temp.: 110 jC, Fe powder: 0.075 g/g ore, time of Fe addition: 20 min.
small value of about 2% after 120 min and then remained almost unchanged at longer time. On the other hand, the total extracted Fe sharply increased after addition of Fe powder, reached a plateau at about 45% between 40 and 60 min and then increased to about 90% after 120 min and then gradually increased to 98% after 6 h. Variations of residual HCl concentrations in the leach liquor were determined during leaching with and without addition of Fe powder and results are shown in Fig. 4. In the absence of Fe powder, the acid concentration decreased from 20% to 15% after 20 min and very slowly decreased at longer time. After 10 min of Fe powder addition, the acid concentration decreased from 15% to 12% and then continuously decreased with time to about 7% after 5 h. The sudden decrease in HCl concentration after addition of Fe powder is due to faster reactions of HCl with the added Fe powder and with the ilmenite ore than without Fe powder addition. It was evident (using the conditions mentioned in Fig. 1) that the added iron is completely dissolved after 10 min. This was revealed by observing the evolution of hydrogen gas that was started when the Fe metal was added and stopped after 10 min. The chemical analysis of the leach liquor before and after 10 min of Fe addition was performed to show the drastic change in the solution composition after the Fe
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Fig. 4. Variation of HCl concentration with time during leaching of ilmenite ore with and without addition of Fe powder. HCl concn.: 20%, ilmenite/acid ratio: 1/7.3 g/g, temp.: 110 jC, Fe powder: 0.075 g/g ore, time of Fe addition: 20 min.
addition due to the rapid leaching. Table 3 shows that the amount of total dissolved titanium was doubled and the amount of total dissolved iron was increased >2.5 times (the added Fe was subtracted) after 10 min of Fe addition. It also shows that 20% of the HCl was consumed at the same time. It is thus clear that by addition of Fe powder the iron and titanium content of the ilmenite ore are mostly dissolved in hydrochloric acid after about 6 h, while the dissolved titanium is continuously separated as hydrolysed TiO2. 3.1.3. Discussion of extraction behaviour and proposed reactions The possible mechanisms of extraction before and after the addition of Fe powder will be presented Table 3 The chemical analysis of the leach liquor before and after 10 minutes of iron powder addition Element
Total Ti Ti(IV) Ti(III) Total Fe Fe(III) Fe(II) HCl(100%)
Weight, g Before Fe addition (retention time = 20 min)
After 10 min of Fe addition (retention time = 30 min)
0.60 0.60 0.00 1.28 0.72 0.56 22.76
1.20 0.87 0.32 3.12 0.00 3.12 18.30
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through several chemical reactions. The ilmenite fraction in the ore is dissolved in hydrochloric acid as follows:
This is consistent with the EMF values estimated from the standard potentials of the half-cell reactions given below (Bard et al., 1985).
FeO TiO2 þ 4HCl ¼ FeCl2 þ TiOCl2 þ 2H2 O ð1Þ
2Hþ þ 2e ¼ H2
and the hematite fraction in the ore is dissolved in hydrochloric acid as follows:
Fe3þ þ e ¼ Fe2þ
Fe2 O3 þ 6HCl ¼ 2FeCl3 þ 3H2 O
ð2Þ
The decrease in the amount of total Ti in the solution after reaching a peak is related to the hydrolysis of the dissolved titanium species. The predominant species of titanium at the used HCl concentration is TiOCl2 (Cservenya´k et al., 1996). The hydrolysis reaction where HCl is released is given as follows: TiOCl2 þ H2 O ¼ TiO2 # þ 2HCl
ð3Þ
The added iron powder will readily react with HCl creating nascent hydrogen and ferrous chloride. Fe þ 2HCl ¼ FeCl2 þ 2H
ð4Þ
A part of the nascent hydrogen forms hydrogen gas. H þ H ¼ H2 z
ð5Þ
The dissolved Fe3 + will be simultaneously reduced to the Fe2 + state by reaction with the nascent hydrogen and/ore with the Fe powder. 2FeCl3 þ 2H ¼ 2FeCl2 þ 2HCl
ð6Þ
2FeCl3 þ Fe ¼ 3FeCl2
ð7Þ
The reduction of all the Fe3 + in the solution to the Fe ions is completed when the colour of the solution changes from yellow-red to violet. The appearance of the characteristic violet colour of the Ti3 + indicates the consequent reduction of the Ti4 + (that is dissolved before the Fe addition) to the Ti3 + state by reaction with the nascent hydrogen and/or the Fe powder. 2+
2TiOCl2 þ 2HCl þ 2H ¼ 2TiCl3 þ 2H2 O
ð8Þ
2TiOCl2 þ Fe þ 4HCl ¼ 2TiCl3 þ FeCl2 þ 2H2 O ð9Þ
ð10Þ
0:00 V
ð11Þ
0:77 V
TiO2þ þ 2Hþ þ e ¼ Ti3þ þ H2 O
0:10 V
ð12Þ
The higher EMF value of the Fe3 +/Fe2 + couple (0.77 V) compared with the TiO2 +/Ti3 + couple (0.1 V) indicates the higher tendency for Fe3 + reduction (Eq. (11)) to proceed than Ti4 + reduction (Eq. (12)). Since the nascent hydrogen will be rapidly removed from the reaction medium as soon as the iron powder is completely dissolved, the formed Ti3 + will keep the reducing conditions in the solution. That is, the formed Ti3 + will reduce any further dissolved Fe3 + to the Fe2 + state. TiCl3 þ FeCl3 þ H2 O ¼ TiOCl2 þ FeCl2 þ 2HCl ð13Þ This sequence is confirmed by the profile of Ti3 + contents in solution as shown in Fig. 3. The consumption of the Ti3 + from the solution with time is indicating the continuous oxidation to the Ti4 + state. Ti3 + does not exist in a solid hydrolysed form and oxidation is the only possible explanation for its consumption. The reductive dissolution of ferric iron increases the reactivity of the ilmenite ore, possibly due to breaking up of the grain structure causing further diffusion of the acid protons to pores created in the ore particles. The measured specific surface areas of the leaching residues after 1 h with and without Fe powder addition were 19.7 and 28.9 m2 g 1, respectively. The increased surface area after Fe addition is considered to be related to increasing pores created by partial dissolution of hematite and ilmenite. Continued rapid leaching of total Fe beyond 60 min seems dependent on the hydrolysis reaction where active HCl is released (Sinha, 1984). This is confirmed by the fact that the rapid dissolution after 60 min coincides with the appearance of a white hydrated TiO2 precipitate in the reaction medium.
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3.2. Proposed flow sheet A promising route for synthetic rutile production can be outlined based on the improved reactivity of ilmenite ore in hydrochloric acid after addition of iron metal without pretreatment. Fig. 5 shows a proposed flow sheet for the production of synthetic rutile from ilmenite by the reductive leaching process in combination with acid regeneration from spent leach liquor. It is designed to be a batch process where leaching and hydrolysis take place simultaneously in one stage. The TiO2 concentrate can be separated by decantation with the help of a flocculating agent. Synthetic rutile is produced by calcination of the TiO2 concentrate at about 900 jC. Hydrochloric acid can be regenerated from the spent liquor by evaporation to separate FeCl2 from which the HCl can be produced by pyrolysis. Ferric oxide is formed in a pellet form which can be used as a feed to steel making, cement production or other uses depending on the location. From the practical point of view, addition of Fe to the HCl should be controlled by some means to overcome evolution of excessive H2 gas. Addition of Fe as a scrap at a low temperature and slowly increasing the temperature could solve this problem. For research reasons, pure Fe powder was used in this study which insures reproducibility of the results. The different stages of the proposed process should be investigated in details (e.g. leaching, solid/liquid separation, acid regeneration, product purification. . .
