Removal of iron from silica sand by leaching with ...

hydrometallurgy

: v i i" .

ELSEVIER

Hydrometallurgy46 (1997) 215-227

Removal of iron from silica sand by leaching with oxalic acid M. Taxiarchou, D. Panias, I. Douni, I. Paspaliaris, A. Kontopoulos * Laboratory of Metallurgy, National Technical Universi O' of Athens P.O. Box 64056, GR 157-10 Zogrqfos, Greece

Received 6 December 1996; accepted 28 February 1997

Abstract The removal of iron from silica sand with oxalic acid has been studied under various experimental conditions in order to optimise the process parameters and reach a high degree of iron removal at minimum operating cost. The parameters studied were: temperature, pH of the solution, oxalate concentration, Ar purging, and ferrous ions addition to the solution. For the specific silica sand sample used, at temperatures varying between 90-100°C the maximum iron extraction that can be achieved is approximately 40%. At temperatures lower than 80°C this extraction is decreased to 30%. At these temperatures purging of the oxatate solution with Ar and ferrous ions addition has no effect on the iron extraction, while at temperatures lower than 25°C iron dissolution is accelerated with the addition of ferrous ions. Iron dissolution is significantly affected by the pH, while it is practically independent of the oxalate concentration and the pulp density. Without the addition of bivalent iron, iron extraction is optimised in high acid solutions; when ferrous ions are added in the oxalate solution, best results are achieved at pH 3. © 1997 Elsevier Science B.V.

1. Introduction Industrial minerals, such as silica/glass sand, china clays and feldspar are often associated with deleterious impurities or unwanted structural components, particularly in the form of iron compounds. These minerals are mainly used as raw materials for the production of optical fibres, glass, ceramics and refractory materials. Iron occurring in them is harmful, as it impairs transmission in optical fibres and the transparency of

* Corresponding author. E-mail: [email protected] 0304-386X/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0304-386X(97)00015-7

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M. Taxiarchou et al. / Hydrometallurgy 46 (1997) 215-227

glasses, it discolours ceramic products and lowers the melting point of refractory materials. For the production of colourless glass or higher premium ceramics, silica sands with less than 100 ~ g / g iron are needed, while the final iron content for optical glass and optical fibres should be less than 10 and 1 p.g/g, respectively. The iron content of industrial minerals can be reduced by a number of physical, physicochemical and chemical methods, namely: attrition processes, aiming at removing iron bearing minerals from the surface of the particles; separation processes (e.g., magnetic separation or flotation) for the separation of iron-rich minerals, and chemical treatment aiming at dissolving iron compounds bonded at the surface or existing as distinct mineral grains. The appropriate method for the removal of iron from an industrial mineral depends on the mineralogical forms and the distribution of iron in the particular ore. Chemical methods involve leaching of the mineral with organic and inorganic acids. The most commonly used organic acids are oxalic [1-3], citric [4] and ascorbic, [5] and inorganic [6,7] are hydrofluoric, hydrochloric, sulphuric and perchloric acid. Studies on the dissolution of pure iron oxides, hematite and magnetite, in acidic oxalate solutions has shown that the dissolution mechanism comprises three steps [8]: (1) Adsorption of organic ligands on system interface: Activation of solid surface, (2) Reductive dissolution of active centres: Generation of ferrous ions in the solution (induction period), (3) Autocatalytic dissolution of active centers. The iron dissolution proceeds through a reductive process characterised by a well-defined induction period, during which generation of ferrous ions in the solution takes place. The induction period is caused by the slow rate of ferrous ions generation in solution. When a sufficient amount of ferrous ions has been formed in solution, the dissolution process is accelerated. In oxalate solutions and in the presence of dissolved oxygen, bivalent iron is easily oxidised to trivalent. In order to avoid this oxidation and decrease the duration of the induction period, the dissolved oxygen has to be removed from the oxalate solution. This can be achieved by purging the solution with an inert gas (e.g., Ar). Moreover, the addition of ferrous ions in the initial oxalate solution has as a result the elimination of the induction period and the increase of the rate of dissolution [9]. The present work aims to study the removal of iron from silica sand with oxalic acid. Based on the above mechanistic observations, an attempt was made to optimise the process parameters and reach a high degree of iron removal at minimum operating cost. The parameters studied were: temperature, pH of the solution, oxalate concentration, Ar purging and ferrous ions addition in solution.

