In the Laboratory
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Reduction of Ilmenite with Charcoal Kristy M. Blyth, Mark I. Ogden, David N. Phillips,* David Pritchard, and Wilhelm van Bronswijk Department of Applied Chemistry, Curtin University of Technology, P.O. Box U1987, Perth, Western Australia 6845; *
[email protected] We have developed a laboratory exercise that introduces students to a heterogeneous reaction and the use of X-ray diffraction to identify mineral phases and to propose reaction schemes. The exercise involves the reduction of ilmenite with charcoal and the efficiency of the reduction process is monitored using wet-chemical methods. It is an original exercise carried out by students in the three-year undergraduate applied chemistry degree course. It forms part of the third-year inorganic chemistry laboratory program that is applied in nature and from which other exercises have been previously reported (1, 2). The mineral ilmenite (FeTiO3) and its processing is responsible for about 85% of the world’s titanium requirements. Most of the ilmenite is used for the production of rutile, TiO2, which, in turn, is used to produce pigment-grade TiO2 and titanium metal. There have been many methods developed for upgrading ilmenite to a rutile substitute. These include high-temperature reduction and direct acid-leaching methods, the bulk of which were developed in the 1960s and early 1970s (3). One of the best-known high-temperature reduction methods for upgrading ilmenite from 55% TiO2 to about 94% TiO2 on a commercial scale is the Becher process (4, 5). The phase composition of a typical feed for the Becher process is 55% ilmenite, 38% pseudorutile (Fe2Ti3O9), and 7% rutile. The raw materials are reacted with charcoal in a rotary kiln at about 1100 C according to the predominant reaction, FeTiO3 + CO
Fe + TiO2 + CO2
(1)
While the reductant is ostensibly solid carbon, it has been shown that at temperatures above 1020 C, the principal reaction involves carbon monoxide produced by the reaction of carbon dioxide with carbon (6): C + O2 CO2 + C
CO2
(2)
2CO
(3)
The reduction product consists of grains of metallic iron embedded in a matrix of titanium dioxide. The iron metal is leached from the reduced product by aerial oxidation at 70 C in aqueous ammonium chloride:
Fe(lattice)
+ − Fe2 (aq) + 2e
O2 + 2H2O + 4e−
4OH−(aq)
(4) (5)
As a consequence iron(II) hydroxide is formed: Fe2+(aq) + 2OH−(aq)
Fe(OH)2(s)
(6)
Since the solution is not buffered, the pH will rise to about
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9, resulting in the formation of magnetite, Fe3O4. Magnetite is the desired product since it can be magnetically separated from the TiO 2. Advanced-level students could be introduced to a Pourbaix (7) diagram at this stage to show how the control of electrode potential and pH leads to the formation of a desired phase. This upgraded ilmenite is approximately 94% TiO2 and may then be submitted to the chloride process for further upgrading to pigment-grade 99.9% TiO2. Further reading on studies carried out on this process, pursuant to the pioneering work of Becher, may be found in refs 8–12. A more complete discussion of the chemistry involved in the Becher process is available in the Supplemental Material.W Exercise This exercise exposes students to a diverse range of techniques and methods. The exercise is composed of three parts: (i) simulation of the reduction process in a tube furnace, (ii) chemical analysis of the total iron in the ilmenite and the metallic-iron content of the reduced product to quantify the degree of reduction, and (iii) an X-ray diffraction study to monitor the phases present in each stage of the process.
Reduction Process A sample of oxidized ilmenite that is reduced more readily under the conditions obtainable in the tube furnace is used. The most reproducible results have been obtained using particle sizes of the oxidized ilmenite and charcoal of 90–150 µm and < 63 µm, respectively. This results in good packing of the solids and diffusivity of gases through the solids. The oxidized ilmenite and charcoal are intimately mixed, placed in a labeled porcelain boat, and the boats given to the laboratory technician for reduction in the tube furnace in a stream of nitrogen to closely simulate the Becher process. The class size is too large to permit the samples to be run by the students during class time. To obtain a range of data from the class, all students use samples from the same source of oxidized ilmenite and are assigned a particular time and temperature of reduction as the variables. Each student sample is run separately under the same conditions with the length of time and temperature being the only variables. The samples are kept desiccated after being run to ensure quality control. Chemical Analysis The students are required to analyze the total-iron content of the oxidized ilmenite and the metallic-iron content of the reduced product. The metallic-iron analysis is a method the students have previously encountered in an analytical chemistry unit in the second year of their course. The students choose the method of total-iron analysis from previous knowledge gained in the course (13, 14). These methods are available in the Supplemental Material.W
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In the Laboratory
X-Ray Diffraction The students are provided with X-ray diffraction patterns of the phases at different stages of the Becher process (Figure 1). They are also provided with data on the d spacings and peak intensities for selected compounds, together with a correlation sheet for conversion of d spacings to degrees 2θ for CuKα radiation. The d spacings for the most intense peaks for each compound are shown in Table 1. Using this information, the students can confirm the presence or absence of compounds in the various stages of the process. It is suggested that the students construct a line diagram for each compound to the same scale as the provided patterns and, as peaks found for compounds are identified, they are color coded until all the major peaks have been assigned.