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etc.). In this work, leaching conditions will be optimised and the product will be characterized. 3.3. Optimisation of leaching conditions The iron content in the synthetic rutile is a very critical parameter that should be minimized to make the product acceptable for chlorination. The leaching conditions, HCl stoichiometry and concentration, temperature, Fe powder addition time and stoichiometry and retention time, were studied to obtain lowest iron content and highest rutile recovery in the solid product. In each experiment, the Fe (as Fe2O3) and Ti (as TiO2) contents in the produced titanium concentrate and in the filtrate were determined and the Fe removal efficiency, TiO2 losses in solution and rutile recovery were calculated. The used ilmenite ore was 100% –75 Am which was found suitable for effective leaching. 3.3.1. Effect of HCl stoichiometry The HCl stoichiometry was calculated according to the Fe and Ti contents in the ilmenite ore applying Eqs. (1) and (2). As shown in Table 4, the Fe content in the synthetic rutile decreased from 2.2% to 0.5% by increasing the HCl stoichiometry from 1 to 1.2 and then gradually decreased at higher HCl stoichiometry. The TiO2 losses in solution increased with increasing stoichiometry of HCl due to the greater inhibition of the titanium hydrolysis and precipitation at higher acid to solid ratio. At the highest acid stoichiometry
Fig. 5. A proposed flowsheet of synthetic rutile production from ilmenite by reductive leaching process.
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Table 4 The effect of HCl stoichiometry on preparation of synthetic rutile HCl stoichiometry
Solid/liquid ratio
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
1.00 1.20 1.45 1.70 1.90
1/6.1 1/7.2 1/8.8 1/10.8 1/11.5
2.2 0.5 0.4 0.3 0.3
97.3 99.4 99.5 99.6 99.7
86.6 88.0 89.8 90.1 90.4
0.5 2.0 2.6 3.3 3.8
96.9 97.5 96.9 96.3 95.9
HCl concentration: 20%, temp.: 110 jC, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
of 1.9 (corresponding to solid/liquid ratio of 1/11.5) the rutile contains only 0.3% Fe (as Fe2O3) but 3.8% TiO2 is lost in the solution. The optimum condition should be selected by comparing the quality of the produced rutile and the TiO2 losses in solution taking in consideration the consumed amount of acid. Thus, an acid stoichiometry of 1.2 was selected as optimum condition from quality and economical point of view. 3.3.2. Effect of HCl concentration A series of experiments was carried out with varying HCl concentration from 15% to 30% at 1.2 HCl stoichiometry. The results in Table 5 revealed that increasing acid concentration very much enhanced the dissolution of iron from the ore. At 15% HCl, the Fe content in the synthetic rutile was 17.3% and this value dropped to 0.1% by increasing the HCl concentration to 30%. That is only 72% of iron was removed using 15% HCl and 99.9% was removed using 30% HCl. On the other hand, increasing the HCl concentration to more than 20% sharply decreased the rutile recovery due to high titanium losses in solution (reaching about 27.5% at 30% HCl). The high acid concentration has a pronounced retarding effect on the Table 5 The effect of HCl concentration on preparation of synthetic rutile HCl Fe in Fe removal TiO2 in TiO2 losses Rutile concentration, rutile, efficiency, rutile, in solution, recovery, % % % % % % 15 18 20 25 30
17.3 2.3 0.5 0.1 0.1
72.2 97.4 99.4 99.9 99.9
64.3 85.7 88.0 89.4 87.9
2.1 2.0 2.0 24.1 27.5
70.1 95.4 97.5 75.8 72.4
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, temperature :110 jC, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
hydrolysis reaction and hence higher amounts of titanium remain soluble in HCl solution. For this reason we selected the optimum HCl concentration to be 20% that is corresponding to 99.4% iron removal and 2.0% TiO2 losses. 3.3.3. Effect of temperature Table 6 shows the results of experiments carried out at 1.2 HCl stoichiometry using 20% HCl but varying the temperature of the reaction mixture from 85 to 110 jC. Increasing the temperature below the boiling point (110 jC) has very little effect on the Fe removal efficiency and rutile recovery. However, drastic increase in these parameters was obtained when the reaction slurry was kept boiling. The Fe content in rutile was as high as 29.2% at 105 jC and dropped to 0.5% by increasing the temperature to 110 jC. This is accompanied by sharp increase in rutile recovery from 27.1% to 97.5%. The hydrolysis reaction of TiOCl2 is known to be much enhanced at higher temperature; this is thought to be the main reason for such a sudden improvement in Fe removal and rutile recovery. Thus the optimum temperature is selected as 110 jC and applied in the subsequent experiments.
Table 6 The effect of temperature on preparation of synthetic rutile Temperature, Fe in Fe removal TiO2 in TiO2 losses Rutile jC rutile, efficiency, rutile, in solution, recovery, % % % % % 85 95 105 110
31.6 30.5 29.2 0.5
49.2 51.2 52.7 99.4
46.2 47.2 48.4 88.0
29.6 28.4 25.7 2.0
19.6 22.8 27.1 97.5
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
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3.3.4. Effect of addition time of Fe powder Leaching experiments were carried out with addition of Fe powder at different times (ranging from 0 to 90 min) at 1.2 HCl stoichiometry, 20% HCl and 110 jC, and results are shown in Table 7. When the Fe powder is added in the beginning of the leaching experiment (at zero time), the content of iron in the produced rutile was as high as 4% with about 93% rutile recovery. However, addition of Fe powder after 15 min of leaching improved the rutile quality (the product contains 0.6% Fe, as Fe2O3, and 87.5% Ti, as TiO2), and increased the rutile recovery to about 97%. However, addition of Fe powder at longer times had little effect. Because the amount of Ti4 + leached increased with time, it is deduced that addition of Fe powder at the beginning of the reaction forms a smaller amount of Ti3 + than when Fe powder is added after longer times of leaching. The improvement of the rutile quality at longer time of Fe addition is thus seems to be related to the presence of larger amounts of Ti3 + in the solution. The optimum time of Fe addition is 30 min corresponding to 0.2% Fe (as Fe2O3) and 89.6% Ti (as TiO2) in the rutile. 3.3.5. Effect of iron powder stoichiometry The effect of the stoichiometric amount of iron powder (corresponding to the content of Fe2O3 in the ilmenite ore that will dissolve and reduced with Fe according to Eq. (7)) added after 30 min was studied in the range from 0.0 to 1.5 at 1.2 HCl stoichiometry, 20% HCl, and 110 jC and results are listed in Table 8. When leaching was carried out without addition of Fe powder, the product had very poor quality; containing 32.5% Fe (as Fe2O3) and 49.4% Ti (as TiO2) with
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Table 8 The effect of Fe powder stoichiometry on preparation of synthetic rutile Fe Fe in Fe removal TiO2 in stoichiometry rutile, efficiency, rutile, % % %
TiO2 Rutile losses in recovery, solution, % %
0.0 0.3 0.5 0.8 0.9 1.0 1.1 1.2 1.5
13.4 7.15 2.08 1.73 1.64 1.58 1.65 1.62 1.59
32.5 15.2 4.8 0.9 0.8 0.6 0.5 0.4 0.2
39.4 77.9 94.1 99.1 99.3 99.5 99.6 99.8 99.8
49.4 67.4 83.4 88.9 89.1 89.3 89.6 89.6 89.6
26.00 70.8 92.0 97.4 97.6 97.8 97.9 98.1 98.2
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe addition time: 30 min.