2. Materials and experimental procedure 2.1, M a t e r i a l s

The chemical composition of the silica sand sample used in this study is presented in Table 1. It consists mainly of a-quartz and feldspar with small quantities of rutile,


217

Table 1 Chemical analysis of dry silica sand Component

%

SiO 2 AI203 KzO Fe203 TiO 2 Cr203 CaO MgO Na~O

99.400 0.270 0.235 0.029 0.033 5.5/xg/g 0.001 0.002 0.030

anatase, muscovite, microcline and biotite. 5.5% of the iron contained in sand is in the form of magnetite. S E M / E P M A analysis has shown the existence of two different types of iron-bearing minerals: iron oxides and chromium ferrites. The sand sample used has a particle size distribution within a narrow size range with 83% being in the - 0 . 3 5 + 0.18 mm fraction.

2.2. Experimentalprocedure All experiments were conducted in 500 mL mechanically stirred spherical glass reactors equipped with a thermostatically controlled heating mantle connected with mercury contact thermometer. The experimental procedure was the following: 400 mL of buffer H2C204/K2C204 solution with constant total oxalate concentration and pre-adjusted pH value was heated in the glass reactor at a pre-selected temperature. The solution was agitated at a speed of 600 m i n - 1. Then, a pre-weighed amount of dry sand was added to the solution, creating a suspension with the appropriate pulp density. Tests with the addition of ferrous ions in the initial solution were also performed. In these tests, during the heating period and several minutes before the addition of sand and ferrous ions, the buffer solution was purged with argon in order to avoid the oxidation of ferrous to ferric ions by the dissolved oxygen. Ferrous ions were added to the solution in the form of extra pure (99%) ammonium iron(II)-sulphate-6-hydrate. Since iron dissolution is a photochemical process [10,11], the experiments had to be performed under controlled light conditions. Therefore, a light isolated box was constructed containing two 15 W white light sources. The experimental apparatus was placed in this box and all the experiments were performed under similar visible light conditions. In each test, the total and bivalent iron concentration in solution were measured as a function of time. Total iron chemical analysis was carried out with flame atomic absorption spectroscopy using a Perkin Elmer 2100 atomic absorption spectrophotometer. Bivalent iron chemical analysis was performed with UV spectrophotometry using a Hitachi U 1100 spectrophotometer and 1,10-phenanthroline as complexing agent [ 12].


218

3. Experimental results

3.1. Effect of temperature Tests were carried out at temperatures varying between 25 and 100°C, constant pH of 1, total oxalate concentration of 0.3 M and pulp density 20% (w/v). The dissolution of iron as a function of time for the above tests is presented in Figs. 1 and 2. It can be seen that the shape of the kinetic curves is similar at all the temperatures studied and the dissolution rate increases as temperature increases. From these results it is observed that, under the experimental conditions studied, the maximum iron dissolution that can be achieved is approximately 40%. This extraction is obtained at temperatures 90-100°C and 2 days retention time. At lower temperatures, varying between 25-80°C, the highest iron extraction observed is approximately 30% and can be achieved either at relatively high temperatures (70°C) and 1 day retention time or at ambient temperature (25°C) and retention time higher than 4 days (Fig. 2, after extrapolation of the experimental data). This is the maximum iron extraction that can be obtained at these temperatures (25-80°C), since at 70°C an increase of the leaching time up to 4 days has no significant effect on the iron extraction (Fig. 2). The iron dissolution is significantly accelerated as temperature increases from 25 to 60°C, while the effect of temperature changes between 60-80°C on the dissolution rate is negligible, as is seen in Fig. 1.

