Drum leaching tests in iron removal from quartz using oxalic and sulphuric acids

ELSEVIER

Int. J. Miner. Process. 54 (1998) 183–200

Drum leaching tests in iron removal from quartz using oxalic and sulphuric acids F. Veglio´ a,Ł , B. Passariello b , M. Barbaro b , P. Plescia b , A.M. Marabini b a

Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita´ degli Studi de l’Aquila, 67040 Monteluco di Roio, L’Aquila, Italy b Istituto per il Trattamento dei Minerali (C.N.R.), via Bolognola 7, 00138 Rome, Italy Received 5 May 1997; accepted 22 April 1998

Abstract A study has been made of a leaching process to remove iron from quartziferous industrial minerals using oxalic and sulphuric acid in a drum reactor. The experimental work was necessary because of the paucity of literature on the use of oxalic acid in the treatment of industrial minerals containing iron as an impurity. The positive effect of oxalic acid on the iron extraction yield is clearly observed during leaching in a drum reactor. Iron extraction yields of 35–45% are obtained on treating the quartz with 3 kg=t oxalic acid and 2 kg=t sulphuric acid at 90ºC for 4–5 h. Under the same conditions but without oxalic acid the iron extraction yield ranges from 3 to 9%, depending on the sulphuric acid content. Chemical and mineralogical analyses were run on the ore to ascertain where the iron compounds occur on the different mineral components. These analyses were carried out on an as-is sample and on three other samples obtained by magnetic separation. The experimental results indicate that 52% of the iron is found in the mica fraction. These results explain why the maximum iron extraction yield is only 35–45%. A flow-sheet of the process is proposed together with a rough material balance in order to estimate oxalic acid, sulphuric acid and water consumption.  1998 Elsevier Science B.V. All rights reserved. Keywords: quartz; iron removal; oxalic acid; leaching

1. Introduction Industrial minerals such as quartz, usually contain some iron, indeed many major deposits are contaminated by small amounts (0.5–3%) of iron minerals (Groudev et al., 1978) which render it completely unsaleable, or saleable at only at a reduced price. Ł

Corresponding author. E-mail: [email protected]

c 1998 Elsevier Science B.V. All rights reserved. 0301-7516/98/$19.00 PII: S 0 3 0 1 - 7 5 1 6 ( 9 8 ) 0 0 0 1 4 - 3