26% rutile recovery. By increasing the amount of added Fe powder to 1.1 the quality of the rutile noticeably improved (0.5% Fe (as Fe 2 O 3 ) and 89.6% Ti (as TiO2) with 97.9% rutile recovery). Higher Fe stoichiometries than 1.1 had little further effect. The beneficial effect of increasing amount of Fe powder is related to the increasing amount of the produced Ti3 + in solution. This was proved by determining the Ti3 + contents in the leach liquor with time at different Fe powder stoichiometries, as shown in Fig. 6. It was noticed that Ti3 + was not detected before the addition of Fe powder and suddenly increased immediately after Fe powder addition, then
Table 7 The effect of addition time of Fe powder on preparation of synthetic rutile Time, min
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
0 15 30 60 90
4.00 0.59 0.22 0.32 0.47
95.1 99.3 99.8 99.6 99.4
83.8 87.5 89.6 88.9 88.0
2.15 1.98 1.59 1.78 1.95
93.0 97.4 98.2 97.9 97.5
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe stoichiometry: 1.5, retention time: 6 hrs.
Fig. 6. The effect of Fe powder stoichiometry on Ti(III) concentration. HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe addition time: 30 min.
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slowly decreased with time. The higher the added Fe stoichiometry the higher the content of Ti3 +. As stated previously, the presence of Ti3 + enhances the dissolution of ilmenite through keeping the dissolved iron in the divalent state. Higher Ti3 + contents keep the reducing conditions for longer times. An Fe stoichiometry of 1.1 was selected as optimum value since it keeps the Ti3 + till the end of reaction time and gives better product quality. 3.3.6. Effect of retention time Leaching experiments were carried out for different periods (1.5 – 6.0 h) at 1.2 HCl stoichiometry, 20% HCl, 110 jC with Fe stoichiometry 1.1 added after 30 min. It is clear from Table 9 that increasing retention time increased the rutile recovery and Fe removal efficiency during the first 3 h reaching 95.9% and 98.3%, respectively. At longer times, these values increased slowly. The selected optimum retention time was 5 h, corresponding to 97.7% and 99.4% for rutile recovery and Fe removal efficiency, respectively. 3.4. Quality of the produced synthetic rutile The produced titanium dioxide concentrate at the optimum conditions was separated, washed, dried at 110 jC and calcined at 900 jC. The X-ray diffraction pattern of the dried product shows broad bands of low crystalline synthetic rutile while the calcined product shows sharp bands of high crystalline synthetic rutile. The calcined synthetic rutile product was chemically analysed and the results are shown in Table 10. The produced rutile contains about 90% TiO2 and only 0.8% iron as Fe2O3. The colouring metals
Table 10 The chemical analysis of the produced synthetic rutile Component
Weight, %
Component
Weight, %
TiO2 Fe2O3 CaO MgO Al2O3 MnO2
89.45 0.77 0.01 0.04 0.03 0.02
Cr2O3 V2O5 SiO2 Moisture Acid insoluble
0.01 0.10 5.80 1.52 2.25
(MnO2, Cr2O3 and V2O5) in the produced material are not exceeding 0.13%. This content of colouring metals is low enough to be processed for production of white pigments by chlorination. The sum of MgO, CaO and Al2O3 (the chlorine consuming components) is about 0.08% which is considered acceptable for the chlorination process. It is significant that the acid leach removes most of the colouring and other deleterious elements from the ilmenite feed, which are subsequently fixed in the waste. The product also contains 5.8% SiO2 which can be minimized by further purification. Silica is not a major problem in the chloride process; it chlorinates but oxidises to form a white powder. Particle size analysis of the produced synthetic rutile showed that it was 99% < 2.5 Am, substantially smaller than optimal for chlorination. Further studies are required to control the particle size of the synthetic rutile to be mostly >50 Am. This may be carried out by seeding the solution with calcined TiO2 or by sintering the particles that would give some increase in size.
4. Conclusions Table 9 The effect of retention time on preparation of synthetic rutile Time, hr
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
1.5 2.0 3.0 4.0 5.0 6.0
7.4 3.3 1.5 1.2 0.5 0.5
90.9 96.4 98.3 98.6 99.4 99.5
80.0 85.9 87.2 88.4 89.5 89.6
7.5 3.4 2.4 2.1 1.7 1.7
83.5 93.0 95.9 96.6 97.7 97.8
HCl stoichiometry: 1.2, HCl concentration: 20%, solid/liquid: 1/7.23 g/g, temperature: 110 jC, Fe addition time 30 min, and Fe stoichiometry: 1.1.
Synthetic rutile was produced from a medium grade Egyptian ilmenite ore by a process that eliminates the high temperature pretreatment and pressure leaching. The process includes using hydrochloric acid as a leaching agent and iron powder as a reducing agent. The addition of iron powder during leaching of ilmenite creates the Ti3 + in solution which was expected to have a main role in accelerating the leaching of ilmenite ore through keeping the dissolved iron in the ferrous state. The optimum leaching conditions were as follows: HCl stoichiometry: 1.2, HCl concentration: 20%, temperature: 110 jC, Fe powder
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stoichiometry: 1.1 added after 30 min and retention time: 5 h. The calcined produced rutile at the optimum conditions contains about 90% TiO2 and only 0.8% iron as Fe2O3 with 0.12% colouring metals and 5.8% SiO2. This product can be improved by purification to minimize the SiO2 content. The particle size of the produced synthetic rutile is 99% < 2.5 Am which is finer than the required specification for the chlorination process (>50 Am).
References Abdel-Aal, E.A., Ibrahim, I.A., Afifi, A.A.I., Ismail, A.K., 2000. Production of synthetic rutile from Egyptian ilmenite ore by a direct hydrometallurgical process. In: Mishra, B., Yamauchi, C. (Eds.), 2nd International Conference on Processing Materials for Properties. The Minerals, Metals & Materials Society, pp. 955 – 960. Afifi, A.A.I., 1994. A contribution to the hydrometallurgy of Egyptian ilmenite ore for titanium dioxide production, MSc thesis, Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt. Balderson, G.F., MacDonald, C.A., 1999. US Patent 5,885,324, March 23. Bard, A.J., Roger, P., Jordan, J., 1985. Standard potentials in aqueous solution, Marcel Dekker, New York, pp. 41, 391, 539. Becher, R.G., 1963. Australian Patent 247, 110. Cservenya´k, I., Kelsall, G.H., Wang, W., 1996. Reduction of TiIV species in aqueous sulfuric acid and hydrochloric acids: titanium speciation. Electrochimica Acta 41, 563 – 572. Diebold, U., 2003. The surface science of titanium dioxide. Surface Science Reports 48, 53 – 229. El-Khaliny, N.M., 1967. Preparation of titanium dioxide from the Egyptian ilmenite ore, MSc thesis, Faculty of Science, Ain Shams University, Cairo, Egypt. Farrow, J.B., Ritchie, I.M., 1987. The reaction between reduced ilmenite and oxygen in ammonium chloride solutions. Hydrometallurgy 18, 21 – 38. Furman, N.H., 1975. Standard Method of Chemical Analysis, 6th ed., vol. 1, pp. 1104 – 1105.