3.2. Effect of pH To study the effect of pH, a series of tests was performed at constant pH of 1, 2, 3 and 5, total oxalate concentration of 0.3 M and pulp density 20% (w/v). At each pH value, three different temperatures (25, 60 and 80°C) were examined. The results are summarised in Fig. 3. It is observed that the iron dissolution rate is higher in high acid solutions irrespective of the leaching temperature, indicating the stability of the dissolution mechanism in this temperature region.

35i

30 ]

i....,-- 25"C • 40~C i~ 50oC

2s i

i

20 i

X 60 C o 70°C

15-

•

80'C

---av- 90°C !~ IO0°C

10 -

ol 0

so

loo

1so

200

250

3oo

Time, rain Fig. 1. Iron extraction as a function o f time at different temperatures (pH 1, Cox 0.3 M, pulp density 20%).


219

4O 35 30-

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25

25°C i •-m---70~C I i~90Ci

20 I 151 I 101 5

oi 0

i

1000

2060

3000

4000

5000

6000

Time, min Fig. 2. Iron extraction as a function of time at different temperatures (pH 1, cox 0.3 M, pulp density 20%).

3.3. Effect of oxalate concentration The effect of total oxalate concentration was studied through a series of tests conducted at 80°C, constant pH of 1 and pulp density 20% ( w / v ) (Fig. 4). It can be clearly seen that for the concentrations studied (0.1-0.5 M), the total oxalate concentration has almost no effect on the iron dissolution rate.

3.4. Effect of Ar purging and ferrous ions addition The results of the above tests have shown that the maximum iron extraction achieved under the prevailing experimental conditions is approximately 30-40%, depending on the temperature region in which leaching is performed. Based on the conclusions drawn from the study on the dissolution of pure iron oxides in acidic oxalate solutions [9,13], an attempt was made to increase the iron extraction by: (a) purging the solution with Ar, and (b) adding ferrous ions to the initial solution. Two tests were carried out at 70°C, pH 1, total oxalate concentration of 0.3 M and pulp density 20% ( w / v ) . In both tests the oxalate solution was purged with Ar, while in the second one 10 m g / L Fe 2÷ was also added to the initial solution. The results are shown in Fig. 5. In the same figure the results of a similar test without Ar purging and ferrous ions addition are also presented. At 70°C, neither Ar purging nor ferrous ions addition increased the iron extraction achieved and the rate of iron dissolution, although the study so far showed that these two parameters significantly affect the iron dissolution mechanism, drastically increasing the dissolution rate [9,13]. Based on all the above results, a series of experiments was performed with the addition of ferrous ions in the initial solution at ambient or even lower temperature conditions, aiming to decrease the leaching temperature and, therefore, to minimise the operating cost. The parameters studied were the concentration of ferrous ions added to the solution, pulp density, temperature and the pH of the solution.


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H--T

i • P !=---I--pH=2 [ ~ pH=3 ! ~ • pH-5 /

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lo 5 Ol 0

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100

150

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Time, min Fig. 3. Iron extraction as a function of time at different temperatures and pH values (Cox 0.3 M, pulp density

2O%). 3.5. Effect o f the concentration o f Fe e + added Tests were performed with the addition of 5 and 10 m g / L Fe 2+ to the solution at 10°C, constant pH of 3, total oxalate concentration of 0.2 M and pulp density 20% ( w / v ) . One comparative test was also carried out at the same experimental conditions without the addition of Fe z+. The iron dissolution as a function of time for these tests is presented in Fig. 6. From these tests, it can be seen that, at these low temperatures, the


221

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25

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i.0.2M i I ,

II

oll

0

50

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~

1O0

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Time, min Fig. 4. Iron extraction as a function of time for different total oxalate concentrations (80°C, pH 1, pulp density 20%).

25

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F i ~' without Ar, Fe(2+) ! ll Ar purging ! • 10mg/L Fe(2+)

i

I 50

I 1oo

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Time, min Fig. 5. Iron extraction as a function of time with and without Ar purging and Fe 2+ addition (70°C, pH 1, Cox 0.3 M, pulp density 20%).