184

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

Hence the dissolution of ferric oxides is a problem of particular interest to the producers of such industrial minerals. The presence of submarginal quantities of ferric and=or ferrous iron in kaolin, quartz sand and clay can prevent their use in many industrial fields: ferric iron imparts an orange tint to kaolin, thus greatly lowering their market value (Yan et al., 1978). Ferrous iron is also undesirable in ceramics because it is oxidised to ferric iron when the products are kilned. In general, iron which occurs in such raw materials is harmful because: (1) it impairs the transparency of colourless container glass and flat=clear (flat) glass; (2) it impairs the transparency of high-quality glass, e.g. for tableware, and optical glass; (3) it impairs transmission in optical fibres; (4) it adversely affects the production of pure silicon products, e.g. silicon metal, silicon carbide and sodium silicate. Considerable efforts have been directed towards removal of iron contaminants by physical and chemical means. However, physical separation techniques such as flotation are generally less effective for iron removal than chemical leaching. HGMS (high gravity magnetic separation) or blending processes are also employed in the beneficiation of industrial minerals (Guimares et al., 1987). Acidic reductive leaching is one of the best known and most widely employed chemical processes for beneficiating minerals of industrial interest. Conventional acidic leaching with sodium dithionite (Na2 S2 O4 ), employed to leach industrial minerals used in the production of ceramics, glassware, paper, etc., does not always allow products of high quality to be obtained (Conley and Lloyd, 1970). Other techniques are also available such as those based on sulphur dioxide and conventional mineral acid leaching (H2 SO4 or HCI), but these are expensive and the ensuing effluents are environmentally unacceptable. Thus there is considerable interest in the development of alternative technological methods such as organic-acid leaching which may be more effective and environmentally acceptable (Veglio´ et al., 1996). The use of different inorganic and organic acids, exerting complexing or reductive action, for the dissolution of iron compounds, has been the subject of a number of theoretical and experimental studies. Most of these are focused on the mechanism of hematite and magnetite dissolution, using various chemicals and experimental conditions. In particular it is known that in the dissolution of iron oxides, protonation of surface sites weakens Me–O lattice bonds, accelerating the rate of detachment (Stone and Morgan, 1987; Chiarizia and Hortwiz, 1991), and, for example, HCl leaching has been proposed for the removal of iron from bauxite (Patermarakis and Paspaliaris, 1989). Iron dissolution can also be achieved using reducing or complexing compounds. In all cases, there is always greater solubility of iron in solution, due either to its reduction to much more soluble Fe(II), or to the formation of complexes. Reductive dissolution of iron oxide minerals by organic reagents has been investigated by several workers, and many experimental studies indicating the basic mechanism involved are reported in the literature (Blesa and Maroto, 1986; Borghi et al., 1989; Dos Santos Afonso et al., 1990; Torres et al., 1990). Blesa et al. (1984) and Litter and Blesa (1988) used EDTA, Waite and Morel (1984) citric acid, Blesa and Maroto (1986) thioglycolic acid, Dos Santos Afonso et al. (1990) ascorbic acid, and Baumgartener et al. (1983), Blesa et al. (1984) and Torres et al. (1990) oxalic acid.

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

185

In addition to these studies using synthetic iron compounds various process investigations have been performed on iron dissolution from minerals of industrial interest using carbohydrates as reducing agents (Veglio´ and Toro, 1993, 1994) and also microorganisms (Groudev et al., 1978; Toro et al., 1992). Further works have been reported in the recent literature on the use of oxalic acid in the iron removal from industrial minerals (Bonney, 1994; Ubaldini et al., 1996; Veglio´ et al., 1996). The interest for this kind of process is testified by a number of industrial projects financed by the E.U. (contracts MA2M-CT90-0014, BRE2-CT92-0215, BRPR-CT96-0156) in which important industrial partners have been involved (Bonney, 1994). It is known that oxalic acid reacts with surface Fe(III) ions to form complexes (Blesa and Maroto, 1986). Once the surface complex has formed, the dissolution mechanism differs depending on the iron mineral concerned. In the case of magnetite, where both ferric and ferrous ions are present on the surface (Baumgartener et al., 1983; Litter and Blesa, 1988), the mechanism involves the reductive dissolution of surface Fe(III) ions, and an autocatalytic process involving the formation of ferrous oxalate has been observed. Other workers have reported that the dissolution of goethite by oxalate occurs through release of the ferric oxalate surface (Martell and Smith, 1974). It would seem that of all the possible complexing carboxylic ligands, oxalic acid is the most effective for iron dissolution (Passariello, 1989; Toro et al., 1990). Perusal of the relevant literature reveals that considerable attention has been paid to mechanism studies based on synthetic minerals such as hematite, goethite and magnetite, while ignoring the complex interactions that can occur in iron dissolution from industrial minerals such as bauxite, kaolin, silica and feldspathic sands. Therefore, the work reported here deals with leaching by means of oxalic acid (OA), under a variety of experimental conditions, for dissolving contaminating iron oxides from a quartz sample. The use of the oxalic acid was investigated because there is little literature regarding its application for the leaching of quartz and kaolins. Moreover, its use could be environmentally more acceptable than other kinds of reagents such as sodium hypochlorite, sulphur dioxide and sodium dithionite (Marabini et al., 1993; Veglio´ and Toro, 1994; Dudeney et al., 1995; Veglio´ et al., 1996). In general, the reactions involved in the process can be summarised as follows: Fe2 O3 C 6H2 C2 O4 ! 2Fe.C2 O4 /33 C 6HC C 3H2 O