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Jackson, J.S., Wadsworth, M.E., 1976. A kinetic study of the dissolution of Allard Lake ilmenite in hydrochloric acid. Light Metals 1, 481 – 540. Kahn, J.A., 1984. Non-rutile feedstocks for the production of titanium. Journal of Metals 36, 33 – 38. Lanyon, M.R., Lwin, T., Merritt, R.R., 1999. The dissolution of iron in the hydrochloric acid leach of an ilmenite concentrate. Hydrometallurgy 51, 223 – 299. Lu, Z.Y., Muir, D.M., 1988. Dissolution of metal ferrites and iron oxides by HCl under oxidising and reducing conditions. Hydrometallurgy 21, 9 – 21. Mackey, T.S., 1994. Upgrading ilmenite into a high-grade synthetic rutile. Journal of Metals, April, 59 – 64. Natziger, R.H., and Elger, G.W., 1987. ‘‘Preparation of titanium feedstock from Minnesota ilmenite by smelting and sulfation – leaching’’ US Bureau of Mines, Report Invest No. 9065 Newman, E.G., Balderson, G.F., 1993. Pivot Mining NL. Continuous leaching of treated titaniferous ores with interstage evaporation, World Patent 9318192, 1993 September 16, Application number: PCT/AU93/00095; 9 March 1993. Sinha, H.N., 1973. MURSO process for producing rutile substitute. In: Jaffe, R.I., Burte, H.M. (Eds.), Titanium Science and Technology. Cambridge, Massachusetts, pp. 233 – 244. May 2 – 5, 1972, A publication of the Metallurgical Society of AIME, Plenum Press, New York - London. Sinha, H.N., 1979. Fluidized-bed leaching of ilmenite. In: Jones, M.J. (Ed.), Proceedings of the Eleventh Commonwealth Mining and Metallurgical Congress. Institute of Mining and Metallurgy, London, pp. 669 – 672. Sinha, H.N., 1984. Hydrochloric acid leaching of ilmenite. Proceeding of the Symposium on Extractive Metallurgy. The Australian Institute of Mining and Metallurgy, Melbourne, pp. 163 – 168. van Dyk, J.P., Vegter, N.M., Pistorius, P.C., 2002. Kinetics of ilmenite dissolution in hydrochloric acid. Hydrometallurgy 65, 31 – 36. Vogel, A.I., 1978. A Textbook of Quantitative Inorganic Analysis, 4th ed., Longman, London, pp. 742, 750. Walpole, E.A., 1995. Acid regeneration, Australian Patent 659535, Almeth. and Austpac Technology, September. 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 1997. SAIMM, Johannesburg, pp. 169 – 174.
Reductive leaching of ilmenite ore in hydrochloric acid for preparation of synthetic rutile M.H.H. Mahmoud *, A.A.I. Afifi, I.A. Ibrahim Central Metallurgical R&D Institute, P.O. Box 87, Helwan, Cairo 14421, Egypt Received 28 June 2003; received in revised form 13 August 2003; accepted 14 August 2003
Abstract The reactivity of ilmenite ore during leaching with hydrochloric acid can be greatly enhanced by reduction in solution using metallic iron. Addition of a particular amount of iron powder after a certain time of reaction will reduce all the dissolved Fe3 + to Fe2 + and reduce a portion of the dissolved Ti4 + to Ti3 +. As the leaching continues, any further dissolved Fe3 + will be simultaneously reduced by the action of the formed Ti3 + to produce Fe2 + and Ti4 +. As a result, dissolution of the iron and titanium contents from the ore will be improved. Meanwhile, the dissolved Ti4 + will be hydrolysed and precipitated leaving mainly ferrous chloride in the solution. The hydrolysis is accompanied by fast dissolution of the unreacted ore. A reductive leaching process is proposed taking advantages of the recent advances in HCl regeneration from spent leach liquor. The different leaching parameters were optimised to obtain as little iron as possible in the titanium dioxide product. Synthetic rutile of about 90% TiO2 and 0.8% Fe2O3 was obtained from an Egyptian medium-grade ilmenite ore of grain size 100% – 75 Am using 20% HCl at boiling temperature (110 jC), with addition of 0.11 g iron powder/g ore for 5 h. The product has a low content of colouring elements (total MnO2, Cr2O3 and V2O5 = 0.12%) and chlorine consuming elements (total CaO, MgO and Al2O3 = 0.08%) but contain high silica (SiO2 = 5.8%). The process is applicable for higher grade ilmenite ores which will produce higher quality synthetic rutile. D 2004 Elsevier B.V. All rights reserved. Keywords: Ilmenite ore; Acid leaching; Reduction in solution; Hydrochloric acid; Synthetic rutile
1. Introduction Titanium dioxide is an important intermediate in the manufacture of paints, pigments, welding-rod coatings, ceramics, papers, and in other areas of chemical industry (Diebold, 2003). White titanium dioxide pigments have been produced by two pro-
* Corresponding author. Tel.: +20-2-501-0642; fax: +20-2-5010639. E-mail address: [email protected] (M.H.H. Mahmoud). 0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2003.08.001
cesses, namely the sulfate process, and the dry chlorination process. The sulfate process which utilises ilmenite (FeTiO3) as a raw material is well known and widely applied but it is lengthy, costly and the byproduct ferrous sulfate is less marketable (Afifi, 1994; Abdel-Aal et al., 2000). The dry chlorination process which utilises rutile (TiO2) as a raw material presently enjoys more favourable economics and generates less waste materials (Mackey, 1994). Shortage of natural rutile has encouraged research efforts to convert ilmenite into synthetic rutile for the chlorination process.
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There are several processes for the production of synthetic rutile from ilmenite, most of which fall into one of five classifications: (1) Smelting processes, where the iron part of the ilmenite is reduced and melted to separate the iron from titanium (Natziger and Elger, 1987). The titania slag is then leached with the sulfuric acid or with hydrochloric acid at elevated temperatures. (2) Reduction of ilmenite to convert ferric iron partially to the ferrous form or completely to the metallic iron form followed by acid leaching (Balderson and MacDonald, 1999; Kahn, 1984). (3) Reduction of the iron content of the ilmenite to the metallic iron followed by corrosion with oxygen and ammonium chloride (Becher, 1963; Farrow and Ritchie, 1987). (4) Oxidation and reduction of ilmenite followed by hydrochloric acid leaching, MURSO process (Sinha, 1973, 1979). (5) Roasting and magnetic separation followed by hydrochloric acid leaching, ERMS process (Walpole, 1997). All these processes depend mainly on reductive and/or oxidative thermal pretreatment of ilmenite which is an extensive energy consuming stage. Reductive pretreatment aims at converting the ferric iron in ilmenite into the ferrous state which is more soluble in hydrochloric acid. It was stated elsewhere that the dissolution of ilmenite in hydrochloric acid could be enhanced by reduction in solution using metals such as iron, zinc and tin (El-Khaliny, 1967). However, this concept was not practiced to get benefit for dissolution of ilmenite and preparation of synthetic rutile. In the present work, we propose a process for the production of synthetic rutile based on hydrochloric acid leaching of ilmenite ore at ambient pressure in the presence of metallic iron as a reducing agent. The iron metal can be added as a pure powder or as scrap. This trend for synthetic rutile production is considered to be advantageous compared with the other mentioned processes in view of the avoidance of high temperature pretreatment or complicated pressure leaching. The acid consumption could be higher in the proposed process but recent advances in acid regeneration technology from the spent leaching liquor
have made acid leaching processes more attractive (Walpole, 1995; Newman and Balderson, 1993). Also, the by-product iron oxide may have commercial uses and add value to the process. Large ilmenite deposits are present in the South Eastern desert of Egypt. Among these are reserves in Abu Ghalaga ilmenite region, estimated to be about 50 million tons. The present work aims at studying the preparation of synthetic rutile by reductive leaching of Abu Ghalaga ilmenite ore in hydrochloric acid. The study outlines the expected reaction mechanism, optimisation of parameters affecting the process and evaluation of the product.