2O c '~

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;~ i 1

i a 5 mg/L Fe(2+) / AtA110mg/L Fe(2+)i

o~ 50

lO0 Time, rain

150

200

Fig. 6. Iron extraction as a function of time with and without the addition of Fe 2+ to the solution (10°C, pH 3, Cox 0.2 M, pulp density 20%).

M. Taxiarchou et al. / Hydrometallurgy 46 (1997) 2 1 5 - 2 2 7

222

30. 25.

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Time, min Fig. 7. Iron extraction as a function of time at different temperatures (pH 3, Cox 0.2 M, pulp density 60%, 5 mg/L Fe2+ ). dissolution of iron is strongly affected by the addition of ferrous ions. At 10°C, approximately 25% of iron is removed in less than 3 h when 10 m g / L Fe 2+ are added to the solution, while without Fe 2+ addition temperatures higher than 50-60°C are needed for the same percentage of iron dissolution.

3.5.1. Effect of pulp density To study the effect of pulp density, three tests were conducted at 10°C, constant pH of 3, total oxalate concentration 0.2 M with the addition of 10 m g / L Fe 2+ to the solution. For the values studied (20-80%, w / v ) , pulp density has insignificant effect on the removal of iron. At pulp density 20%, a slightly higher dissolution rate is observed, which can be attributed to the higher reagent excess in the solution. Therefore, a pulp density of 60% was used in the subsequent experiments.

3.5.2. Effect of temperature Tests were performed at 5, 10, 15 and 20°C, constant pH 3, total oxalate concentration 0.2 M with the addition of 5 m g / L Fe 2÷ to the solution and pulp density 60% (w/v). The results are summarised in Fig. 7. Under the prevailing experimental conditions, temperature changes varying between 5 and 20°C have insignificant effect on the iron removal. The maximum iron removal that can be achieved is approximately 25%, at leaching temperatures as low as 5°C and 4 - 5 h retention time.

3.5.3. Effect of pH The effect of pH was studied through a series of tests conducted at 10°C, pulp density 60% ( w / v ) , in a 0.2 M oxalate solution containing 5 m g / L Fe 2+ with pH values varying between 1 and 5. The iron dissolution versus time is plotted in Fig. 8. The optimum iron extraction is observed at pH 3, while in higher and less acid solutions the dissolution rate decreases significantly. These results are in disagreement with those from the tests without the addition of ferrous ions, where the dissolution increased as pH decreased (Fig. 3).

M. Taxiarchou et al. / Hydrornetallurgy 46 (1997) 215-227

223

20 18

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~

12

~

I0-

--B- pH=3 [

e. 66

LP__p~:Si

42-

o!

~ 50

1(30

150

200

Time, min

Fig. 8. Iron extraction as a function of time for different pH values (10°C, cox 0.2 M, pulp density 60%, 5 m g / L Fe 2+ ).

3.6. Continuous leaching tests Continuous leaching tests were carried out in columns containing 7 kg of silica sand. The column was designed so as to permit the development of an inert atmosphere during leaching. Two tests were performed at 10°C, constant pH 3, total oxalate concentration of 0.2 M with the addition of 10 m g / L Fe 2÷, under light conditions similar to those used at the batch experiments. The flow rate of the leaching solution was 10 and 100 L / d , respectively. One more test was carried out at 10°C, constant pH 1, total oxalate concentration of 0.2 M, 10 L / d solution flow rate without the addition of ferrous ions to the solution. The results are presented in Fig. 9. As can be seen, at 10°C without the addition of ferrous ions only 11% of the contained iron was removed in 7 days, while with the addition of 10 m g / L Fe z÷ the same percentage of iron dissolution is achieved in less than 2 h. In this test, approximately 30% of iron is extracted in 5 days. This is the maximum iron dissolution that can be obtained under these experimental conditions, as already deduced from the batch experiments. A 10 times increase of the leaching solution flow rate accelerates the iron dissolution process.

3O

25

without Fe(2+), I0 L/d

2o

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010 mg/L Fe(2+), 10 L/d

15

A 10 mg/L Fe(2+), 100 L/d!