(1)

Fe.C2 O4 /33 C 6HC C 4H2 O ! 2FeC2 O4 Ð 2H2 O C 3H2 C2 O4 C 6CO2

(2)

Fe2 O3 C 3H2 C2 O4 C H2 O ! 2FeC2 O4 Ð 2H2 O C 2CO2

(3)

As emerges from the overall reactions, the action mechanism of OA may be that of complexing and=or reducing Fe(III) to Fe(II). It has been demonstrated that low values of pH (<1.5) produce a reductive pathway for the process, whereas at pH 2–3 the complexing action of OA prevails (Veglio´ et al., 1996). Furthermore, the presence of Fe(II) in solution and light (Waite and Morel, 1984) may increase the process kinetics

186

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

because it has been demonstrated to have an auto-catalytic effect on Fe(III) detachment from the mineral under study (Baumgartener et al., 1983). The aim of this work was to define the process conditions of iron removal from a quartz of industrial interest, supplied by Fife Silica Sand Ltd. (referred to in the following as FSSC) in a leaching process involving the use of oxalic and sulphuric acids in a lab-scale rotating drum reactor. This kind of reactor was used in order to test experimental conditions close to USA and UK silica-sand leaching practice. In this last case, sulphuric acid solutions are in general used in a rotatory drum reactor with an acid consumption of about 12 kg=t at about 100ºC. Then, the objective is that of developing a commercially viable process for the removal of iron from quartz by acid leaching in the presence of reducing and=or complexing agents such as oxalic acid: the use of oxalic acid seems to be attractive, mainly for its better action on the iron removal with respect the mineral acids, and for less environmental impact (Bonney, 1994). The experimental work was performed with a view to comparing the results obtained by the usual techniques utilised in the UK to remove iron from quartziferous ores by sulphuric acid alone. For this reason this study was mainly aimed to obtain process data and no study was performed on the dissolution mechanism, to verify for example the effect of Fe(II) or light (U.V.) on the kinetic dissolution of iron from the ore.

2. Materials and methods 2.1. Ore The ore was supplied by Fife Silica Sands Ltd. UK. Complete physical, chemical and mineralogical characterisation was carried out before performing the leaching tests. Elemental analysis of the ore was performed by an Inductively Coupled Plasma Spectrometer ICP Perkin Elmer 6500 after HF attack. The chemical composition of the ore is shown in Table 1. The particle-size analysis was ascertained by means of a Laser Granulometer Sympatec Helios; the results are shown in Fig. 1. 2.2. Magnetic separation The FSSC material in the ‘as-is’ state was treated first by means of the isodynamic Franz magnetic separator with coil current of 1.5 A, inclination angle of 30º and axial Table 1 Elemental composition of sample FSSC Compound

(%)

Si Al Ca Fe as Fe2 O3 O

46.640 0.060 0.040 0.0180 0.0259 53.000

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

187

Fig. 1. Particle size distribution of FSSC sample.

angle of inclination of 5º. Each sample was given ten passes to improve the degree of precision of the test. In this manner three fractions were prepared for each sample, namely non-magnetic (NMF), magnetic (MF) and highly magnetic (HMF). 2.3. XRD analysis The FSSC material in the ‘as-is’ state and the samples from an magnetic separation test were ground for 60 min, mixed with a dispersant (cellulose) and smeared on bronze specimen holders. It was found useful to employ a dispersant because the main minerals involved suffering from very marked preferential iso-orientation. The presence of the cellulose does not create any analytical problems because its contribution is removed in the data-analysis phase. These samples were then subjected to X-ray analysis using a Siemens D-500 diffractometer with a Cu tube, a graphite crystal monochromator with a proportional detector, and a digital data analysis system involving an interfaced computer. Positioning and attribution of the diffractogram reflections were done with the aid of a JCPDS Data Bank, while the semi-quantitative calculation was performed via a least squares regression program, with the internal standard method (Chung method). The Chung method is based on an evaluation of volume fraction of unknown