2. Experimental 2.1. Materials and procedure One ton of ilmenite ore from Abu Ghalaga region, Red Sea, was thoroughly mixed and a representative sample of about 15 kg was crushed and ground to 100% –75 Am and used for conducting this work. The particle size analysis of the used ilmenite ore is given in Table 1. Acid leaching of ilmenite ore was carried out using a 250 cm3 three necked glass reactor provided with a reflux condenser and a mechanical agitator with Teflon-coated stirring rod. A stirring speed of 400 rpm was applied to keep the slurry suspended during the leaching experiment. The desired volume of hydrochloric acid of the required concentration was poured in the reactor and heated using a thermostatically controlled glycerol/water bath. After reaching the desired temperature, a 20 g ilmenite ore sample was added and after a certain time a weighed amount of iron powder was added. In the preliminary investigations, samples of 1 cm3 were withdrawn from the leach liquor to determine the dissolved contents of Fe2 +, Fe3 +, Ti4 +, Ti3 + and the residual HCl concentration. After the required reacTable 1 The particle size distribution of ilmenite ore Size, Am
Cumulative percentage passed
75 63 53 45
100.0 81.8 60.7 33.8
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tion period, the slurry was filtered off and the solid titanium concentrate was thoroughly washed with 3% HCl to minimize the concentration of iron in the liquor that is entrained within the porous solid. The washed titanium concentrate was dried in air at 110 jC and calcined at 900 jC to completely dehydrate the sample. 2.2. Materials characterization Phase identification of the ilmenite ore and the titanium concentrate were carried out using X-ray diffraction. Analysis of the Abu Ghalaga ilmenite ore showed that the strongest lines correspond to the typical ilmenite structure (card #29-733) with lines of lower but still appreciable intensities indicating the presence of free hematite (card #13-534 and 24-27). Photomicrographs of the Abu Ghalaga ilmenite ore are shown in Fig. 1. It can be seen that ilmenite grains enclose exsolution bodies of hematite. It also reveals that minute hematite lamellae are segregated forming elongated bodies of hematite within the ilmenite host (plate A). Small grains of hematite are exsolved and segregated at the borders of ilmenite grains (plate B). Thus, the ore is mainly formed from ferri-ilmenite together with small quantities of titano-hematite. For the chemical analysis of the ilmenite ore, a weighed sample of finely ground ore was fused with potassium pyrosulfate, dissolved in 1:1 H2SO4 and filtered. The metallic contents were analysed in the filtrate and the SiO2 content was analysed in the insoluble residue by evaporation with HF. Titanium was determined spectrophotometrically by the hydro-
101
Table 2 The chemical analysis of the ilmenite ore Component
Weight, %
Component
Weight, %
TiO2 FeO Fe2O3 Fe Total Al2O3 MgO CaO MnO
41.10 24.40 28.60 39.01 0.63 0.64 0.15 0.36
Cr2O3 V2O5 SO3 CO2 P2O5 SiO2 Moisture
0.36 0.40 0.11 0.65 0.02 2.43 0.15
gen peroxide method at wavelength 410 nm (Vogel, 1978). Titanium (III) was determined by titration against FeCl3 using KSCN as indicator (Furman, 1975). Total iron and ferrous iron were determined spectrophotometrically using the phenanthroline method at wavelength 515 nm (Vogel, 1978). Ferric iron was calculated by difference between total and ferrous iron contents. For the chemical analysis of the titanium concentrate, a weighed sample of the calcined concentrate was dissolved in ammonium sulfate and concentrated sulfuric acid. The content of titanium in the filtrate was determined spectrophotometrically; iron and other metallic impurities were determined by atomic absorption spectrometer (Perkin Elmer 3100). The SiO2 content was determined by the HF method. The chemical composition of the ore sample is given in Table 2. The ore contains 41.1% TiO2, 24.4% FeO and 28.6% Fe2O3. This ore is considered as medium grade since it contains a relatively small amount of titanium. The ore also contains 2.4% SiO2, 0.02 P2O5 and minor elements such as Mn, V, Cr and Mg. Most of the impurities in the ilmenite ore
Fig. 1. Photomicrographs of Abu Ghalaga ilmenite ore.
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are expected to be dissolved by acid leaching while SiO2 content will be enriched in the final product. Silica content in the ore can be minimized by gravity concentration which will yield more acceptable product, but this work utilises as received ore. The size distribution of the produced synthetic rutile was determined using a laser sizing (Analysete 22, FRITCH). Surface areas were measured using high-speed gas sorption analyzer (NOVA 2200, version 7.11 Quantachrome).
3. Results and discussion 3.1. Preliminary investigation and proposed mechanism Several studies have been performed to illustrate the mechanism of ilmenite leaching in hydrochloric acid (van Dyk et al., 2002; Lanyon et al., 1999; Jackson and Wadsworth, 1976; Sinha, 1984). Most of these studies include reduction or oxidation pretreatment. Here we will investigate the direct leaching of ilmenite without and with addition of iron powder as a reducing agent. 3.1.1. Leaching without reducing agent Direct leaching of ilmenite ore was performed at a typical ilmenite/20% HCl ratio of 1:7.3 g/g at the boiling temperature, 110 jC. The extent of extraction of total Fe and Ti was measured at different time intervals based on the content of the dissolved metals. As shown in Fig. 2, the extraction increased with time reaching values of about 18% and 13% for total Fe and Ti after 30 min, respectively. The extraction of total Fe slowly increased at times longer than 30 min and reached only about 29% after 5 h. However, the values of Ti extraction slightly decreased with times longer than 30 min and reached about 10% after 5 h. This decrease may be due to transformation of a portion of the dissolved titanium into the white insoluble hydrated titanium dioxide. This was obvious by comparing the black colour of the ore and the dark grey colour of the residue. It is clear from these results that the ore is only partially leached; this may be due to the limited dissolution of hematite in hydrochloric acid. This will adversely affect the leaching process since the hema-
Fig. 2. The leaching of ilmenite ore with HCl. HCl concn.: 20%, temp.: 110 jC, ilmenite/acid ratio: 1/7.3 g/g.