N 5

OI

0

50

100

150

200

Time, h

Fig. 9. Iron extraction as a function of time for the column leaching tests (10°C, pH 1 and 3, Cox 0.2 M).

224


4. Discussion

This study has shown that the most important parameters affecting the dissolution of iron from silica sand in acidic oxalate solutions are temperature, pH and ferrous ions addition to the solution. Under the experimental conditions studied, two different temperature regions can be observed. In the first one, ranging between 90-100°C, the maximum iron extraction that can be achieved is 40%. In the second region, comprising temperatures lower than 80°C, the maximum percentage of iron that can be dissolved decreases to approximately 30%. This extraction is obtained either at low (ambient or even lower) temperatures and longer leaching time or at higher temperatures and shorter retention time. Temperature changes between 60-80°C have no effect on iron dissolution. Moreover, at these temperatures the iron extraction is not increased by the addition of ferrous ions in the oxalate solution. On the other hand, at temperatures as low as 10°C the addition of 5 - 1 0 m g / L Fe 2÷ to the initial solution strongly accelerates the iron dissolution process. From the above phenomena it is deduced that thermodynamic and kinetic inhibitions are taking place during leaching of silica sand in oxalate solutions. Kinetic inhibitions hinder the iron dissolution at low temperatures, while as temperature increases or with the addition of ferrous ions to the solution, they are eliminated and the dissolution process is accelerated. At 60°C, kinetic inhibitions are eliminated and iron dissolution rate becomes independent of temperature, up to 80°C, and ferrous ions addition. On the other hand, the existence of different iron mineral phases that can be dissolved at temperatures higher than 90°C, while they are practically insoluble at lower temperatures, determine the maximum percentage of iron that can be dissolved in each temperature region. In order to optimise the iron dissolution process, the pH of the solution has to be taken into consideration. The optimum pH value depends on the presence of ferrous ions in the initial solution. During dissolution, the hydrogen ions are adsorbed on sites of the solid surface, creating surface active centres, on which the main reaction of dissolution takes place [8]. As the hydrogen ion concentration in solution increases, the amount of adsorbed hydrogen ions also increases, according to the adsorption theory. An increase in the number of active centres results in a corresponding increase of the iron dissolution rate. Therefore, without the addition of ferrous ions to the solution, iron dissolution proceeds faster in more acidic solutions. When ferrous ions are added to the oxalate solution, pH affects their speciation in the solution, as seen in Fig. 10. In high acid solutions, the ferrous ions remain uncomplexed. As pH increases the concentration of the Fe(C204) 2- complex ion in solution also increases; at pH higher than 2.5 all ferrous ions are in the form of Fe(C204)22-. The presence of Fe(C204) 4-, and of all the other hydroxo complex ions, can be neglected due to its very low concentration at all pH values [14]. So, from the above ferrous ions, only the complex Fe(C204) 2- can be adsorbed onto the solid surface. The behaviour of iron dissolution when bivalent iron is added to the solution is attributed to two competitive phenomena taking place during the dissolution process: the adsorption of hydrogen ions and complex Fe(C204 )2- ions on the particle surface. The first one creates the surface active centres, while the second one accelerates the


225

100

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80

20 )22.

60

40

1

2

3

pH

i

i

4

5

6

4

5

6

0.035 0.03 0.025 .--~

0.02 0.015 0.01 0.0(~3 0

1

2

3

pit

Fig. 10. Speciationof ferrous ions in an oxalate solution as a function of pH (co~ 0.1 M). dissolution reaction through these active centres. These two phenomena are intensified under extreme experimental conditions. In high acid solutions (pH 1), the number of active sites on the solid surface is high, but the amount of Fe(C204) ~- ions adsorbed on active sites is very small due to their low concentration in the solution. Therefore, the addition of ferrous ions to the solution is not effective. In less acid solutions (pH 5), the number of active sites on the solid surface is low and so is the amount of Fe(C204) ~- ions adsorbed on active sites, although, under these conditions, the Fe(CeO4) ~- ions concentration is very high. As a result, most of the Fe(C204) ~- ions are adsorbed on non-activated sites of the solid surface and the iron dissolution is inhibited [9]. Therefore, under these extreme conditions, the addition of ferrous ions to the solution is also not effective. From the above, it is deduced that the iron dissolution is optimised when a compromise is reached between the above competitive phenomena. This is found experimentally to be at a pH value around 3. The application of the above observations to silica sand column leaching gave very good results. The most important was the significant decrease of the leaching time with the addition of ferrous ions to the oxalate solution. Approximately 30% of iron is extracted in 5 days, which is the maximum iron dissolution that can be achieved under these experimental conditions.