188

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

substance with the calculation of its RiR (reference intensity ratio) (De Wolff and Visser, 1964). The RiR is the ratio between the intensity of the strongest peak of corundum (α-Al2 O3 , internal standard) and the strongest peak of analyte. Prior to the evaluation, the ratio must be determined with a standard of substance mixed (1 : 1) with corundum. The evaluation is only semi-quantitative, but is recommended in the studies on complex mixtures. 2.4. Scanning electron microscope analysis (SEM) The samples to be analysed by the SEM were smeared on conductive, adhesive specimen holders and then metallised in graphite and gold. Semiquantitative determinations of the types of phases present were made by electronic image analysis. These determinations were then utilised in the normative analysis and the mineralogical reconstruction. The chemical composition of all the principal minerals was ascertained by EDS analysis performed using a ZEISS DSM960 digital electronic microscope and thin-window PGT microanalysis. 2.5. Leaching tests All tests were conducted in 50-ml rotating glass cylinders, the external temperature of the device being 130ºC. The experimental apparatus is shown in Fig. 2. The glass cylinders are connected in a rotating drum (50–100 rpm) heated by hot air with a thermostat. In general, the solid=liquid ratio used was 1.5 (30 g of mineral and 20 ml of oxalic acid solutions at various concentrations), which was the maximum possible to have an acceptable iron extraction yield. The internal temperature was 98–100ºC and treatment time varied from 1 to 5 h. Samples were collected during the leaching process stopping the rotating drum, to determine the iron extraction yield vs. time. The samples were analysed by an inductively coupled plasma spectrometer, ICP Perkin Elmer 6500.

Fig. 2. Experimental system used in the leaching drum tests.

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

189

2.6. Data analysis Analysis of the variance was used to establish the significance of the factors investigated (Davies et al., 1979). According to the basic principle of the statistical design of experiments the leaching tests were randomised (Montgomery, 1991)

3. Results 3.1. Ore characterisation By means of magnetic separation it has been possible to make a quantitative estimate of the non-magnetic (NMF), the magnetic (MF) and the highly magnetic (HMF) fractions of the FSSC sample. The experimental results are summarised in Table 2. The FSSC sample contains 0.14% of highly magnetic material and 2.68% of magnetic material, while the remaining 97.18% is non-magnetic. From the XRD analysis, the ‘as-is’ sample consists essentially of quartz, together with a very small amount of muscovite. A semiquantitative estimate of the mineral forms has been performed by the internal standard method: this is given in Table 3. The experimental results indicate that in the as-is ore, the mica content is 4:5 š 1% and the quartz content 95:5 š 1%. Other minerals do not occur in quantifiable amounts. In the highly magnetic fraction the mica content increases to as much as 22:5 š 1% (Table 3). Pyroxene (augite) is also present to the extent of 3.5š1%. Where the magnetic fraction is concerned, mica is still present but in a smaller amount (6:7 š 1%), with quartz accounting for the rest. In the non-magnetic fraction the mica content is 5:5 š 1% and the quartz 94:5 š 1%. SEM–EDS analysis indicates that the as-is FSSC material takes the form of poorly Table 2 Experimental results of magnetic separation Fractions

(%)

Highly magnetic (HMF) Magnetic (MF) Non-magnetic (NMF)

0.14 2.68 97.18

Table 3 Semi-quantitative XRD analysis of the as-is FSSC and after magnetic separation Sample

As-is

NMF

MF

HMF

Mica Quartz Pyroxene Magnetite

4.5 š 1 95.5 š 1
5.5 š 1 94.5 š 1
6.7 š 1 93.3 š 1
22.5 š 1 74.0 š 1 3.5 š 1
a

Lower limit of detectability.