Two periods of laboratory time (total of 8 hours) are devoted to the exercise. In the first week, the students prepare the oxidized ilmenite兾charcoal mixture and determine the total iron in the oxidized ilmenite. The sieving and subsequent analysis of the metallic-iron content of the reduced product are carried out in the second week. The students carry out the interpretation of the X-ray diffraction patterns out of class time. Equipment and Samples A tube furnace operating to 1200 C is necessary to carry out the reduction procedure. Ilmenite samples were obtained from Iluka Resources, Capel, Western Australia. Alternatively, the ilmenite may be purchased from companies that retail standard analyzed samples.
Response (Count)
7–h leach
1–h leach
reduced
oxidized
raw 20
30
40
50
60
70
2θ / deg Figure 1. X-ray diffraction patterns of the phases at different stages of the Becher process.
Table 1. X-Ray Diffraction Data for Selected Compounds Ilmenite
Pseudobrookite
Iron
FeTiO3
Fe2TiO5
Fe
d
I/I0
Rutile TiO2
d
I/I0
d
I/I0
d 3.25
I/I0
2.74
100
3.48
100
2.03
100
1.72
100
2.74
80
1.43
20
1.68
60
1.50
85
---
---
---
---
2.49
50
2.54
85
---
---
---
---
2.19
25
Anosovite
Pseudorutile Fe2Ti3O9 d
Haematite α-Fe2O3
Ti3O5
I/I0
100
Magnetite Fe3O4
d
I/I0
d
I/I0
d
I/I0
1.69
100
3.46
100
2.69
100
2.53
100
2.39
60
2.70
100
1.69
60
1.61
85
2.19
50
4.78
60
2.51
50
1.48
85
---
---
---
---
---
---
2.10
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In the Laboratory
Hazards The tube-furnace assembly and muffle furnace should be set up in fume hoods. A protective face shield should be used when operating both the tube and muffle furnaces. Eye protection should be used at all times when carrying out the analytical procedures. Proper attention and caution should be applied when handling many of the reagents in this laboratory exercise. Concentrated sulfuric acid and sodium peroxide are both powerful oxidizing agents and react exothermically when added to water. Sodium sulfite is a reducing agent and its reaction with hydrochloric acid produces sulfur dioxide. The reduction of iron(III) with sodium sulfite should therefore be carried out in a fume hood. The sodium hydroxide used in this experiment is moderately alkaline and care should be exercised when using it to neutralize the hydrochloric acid. The other reagents used in this exercise do not pose any significant hazard. MSDS sheets, which may be obtained from the Merck and Sigma Aldrich companies, must be consulted for each chemical prior to its handling.
Fraction Reduced (%)
100
1100 °C
80
1000 °C 60
900 °C
40
20
0 0
50
X-Ray Diffraction The major phases that are identified in the X-ray diffraction patterns are shown in Table 2. The pattern of the raw ilmenite shows intense peaks representative of the starting materials ilmenite, pseudorutile, and rutile at 32.7 (2θ), 54.3 (2θ), and 27.5 (2θ) respectively. In the oxidized ilmenite pattern there is seen the retention of rutile peaks and the disappearance of ilmenite peaks, while there is the emergence of peaks characteristic of pseudobrookite at 25.6 (2θ). This represents the oxidation of Fe(II) in the ilmenite to the Fe(III) state in pseudobrookite. The pattern of reduced ilmenite shows the almost complete disappearance of the pseudobrookite peaks. There is also a marked emergence of metallic-iron peaks at 44.6 (2θ) and 65.3 (2θ), together with a significant increase in the intensity of the rutile peaks, as Fe(III) is reduced to the metallic state with the corresponding formation of rutile. The reduced product leached for 1 hour shows a significant reduction in the metallic-iron peaks and the emergence of strong peaks at 35.6 (2θ), 57.2 (2θ) and 62.8 (2θ) as a result of the oxidation of the metallic iron to magnetite, Fe3O4. The rutile peaks remain significant. There is a steady trend seen in the metallic-iron and magnetite peaks as the leaching process is extended from 1 to 7 hour. There are no peaks to be found in the diffractograms for the anosovite or haematite. The diffraction patterns also display the general
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150
200
250
300
Time / min Figure 2. Percentage iron reduction as a function of temperature and time.
Results and Discussion
Chemical Analysis Typical data that may be achieved by a student in this laboratory exercise are shown in Figure 2. The data show that reduction is rapid in the early stages of the process. The initial slope of the reaction curve increases as the temperature is increased from 900 C to 1100 C. Maximum iron(0) formation is achieved after 300 min at 1100 C. The reaction becomes more diffusion controlled over longer periods of time.