tite is finely disseminated in the ilmenite (as shown in the microscopic investigations in Fig. 1). 3.1.2. Leaching with Fe powder as a reducing agent It is known that the hematite dissolution in hydrochloric acid can be enhanced in reducing medium (Lu and Muir, 1988). Thus, reduction in solution is suggested here to promote the leaching of ilmenite ore in hydrochloric acid. Iron metal is considered the most suitable reductant for this purpose since no foreign ions will be introduced to the reaction medium and the ferrous chloride formed can be separated from the solution and used for regeneration of HCl. A leaching experiment was carried out at the same conditions as the previous test (ilmenite/20% HCl ratio of 1:7.3 and 110 jC) but with the addition of Fe powder, as 0.075 g Fe/g ore, after 20 min of reaction. Plots of extracted percentages of total Fe, total Ti and Ti3 + are shown in Fig. 3. The added Fe powder was subtracted when the total Fe extraction is calculated. In this case, Ti3 + started to appear in the solution once the Fe powder was added and suddenly increased with time. Then, the Ti3 + content gradually decreased and was absent after about 90 min. It is clear that the extraction of total Fe and Ti in Fig. 3 are markedly changed by adding Fe powder, compared to the extractions without Fe powder addition, Fig. 2. The total extracted Ti rapidly increased after addition of the Fe powder, reached a peak value of about 28% after 40 to 60 min and then started to decrease to a
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Fig. 3. The Leaching of ilmenite ore with HCl in presence of iron powder. HCl concn.: 20%, ilmenite/acid ratio: 1/7.3 g/g, temp.: 110 jC, Fe powder: 0.075 g/g ore, time of Fe addition: 20 min.
small value of about 2% after 120 min and then remained almost unchanged at longer time. On the other hand, the total extracted Fe sharply increased after addition of Fe powder, reached a plateau at about 45% between 40 and 60 min and then increased to about 90% after 120 min and then gradually increased to 98% after 6 h. Variations of residual HCl concentrations in the leach liquor were determined during leaching with and without addition of Fe powder and results are shown in Fig. 4. In the absence of Fe powder, the acid concentration decreased from 20% to 15% after 20 min and very slowly decreased at longer time. After 10 min of Fe powder addition, the acid concentration decreased from 15% to 12% and then continuously decreased with time to about 7% after 5 h. The sudden decrease in HCl concentration after addition of Fe powder is due to faster reactions of HCl with the added Fe powder and with the ilmenite ore than without Fe powder addition. It was evident (using the conditions mentioned in Fig. 1) that the added iron is completely dissolved after 10 min. This was revealed by observing the evolution of hydrogen gas that was started when the Fe metal was added and stopped after 10 min. The chemical analysis of the leach liquor before and after 10 min of Fe addition was performed to show the drastic change in the solution composition after the Fe
103
Fig. 4. Variation of HCl concentration with time during leaching of ilmenite ore with and without addition of Fe powder. HCl concn.: 20%, ilmenite/acid ratio: 1/7.3 g/g, temp.: 110 jC, Fe powder: 0.075 g/g ore, time of Fe addition: 20 min.
addition due to the rapid leaching. Table 3 shows that the amount of total dissolved titanium was doubled and the amount of total dissolved iron was increased >2.5 times (the added Fe was subtracted) after 10 min of Fe addition. It also shows that 20% of the HCl was consumed at the same time. It is thus clear that by addition of Fe powder the iron and titanium content of the ilmenite ore are mostly dissolved in hydrochloric acid after about 6 h, while the dissolved titanium is continuously separated as hydrolysed TiO2. 3.1.3. Discussion of extraction behaviour and proposed reactions The possible mechanisms of extraction before and after the addition of Fe powder will be presented Table 3 The chemical analysis of the leach liquor before and after 10 minutes of iron powder addition Element
Total Ti Ti(IV) Ti(III) Total Fe Fe(III) Fe(II) HCl(100%)
Weight, g Before Fe addition (retention time = 20 min)
After 10 min of Fe addition (retention time = 30 min)
0.60 0.60 0.00 1.28 0.72 0.56 22.76
1.20 0.87 0.32 3.12 0.00 3.12 18.30
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through several chemical reactions. The ilmenite fraction in the ore is dissolved in hydrochloric acid as follows:
This is consistent with the EMF values estimated from the standard potentials of the half-cell reactions given below (Bard et al., 1985).
FeO TiO2 þ 4HCl ¼ FeCl2 þ TiOCl2 þ 2H2 O ð1Þ
2Hþ þ 2e ¼ H2
and the hematite fraction in the ore is dissolved in hydrochloric acid as follows:
Fe3þ þ e ¼ Fe2þ
Fe2 O3 þ 6HCl ¼ 2FeCl3 þ 3H2 O
ð2Þ
The decrease in the amount of total Ti in the solution after reaching a peak is related to the hydrolysis of the dissolved titanium species. The predominant species of titanium at the used HCl concentration is TiOCl2 (Cservenya´k et al., 1996). The hydrolysis reaction where HCl is released is given as follows: TiOCl2 þ H2 O ¼ TiO2 # þ 2HCl
ð3Þ
The added iron powder will readily react with HCl creating nascent hydrogen and ferrous chloride. Fe þ 2HCl ¼ FeCl2 þ 2H
ð4Þ
A part of the nascent hydrogen forms hydrogen gas. H þ H ¼ H2 z
ð5Þ
The dissolved Fe3 + will be simultaneously reduced to the Fe2 + state by reaction with the nascent hydrogen and/ore with the Fe powder. 2FeCl3 þ 2H ¼ 2FeCl2 þ 2HCl
ð6Þ
2FeCl3 þ Fe ¼ 3FeCl2
ð7Þ
The reduction of all the Fe3 + in the solution to the Fe ions is completed when the colour of the solution changes from yellow-red to violet. The appearance of the characteristic violet colour of the Ti3 + indicates the consequent reduction of the Ti4 + (that is dissolved before the Fe addition) to the Ti3 + state by reaction with the nascent hydrogen and/or the Fe powder. 2+
2TiOCl2 þ 2HCl þ 2H ¼ 2TiCl3 þ 2H2 O
ð8Þ
2TiOCl2 þ Fe þ 4HCl ¼ 2TiCl3 þ FeCl2 þ 2H2 O ð9Þ
ð10Þ
0:00 V
ð11Þ
0:77 V
TiO2þ þ 2Hþ þ e ¼ Ti3þ þ H2 O
0:10 V
ð12Þ
The higher EMF value of the Fe3 +/Fe2 + couple (0.77 V) compared with the TiO2 +/Ti3 + couple (0.1 V) indicates the higher tendency for Fe3 + reduction (Eq. (11)) to proceed than Ti4 + reduction (Eq. (12)). Since the nascent hydrogen will be rapidly removed from the reaction medium as soon as the iron powder is completely dissolved, the formed Ti3 + will keep the reducing conditions in the solution. That is, the formed Ti3 + will reduce any further dissolved Fe3 + to the Fe2 + state. TiCl3 þ FeCl3 þ H2 O ¼ TiOCl2 þ FeCl2 þ 2HCl ð13Þ This sequence is confirmed by the profile of Ti3 + contents in solution as shown in Fig. 3. The consumption of the Ti3 + from the solution with time is indicating the continuous oxidation to the Ti4 + state. Ti3 + does not exist in a solid hydrolysed form and oxidation is the only possible explanation for its consumption. The reductive dissolution of ferric iron increases the reactivity of the ilmenite ore, possibly due to breaking up of the grain structure causing further diffusion of the acid protons to pores created in the ore particles. The measured specific surface areas of the leaching residues after 1 h with and without Fe powder addition were 19.7 and 28.9 m2 g 1, respectively. The increased surface area after Fe addition is considered to be related to increasing pores created by partial dissolution of hematite and ilmenite. Continued rapid leaching of total Fe beyond 60 min seems dependent on the hydrolysis reaction where active HCl is released (Sinha, 1984). This is confirmed by the fact that the rapid dissolution after 60 min coincides with the appearance of a white hydrated TiO2 precipitate in the reaction medium.