190

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

rounded grains, apparently free from coatings. In decreasing order of abundance, the minerals recognised are: (1) quartz (average grain size 350 µm); (2) muscovite mica (average grain size 500 µm, with a few plates exceeding one millimetre). SEM analysis reveals a significant difference in the grain size of the highly magnetic and the magnetic samples mainly as regards the mica. In addition to quartz, the highly magnetic sample contains other minerals. In decreasing order of these are: (1) quartz, in grains often with coatings rich in Ca and Fe; (2) iron-rich micas (muscovite in grains exceeding 300 µm in size); (3) augite (grain size between 100 and 500 µm); (4) accessory minerals such as Ti-magnetic (grains <50 µm), monazite (30–50 µm), rutile (inclusions in quartz, 5–10 µm), pyrite (grains enclosed between quartz crystals) (size 100 µm), and zircon (grains on quartz, 100–200 µm). The magnetic sample contains clear uncoated quartz, muscovite not so rich in Fe, occasional pyroxenes and a few accessory minerals, particularly rutile and zircon. SEM–BSE reveals many minerals rich in heavier elements (Fe, Ca and Zr) and the presence of large flakes of muscovite. Table 4 gives the results of the SEM–BSE image analysis performed to determine phases present in the as-is FSSC, non-magnetic, magnetic and highly magnetic fractions. Each phase was recognized by morphology and chemical data. EDS analysis was performed on the samples resulting from magnetic separation to determine the iron content (expressed as Fe2 O3 ) of each mineral. The experimental results demonstrate the presence of iron-rich muscovite (5.6% Fe2 O3 ) in the highly magnetic fraction (Table 5) and low-iron muscovite (3.5% Fe2 O3 ) in the magnetic fraction. Analyses of the mica in the non-magnetic fraction always indicate less than 1% as Fe2 O3 . They also reveal the presence of a mica with traces of kaolinisation in which potassium is completely absent. The augite has an iron content of 11.3% Fe2 O3 (Table 5). Image analysis carried out on the three samples obtained by magnetic separation (see Table 4) give the percentage breakdowns shown in the first three columns of Table 5. In particular the highly magnetic fraction (HMF) contains 66 š 2% quartz, 24 š 2% muscovite, 6 š 2% augite and 1 š 2% magnetite, while the magnetic fraction (MF) contains 91 š 2% quartz, 7 š 2% muscovite and 1 š 2% augite. The results obtained indicate that the main sources of iron are the muscovite mica and the occasional quartz grains that have a weak coating of oxides and carbonates; pyroxene and magnetite are secondary factors. Table 4 SEM–BSE Image analysis: percent grains on 500 readings Sample

As-is

NMF

MF

HMF

Mica Quartz Pyroxene Magnetite Zircon Rutile

3 93 1 1 1 2

5 95 0 0 0 0

7 91 1 0 1 1

24 66 6 1 1 1

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

191

Table 5 Distribution of Fe2 O3 in the minerals in the FSSC sample Mineral

HMF (%)

MF (%)

NMF (%)

Fe2 O3 a (%) ‘as is’

Fe2 O3 (%)

Relative Fe2 O3 (%)

Mica–Fe Mica–Fep Mica–Few Pyroxene Magnetite Total

24 – – 6 1

– 7 – 1 –

– – 5 – –

5.6 3.5 0.2 11.3 86.0

0.0019 0.0066 0.0089 0.0039 0.0120 0.0333

52.3 b 52.3 b 52.3 b 11.7 36.3 100

Mica–Fep D mica with a poor iron content; mica–Few D mica without iron. a % Fe as Fe O assessed on individual phases by EDS microanalysis. 2 3 b Fe O contribution present in the mica fraction. 2 3