100
Table 2. Phases Identified from X-Ray Diffraction Patterns Material
Phases
Raw ilmenite
Ilmenite, pseudorutile, rutile
Oxidized ilmenite
Pseudobrookite, pseudorutile, rutile
Reduced ilmenite
Iron, rutile
Leached for 1 hour
Iron, rutile, magnetite
Leached for 7 hours
Iron, rutile, magnetite
principle that it is difficult to achieve complete quantitative reactions in an industrial process. There remain small quantities of ilmenite, pseudobrookite, pseudorutile, and metallic iron in the final product after 7 hours of leaching, illustrating why it is not possible to obtain a reduced product analyzing greater than 94% TiO2. However this is a minimum percentage of TiO2 that is required to avoid excessive chlorine losses when this material is converted to 99.9% pigment grade by the “chloride” process.
Prelaboratory Work As prelaboratory work the students proposal a method for the total-iron analysis of the oxidized ilmenite. The justification is expected to contain the reasons for the weight of sample to be taken, the decomposition method, separation of interfering species, and the quantification of the analyte, iron. Points are deducted if the student is unable to produce an acceptable method.
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In the Laboratory
Report The lab report is expected to be in three parts (point allocation out of 10 in parenthesis): • A commentary on the efficiency of the reduction procedure at allocated reduction temperature and time. Include a well-labeled diagram of the tube-furnace assembly, describing the purpose of each component (4 points). • A tabulation of the phases present in each stage of the Becher process from the X-ray diffraction results. Use equations wherever possible to describe what occurs at each stage of the process, paying particular attention to changes in oxidation states (5 points). • A brief discussion of the chloride method by which the upgraded ilmenite from the Becher process may be further refined to produce pigment-grade TiO2 (1 point).
Postlaboratory Activity At the end of the semester when all the reports have been assessed and returned to the students, a postlaboratory period is held where the total data for this exercise (and other exercises in the unit) are presented by the laboratory demonstrator. The results from Figure 2 and Table 2 are discussed, together with the answers to questions required in the report. Student Comments The students enjoy carrying out this exercise. Typical comments are “it is an excellent integration of chemical analysis and a solid-state method of analysis”, “a robust experiment directly based on an industrial process”, “I had the opportunity of applying my previous analytical experience to this exercise”. The exercise is also highly recommended by the Department’s Course Advisory Committee, which is primarily comprised of chemists from industry. Conclusions The reduction of ilmenite with charcoal is an excellent exercise to combine bench chemistry with a solid-state
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method of analysis. The percentage reduction of iron is monitored by wet-chemical analysis while the phase changes in the reduction process are deduced using X-ray diffraction. Pooling of the class results allows for a substantial postlaboratory discussion of the exercise among the whole class after each student’s report has been assessed. W
Supplemental Material
Chemical reactions involved in the Becher process and the experimental procedures for the reduction process, analysis of metallic iron, and total iron are available in this issue of JCE Online. Literature Cited 1. Dunn, J. G.; Phillips, D. N.; Van Bronswijk, W. J. Chem. Educ. 1997, 74, 1188–1190. 2. Dunn, J. G.; Phillips, D. N.; Van Bronswijk, W. J. Chem. Educ. 1997, 74, 1186–1187. 3. Henn, J. H.; Barclay, J. A. Inf. Circ. U.S. Bur. Mines 1970, 8450, 1–27. 4. Becher, R. G.; Canning, R. G.; Goodheart, B. A.; Uusna, S. Proc. Australas. Inst. Min. Metall. 1965, 214, 21–44. 5. Becher, R. G. The Removal of Iron from Ilmenite. Australian Patent No. 247110, 1963. 6. El-Guindy, M. I.; Davenport, W. G. Metall. Trans. 1970, 1, 1729–1734. 7. Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solution; Pergamon: Oxford, 1996. 8. Donnelly, R. P.; Brennan, L. J.; McMullan, W.; Rouillard, A. Australian Mining 1970, 58–65. 9. Jones, D. G. Trans. Inst. Min. Metallurgy 1974, 83, C1–C9. 10. Bracanin, B. F.; Cassidy, P. W.; MacKay, J. M.; Hockin, H. W. Proc. 101st Annual AIME Meeting; San Francisco, CA, 1972; TMS Paper A72–31, 209–259. 11. Grey, I. E.; Jones, D. G.; Reid, A. F. Trans. Inst. Min. Metallurgy 1973, 82, C151–C152. 12. Grey, I. E.; Reid, A. F. Trans. Inst. Min. Metallurgy 1974, 83, C39–C46. 13. Dunn, J. G.; Mullings, L. R.; Phillips, D. N. J. Chem. Educ. 1995, 72, 220–221. 14. Phillips, D. N. Anal.Chem. 2002, 74, 427A–430A.
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