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3.2. Proposed flow sheet A promising route for synthetic rutile production can be outlined based on the improved reactivity of ilmenite ore in hydrochloric acid after addition of iron metal without pretreatment. Fig. 5 shows a proposed flow sheet for the production of synthetic rutile from ilmenite by the reductive leaching process in combination with acid regeneration from spent leach liquor. It is designed to be a batch process where leaching and hydrolysis take place simultaneously in one stage. The TiO2 concentrate can be separated by decantation with the help of a flocculating agent. Synthetic rutile is produced by calcination of the TiO2 concentrate at about 900 jC. Hydrochloric acid can be regenerated from the spent liquor by evaporation to separate FeCl2 from which the HCl can be produced by pyrolysis. Ferric oxide is formed in a pellet form which can be used as a feed to steel making, cement production or other uses depending on the location. From the practical point of view, addition of Fe to the HCl should be controlled by some means to overcome evolution of excessive H2 gas. Addition of Fe as a scrap at a low temperature and slowly increasing the temperature could solve this problem. For research reasons, pure Fe powder was used in this study which insures reproducibility of the results. The different stages of the proposed process should be investigated in details (e.g. leaching, solid/liquid separation, acid regeneration, product purification. . .
105
etc.). In this work, leaching conditions will be optimised and the product will be characterized. 3.3. Optimisation of leaching conditions The iron content in the synthetic rutile is a very critical parameter that should be minimized to make the product acceptable for chlorination. The leaching conditions, HCl stoichiometry and concentration, temperature, Fe powder addition time and stoichiometry and retention time, were studied to obtain lowest iron content and highest rutile recovery in the solid product. In each experiment, the Fe (as Fe2O3) and Ti (as TiO2) contents in the produced titanium concentrate and in the filtrate were determined and the Fe removal efficiency, TiO2 losses in solution and rutile recovery were calculated. The used ilmenite ore was 100% –75 Am which was found suitable for effective leaching. 3.3.1. Effect of HCl stoichiometry The HCl stoichiometry was calculated according to the Fe and Ti contents in the ilmenite ore applying Eqs. (1) and (2). As shown in Table 4, the Fe content in the synthetic rutile decreased from 2.2% to 0.5% by increasing the HCl stoichiometry from 1 to 1.2 and then gradually decreased at higher HCl stoichiometry. The TiO2 losses in solution increased with increasing stoichiometry of HCl due to the greater inhibition of the titanium hydrolysis and precipitation at higher acid to solid ratio. At the highest acid stoichiometry
Fig. 5. A proposed flowsheet of synthetic rutile production from ilmenite by reductive leaching process.
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Table 4 The effect of HCl stoichiometry on preparation of synthetic rutile HCl stoichiometry
Solid/liquid ratio
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
1.00 1.20 1.45 1.70 1.90
1/6.1 1/7.2 1/8.8 1/10.8 1/11.5
2.2 0.5 0.4 0.3 0.3
97.3 99.4 99.5 99.6 99.7
86.6 88.0 89.8 90.1 90.4
0.5 2.0 2.6 3.3 3.8
96.9 97.5 96.9 96.3 95.9
HCl concentration: 20%, temp.: 110 jC, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
of 1.9 (corresponding to solid/liquid ratio of 1/11.5) the rutile contains only 0.3% Fe (as Fe2O3) but 3.8% TiO2 is lost in the solution. The optimum condition should be selected by comparing the quality of the produced rutile and the TiO2 losses in solution taking in consideration the consumed amount of acid. Thus, an acid stoichiometry of 1.2 was selected as optimum condition from quality and economical point of view. 3.3.2. Effect of HCl concentration A series of experiments was carried out with varying HCl concentration from 15% to 30% at 1.2 HCl stoichiometry. The results in Table 5 revealed that increasing acid concentration very much enhanced the dissolution of iron from the ore. At 15% HCl, the Fe content in the synthetic rutile was 17.3% and this value dropped to 0.1% by increasing the HCl concentration to 30%. That is only 72% of iron was removed using 15% HCl and 99.9% was removed using 30% HCl. On the other hand, increasing the HCl concentration to more than 20% sharply decreased the rutile recovery due to high titanium losses in solution (reaching about 27.5% at 30% HCl). The high acid concentration has a pronounced retarding effect on the Table 5 The effect of HCl concentration on preparation of synthetic rutile HCl Fe in Fe removal TiO2 in TiO2 losses Rutile concentration, rutile, efficiency, rutile, in solution, recovery, % % % % % % 15 18 20 25 30
17.3 2.3 0.5 0.1 0.1
72.2 97.4 99.4 99.9 99.9
64.3 85.7 88.0 89.4 87.9
2.1 2.0 2.0 24.1 27.5
70.1 95.4 97.5 75.8 72.4
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, temperature :110 jC, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
hydrolysis reaction and hence higher amounts of titanium remain soluble in HCl solution. For this reason we selected the optimum HCl concentration to be 20% that is corresponding to 99.4% iron removal and 2.0% TiO2 losses. 3.3.3. Effect of temperature Table 6 shows the results of experiments carried out at 1.2 HCl stoichiometry using 20% HCl but varying the temperature of the reaction mixture from 85 to 110 jC. Increasing the temperature below the boiling point (110 jC) has very little effect on the Fe removal efficiency and rutile recovery. However, drastic increase in these parameters was obtained when the reaction slurry was kept boiling. The Fe content in rutile was as high as 29.2% at 105 jC and dropped to 0.5% by increasing the temperature to 110 jC. This is accompanied by sharp increase in rutile recovery from 27.1% to 97.5%. The hydrolysis reaction of TiOCl2 is known to be much enhanced at higher temperature; this is thought to be the main reason for such a sudden improvement in Fe removal and rutile recovery. Thus the optimum temperature is selected as 110 jC and applied in the subsequent experiments.
Table 6 The effect of temperature on preparation of synthetic rutile Temperature, Fe in Fe removal TiO2 in TiO2 losses Rutile jC rutile, efficiency, rutile, in solution, recovery, % % % % % 85 95 105 110
31.6 30.5 29.2 0.5
49.2 51.2 52.7 99.4
46.2 47.2 48.4 88.0
29.6 28.4 25.7 2.0
19.6 22.8 27.1 97.5
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, Fe addition time: 90 min, Fe stoichiometry: 1.5, retention time: 6 hrs.