Microchemical data and semiquantitative analyses of the various phases have been employed to permit a semiquantitative assessment of the amount of Fe contributed by each mineral. The results are summarised in Table 5. Examination of these semiquantitative data reveals that 52.3% of the total iron (as Fe2 O3 ) in the FSSC occurs in the mica fraction, 11.7% is due to the pyroxene and 36% is present as magnetite. In the highly magnetic sample most of the mica is rich in Fe which has substituted Al. There is a smaller proportion of coated quartz grains in the magnetic sample than in the highly magnetic one. Moreover, the mica in the magnetic fraction is not so rich in Fe. The ‘as-is’ FSSC sample contains 0.0259% Fe as Fe2 O3 ; the difference obtained in the calculated Fe2 O3 grade (0.0333%) compared with the Fe2 O3 grade (0.0259%) determined by chemical analysis is due to the semiquantitative approach of the methods employed for this analysis (XRD, image analysis and SEM–EDS microanalysis). The quartz containing only a few ppm of Fe is a stable structure in an acid medium and therefore does not release or exchange ions with the solution. The abrasion pH of the quartz is between 6 and 7, where the Si–O–Si structure tends to break up. The surface oxide coatings, instead, tend to go into solution more readily, especially when they are formed of iron hydroxides. Solubilisation of the Fe of the coatings is attributable also to solubilisation of the carbonate matrices which support the iron oxides and hydroxides. Muscovite mica may undergo various changes, depending on the availability of exchangeable ions in solution or the leaching strength of the solution. Muscovite is leached at a pH greater than 7.8. Ion exchange may also occur at a lower pH with the substitution of K by Na or Ca or other alkaline and alkaline-earth elements. Further investigations are needed to establish the reactions that occur in the structure of the muscovite and the other Fe-containing silicates during an extraction process. 3.2. Leaching tests The main purpose of the leaching process is to obtain a quartz (FSSC) with a lower iron content than in the as-is FSSC sample. In particular the target value is an Fe2 O3 concentration below 0.015% without grinding. An iron extraction yield of 40–45% is thus required from the leaching process. The procedure adopted in current UK practice, suggested by Dr. Wells (pers.

192

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

Fig. 3. Iron extraction yield after one hour of treatment with H2 SO4 alone: T D 90ºC; S=L ratio D 1.5 (w=v).

commun., FIFE Silica Sands Ltd.), has been followed for the OA leaching tests on the FSSC sample. The purpose of the tests was to compare the proposed method which involves the use of sulphuric and oxalic acids (H2 SO4 C OA) and the method currently employed in the UK, namely that based on sulphuric acid alone (12 kg=tonne), and also to ascertain the real possibility of utilising OA under conditions similar to those adopted industrially with H2 SO4 in a drum reactor. Fig. 3 shows the iron extraction yield after one hour’s treatment, using only H2 SO4 in various amounts (kg=tonne) at two different temperatures (20 and 90ºC, respectively). The solid=liquid ratio in the tests was always 1.5 (w=v). It appears that at 20ºC the iron extraction yield is not affected by the sulphuric acid content in the range from 2 to 12 kg=t, being 1–2% in all cases. The higher temperature improves the iron extraction yield significantly from 3 to 9% with a sulphuric acid content of 2 and 12 kg=t, respectively. At 90ºC and the same solid=liquid ratio (S=L D 1:5) leaching tests were performed with and without OA and 0 kg=t, respectively) using a sulphuric acid content of 12 kg=t. The results (see Fig. 4) reveal a very marked improvement in the iron extraction yield obtained in the presence of OA (30–40% against 9–10% obtained with H2 SO4 alone).

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

193

Fig. 4. Effect of OA on iron extraction yield: T D 90ºC; S=L ratio D 1.5 (w=v).