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M. Taxiarchou et aL / Hydrometallurgy 46 (1997) 215-227

5. Conclusions

This work aimed to study and optimise the parameters affecting the removal of iron from silica sand with oxalic acid and has shown that: - At temperatures lower than 80°C the m a x i m u m iron extraction that can be achieved is approximately 30%. - Part of the iron mineral phases contained in silica sand can be dissolved at temperatures higher than 90°C, while they are practically insoluble at lower temperatures. Therefore, at temperatures around 9 0 - 1 0 0 ° C , the m a x i m u m iron extraction that can be achieved increases to 40%. - Under the experimental conditions studied, change of the oxalate concentration between 0 . 1 - 0 . 5 M and pulp density from 20 to 80% has almost no effect on the removal of iron. - At 70°C, purging of the oxalate solution with A r and ferrous ions addition has no effect on the iron extraction, while at low temperatures (lower than 25°C) iron dissolution is accelerated with the addition of ferrous ions. This is attributed to kinetic inhibitions that hinder dissolution at lower temperatures, while they are eliminated at increased temperatures. - Under the prevailing experimental conditions, iron dissolution is significantly affected by pH, Without the addition of bivalent iron, iron dissolution is optimised in high acid solutions; when ferrous ions are added to the oxalate solution, best results are achieved at pH 3. - Continuous leaching testwork has shown that at 10°C, with the addition of 10 m g / L Fe 2÷ to the initial solution, total oxalate concentration 0.2 M and under visible light conditions approximately 30% o f iron can be extracted in 5 days.

Acknowledgements

The financial support of the European Commission within the framework of the Brite-Euram II Program (Contract No. BRE2-CT92-0215) is gratefully acknowledged.

References

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[7] H. Kametani, K. Azuma, Dissolution of calcined ferric oxides, Trans. Metall. Soc. AIME 242 (1968) 1025-1034. [8] D. Panias, M. Taxiarchou, I. Paspaliaris, A. Kontopoulos, Mechanisms of dissolution of iron oxides in oxalic acid solutions, Hydrometallurgy 42 (1996) 257-265. [9] D. Panias, M. Taxiarchou, I. Douni, i. Paspaliaris, A. Kontopoulos, Dissolution of hematite in acidic oxalate solutions: Effect of ferrous ions addition, Hydrometallurgy 43 (1996) 219-230. [10] J.IH. Patterson, S.P. Perone, Spectrophotometric and electrochemical studies of flash-photolyzed trioxalato-ferrate(lll), J. Phys. Chem. 77 (20) (1973) 2437-2440. [11] C.A. Parker, C.G. Hatchard, Photodecomposition of complex oxalates. Some preliminary experiments by flash photolysis, J. Phys. Chem. 63 (1959) 22-26. [12] A.E. Harvey, J.A. Smart, E.S. Amis, Simultaneous spectrophotometric determination of iron(lI) and total iron with 1,10-phenanthroline, Anal. Chem. 27 (1)(1955)26-29. [13] M. Taxiarchou, D. Panias, I. Paspaliaris, A. Kontopoulos, Dissolution of hematite in acidic oxalate solutions, Hydrometallurgy 44 (1997) 287-299. [14] D. Panias, M. Taxiarchou, I. Douni, 1. Paspaliaris, A. Kontopoulos, Thermodynamic analysis of the reactions of iron oxides dissolution in oxalic acid, Can. Metall. Quarterly, in press.