M.H.H. Mahmoud et al. / Hydrometallurgy 73 (2004) 99–109
3.3.4. Effect of addition time of Fe powder Leaching experiments were carried out with addition of Fe powder at different times (ranging from 0 to 90 min) at 1.2 HCl stoichiometry, 20% HCl and 110 jC, and results are shown in Table 7. When the Fe powder is added in the beginning of the leaching experiment (at zero time), the content of iron in the produced rutile was as high as 4% with about 93% rutile recovery. However, addition of Fe powder after 15 min of leaching improved the rutile quality (the product contains 0.6% Fe, as Fe2O3, and 87.5% Ti, as TiO2), and increased the rutile recovery to about 97%. However, addition of Fe powder at longer times had little effect. Because the amount of Ti4 + leached increased with time, it is deduced that addition of Fe powder at the beginning of the reaction forms a smaller amount of Ti3 + than when Fe powder is added after longer times of leaching. The improvement of the rutile quality at longer time of Fe addition is thus seems to be related to the presence of larger amounts of Ti3 + in the solution. The optimum time of Fe addition is 30 min corresponding to 0.2% Fe (as Fe2O3) and 89.6% Ti (as TiO2) in the rutile. 3.3.5. Effect of iron powder stoichiometry The effect of the stoichiometric amount of iron powder (corresponding to the content of Fe2O3 in the ilmenite ore that will dissolve and reduced with Fe according to Eq. (7)) added after 30 min was studied in the range from 0.0 to 1.5 at 1.2 HCl stoichiometry, 20% HCl, and 110 jC and results are listed in Table 8. When leaching was carried out without addition of Fe powder, the product had very poor quality; containing 32.5% Fe (as Fe2O3) and 49.4% Ti (as TiO2) with
107
Table 8 The effect of Fe powder stoichiometry on preparation of synthetic rutile Fe Fe in Fe removal TiO2 in stoichiometry rutile, efficiency, rutile, % % %
TiO2 Rutile losses in recovery, solution, % %
0.0 0.3 0.5 0.8 0.9 1.0 1.1 1.2 1.5
13.4 7.15 2.08 1.73 1.64 1.58 1.65 1.62 1.59
32.5 15.2 4.8 0.9 0.8 0.6 0.5 0.4 0.2
39.4 77.9 94.1 99.1 99.3 99.5 99.6 99.8 99.8
49.4 67.4 83.4 88.9 89.1 89.3 89.6 89.6 89.6
26.00 70.8 92.0 97.4 97.6 97.8 97.9 98.1 98.2
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe addition time: 30 min.
26% rutile recovery. By increasing the amount of added Fe powder to 1.1 the quality of the rutile noticeably improved (0.5% Fe (as Fe 2 O 3 ) and 89.6% Ti (as TiO2) with 97.9% rutile recovery). Higher Fe stoichiometries than 1.1 had little further effect. The beneficial effect of increasing amount of Fe powder is related to the increasing amount of the produced Ti3 + in solution. This was proved by determining the Ti3 + contents in the leach liquor with time at different Fe powder stoichiometries, as shown in Fig. 6. It was noticed that Ti3 + was not detected before the addition of Fe powder and suddenly increased immediately after Fe powder addition, then
Table 7 The effect of addition time of Fe powder on preparation of synthetic rutile Time, min
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
0 15 30 60 90
4.00 0.59 0.22 0.32 0.47
95.1 99.3 99.8 99.6 99.4
83.8 87.5 89.6 88.9 88.0
2.15 1.98 1.59 1.78 1.95
93.0 97.4 98.2 97.9 97.5
HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe stoichiometry: 1.5, retention time: 6 hrs.
Fig. 6. The effect of Fe powder stoichiometry on Ti(III) concentration. HCl stoichiometry: 1.2, solid/liquid: 1/7.23 g/g, HCl concentration: 20%, temperature:110 jC, Fe addition time: 30 min.
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M.H.H. Mahmoud et al. / Hydrometallurgy 73 (2004) 99–109
slowly decreased with time. The higher the added Fe stoichiometry the higher the content of Ti3 +. As stated previously, the presence of Ti3 + enhances the dissolution of ilmenite through keeping the dissolved iron in the divalent state. Higher Ti3 + contents keep the reducing conditions for longer times. An Fe stoichiometry of 1.1 was selected as optimum value since it keeps the Ti3 + till the end of reaction time and gives better product quality. 3.3.6. Effect of retention time Leaching experiments were carried out for different periods (1.5 – 6.0 h) at 1.2 HCl stoichiometry, 20% HCl, 110 jC with Fe stoichiometry 1.1 added after 30 min. It is clear from Table 9 that increasing retention time increased the rutile recovery and Fe removal efficiency during the first 3 h reaching 95.9% and 98.3%, respectively. At longer times, these values increased slowly. The selected optimum retention time was 5 h, corresponding to 97.7% and 99.4% for rutile recovery and Fe removal efficiency, respectively. 3.4. Quality of the produced synthetic rutile The produced titanium dioxide concentrate at the optimum conditions was separated, washed, dried at 110 jC and calcined at 900 jC. The X-ray diffraction pattern of the dried product shows broad bands of low crystalline synthetic rutile while the calcined product shows sharp bands of high crystalline synthetic rutile. The calcined synthetic rutile product was chemically analysed and the results are shown in Table 10. The produced rutile contains about 90% TiO2 and only 0.8% iron as Fe2O3. The colouring metals
Table 10 The chemical analysis of the produced synthetic rutile Component
Weight, %
Component
Weight, %
TiO2 Fe2O3 CaO MgO Al2O3 MnO2
89.45 0.77 0.01 0.04 0.03 0.02
Cr2O3 V2O5 SiO2 Moisture Acid insoluble
0.01 0.10 5.80 1.52 2.25
(MnO2, Cr2O3 and V2O5) in the produced material are not exceeding 0.13%. This content of colouring metals is low enough to be processed for production of white pigments by chlorination. The sum of MgO, CaO and Al2O3 (the chlorine consuming components) is about 0.08% which is considered acceptable for the chlorination process. It is significant that the acid leach removes most of the colouring and other deleterious elements from the ilmenite feed, which are subsequently fixed in the waste. The product also contains 5.8% SiO2 which can be minimized by further purification. Silica is not a major problem in the chloride process; it chlorinates but oxidises to form a white powder. Particle size analysis of the produced synthetic rutile showed that it was 99% < 2.5 Am, substantially smaller than optimal for chlorination. Further studies are required to control the particle size of the synthetic rutile to be mostly >50 Am. This may be carried out by seeding the solution with calcined TiO2 or by sintering the particles that would give some increase in size.
4. Conclusions Table 9 The effect of retention time on preparation of synthetic rutile Time, hr
Fe in rutile, %
Fe removal efficiency, %
TiO2 in rutile, %
TiO2 losses in solution, %
Rutile recovery, %
1.5 2.0 3.0 4.0 5.0 6.0
7.4 3.3 1.5 1.2 0.5 0.5
90.9 96.4 98.3 98.6 99.4 99.5
80.0 85.9 87.2 88.4 89.5 89.6
7.5 3.4 2.4 2.1 1.7 1.7
83.5 93.0 95.9 96.6 97.7 97.8
HCl stoichiometry: 1.2, HCl concentration: 20%, solid/liquid: 1/7.23 g/g, temperature: 110 jC, Fe addition time 30 min, and Fe stoichiometry: 1.1.
Synthetic rutile was produced from a medium grade Egyptian ilmenite ore by a process that eliminates the high temperature pretreatment and pressure leaching. The process includes using hydrochloric acid as a leaching agent and iron powder as a reducing agent. The addition of iron powder during leaching of ilmenite creates the Ti3 + in solution which was expected to have a main role in accelerating the leaching of ilmenite ore through keeping the dissolved iron in the ferrous state. The optimum leaching conditions were as follows: HCl stoichiometry: 1.2, HCl concentration: 20%, temperature: 110 jC, Fe powder
M.H.H. Mahmoud et al. / Hydrometallurgy 73 (2004) 99–109
stoichiometry: 1.1 added after 30 min and retention time: 5 h. The calcined produced rutile at the optimum conditions contains about 90% TiO2 and only 0.8% iron as Fe2O3 with 0.12% colouring metals and 5.8% SiO2. This product can be improved by purification to minimize the SiO2 content. The particle size of the produced synthetic rutile is 99% < 2.5 Am which is finer than the required specification for the chlorination process (>50 Am).
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