After these tests, the effect of the solid=liquid (S=L) ratio was investigated in the drum leaching apparatus. The iron extraction yield after one hour’s treatment at 90ºC using a solution containing OA and H2 SO4 (6 and 12 kg=t, respectively) is shown in Fig. 5 as a function of the S=L ratio. It can be seen that the limit solid=liquid ratio of 2.5 is the maximum for ensuring full efficiency of the reagent as regards the iron extraction yield. In fact, if this ratio is increased from 2.5 to 5 the iron extraction yield decreases from 30% to 17%. After these preliminary experimental runs a mixed factorial experiment was performed to evaluate the effect of the OA, the H2 SO4 and the treatment time on the iron extraction yield. Table 6 indicates the factors and levels investigated. The experimental results are shown in Fig. 6. From the analysis of the variance given in Table 7 (Davies et al., 1979) it is possible to observe that in the range of experimental conditions adopted the sulphuric acid exerts no effect on the iron extraction yield. In particular the following can be noted. (1) The OA concentration is a significant factor: its effect however is very small, the largest iron extraction yields being obtained for an oxalic acid content of 3 and 6 kg=t. In fact as reported elsewhere (Veglio´ et al., 1996) too much oxalic acid reduces the iron extraction yield because its positive effect is a function of pH (Veglio´ et al., 1996) even

194

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

Fig. 5. Effect of solid=liquid ratio (w=v) on iron extraction yield: T D 90ºC; OA D 6 kg=t; H2 SO4 D 12 kg=t; time of treatment D 1 h.

if its action is always positive when compared with the effect of sulphuric acid alone. (2) The effect of sulphuric acid concentration is not significant. (3) The three-order interactions are not significant. (4) The effect of treatment time is significant. Fig. 7 shows the iron extraction yield after one hour’s treatment (T D 90ºC; S=L D 1:5) with three levels of sulphuric acid concentration (2, 3 and 5 kg=t) and various levels of OA including the zero level. Table 6 Factors and levels investigated in the drum leaching tests Factors

Levels

A

OA

B

H2 SO4

C

Time

T D 90ºC; S=L D 1.5 (w=v).

(kg=t) (M) (kg=t) (M) (h)

3 0.05 2 0.031 1

2

6 0.10 3 0.046 3

4

12 0.20 5 0.077 5

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

195

Fig. 6. Iron extraction yield for various oxalic acid (OA) and H2 SO4 concentrations (kg=t): T D 90ºC; S=L D 1:5 (w=v).

In all the experimental conditions tested the maximum extraction yield was always 30–40% and in general the peak values are attained after 3–5 h of treatment (T D 90ºC; S=L D 1:5); this indicates that the total iron is not available to the leachant and that the complexing agent or the inorganic acid cannot reach the iron present in the inner lattice of the ore. Table 7 Analysis of the variance of the results obtained in Fig. 6 SS

SS

d.f.

MS

F

Significance (%)

SSA SSB SSC SSAB SSAC SSBC SSABC SST

36.1 15.3 1055.4 29.5 44.9 30.1 64.9 1276.2

2 2 4 4 8 8 16 44

18.1 7.6 263.8 7.4 5.6 3.8 4.1 a

4.4 1.9 65.0 1.8 1.4 0.9

97.1 81.6 100.0 82.5 72.4 47.8

SS D sum of square; d.f. D degree of freedom; MS D mean square; F values from Montgomery (1991). a The three-order interaction ABC was used as experimental error (Montgomery, 1991).

196

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

Fig. 7. Iron extraction yield after one hour treatment: T D 90ºC; S=L D 1:5 (w=v).

It can be concluded from examination of the results that the best treatment conditions (operating at the minimum OA concentration) are: OA concentration H2 SO4 concentration Treatment time Temperature

3 kg=t 2 kg=t 3–5 h 100ºC

Extraction yields ranging between 38 and 45% can be obtained under these conditions. The target values (in terms of iron extraction yield) can thus be attained. These experimental results highlighted the positive effect of OA with respect to the sulphuric acid alone. The use of OA permits to reduce (or eliminate) the sulphuric acid in the iron removal process in the process conditions utilised in the USA and UK in the current leaching operation. Moreover, Ubaldini et al. (1996) reported iron dissolutions of 46% after 8 days of treatment, with the same mineral adopted in this study, but in column leaching tests. The experimental conditions were: temperature 80ºC; oxalic acid concentration 16 g=l; S=L D 0.5 and pH D 2.5 (adjusted by NaOH). Our results show how the mixing conditions utilised in the drum leaching tests permitted to obtain the same iron extraction yields in 3–4 h of leaching and using a larger solid=liquid ratio (S=L) of 1.5, in the presence of OA concentrations of 4.5 g=l;

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

197

in our tests sulphuric acid was also used although in the tested experimental conditions no effect was observed on the iron extraction yield. Further tests might be carried out to check the iron dissolution in the absence of sulphuric acid. 3.3. Flow-sheet development The flow-sheet for iron removal from FSSC ore is shown in Fig. 8. Table 8 indicates the input conditions for the material balance of the process. The leaching solution of OA is made in pond 1 where precipitation of FeC2 O4 takes place (Dudeney et al., 1995). This iron is present due to the recirculation of the leaching solution from the drum reactors after the liquid=solid separation phase. It is assumed that the residence time of the leaching solution is 24 h, where 100% of the total complexed iron precipitates with the development of free OA (reaction 2). Results obtained by Dudeney et al. (1995) show the possibility of obtaining very high-grade ferrous oxalate precipitates from the leach liquor coming from the quartz leaching process. The ore and the leaching solution are then in contact in the drum reactors. As just mentioned, after the liquid– solid separation, the leaching solution containing the iron extracted from the ore is recirculated to pond 1. The solid phase goes for reblunge treatment involving washing with water. It was assumed that the solid phase retains a certain quantity of leach liquor amounting to 10% of the liquid volume entering the drum reactor. After the washing

Fig. 8. Flowsheet proposed for the FSSC leaching process.

198

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

Table 8 Design data for flow-sheet development 1 2 3 4 5 6 7

FSSC treated Initial Fe2 O3 in the ore Final Fe2 O3 in the ore Plant operation Temperature Leaching time S=L in the drum

150,000 t=yr 0.030% 0.017% 329 d=yr 90ºC 5h 1.5 (w=v)

step, the liquid phase coming from the solid=liquid separation goes to the biological reactor UASB (Dudeney et al., 1995), for degradation of the OA and precipitation of the iron. The cleaned water coming from the UASB system is recirculated for the washing process (to pond 2) and for OA solution preparation (pond 1). Specific consumption estimated from the material balance is OA 1 kg=t, H2 SO4 0.5 kg=t and water 0.6 m3 =t. This approximate material balance can be used to perform a rough economic assessment of the process.

4. Conclusions The experimental results permit an appraisal to be made of the effect of oxalic acid in the process of iron removal from quartz of industrial interest. This is positive. Iron extraction yields of 35–45% were obtained on treating the quartz with 3 kg=t oxalic acid and 2 kg=t sulphuric acid at 90ºC after 4–5 h treatment. Under the same conditions but without oxalic acid the iron extraction yield is in the 3–9% range, depending on the sulphuric acid content. The characterisation study carried out indicates why the maximum iron extraction yield is only about 40–45%, namely because 52% of the iron is in the micaceous fraction and is extremely difficult to remove. A preliminary flow-sheet of the process with a rough material balance has been drawn up to evaluate reagent consumption during the process. The experimental results indicate the technical feasibility of the process which uses oxalic acid for the removal of iron from industrial minerals.

Acknowledgements This work was carried out with financial support of the E.U. as per contract MA2M-CT90-0014. The authors are grateful to Mrs. M.A. Esposito for her helpful collaboration in the experimental work.

References Baumgartener, E., Blesa, M.A., Marinovich, H.A., Maroto, A.J.G., 1983. Heterogeneous electron transfer as a pathway in the dissolution of magnetite in oxalic acid solutions. Inorg. Chem. 22, 2224–2226.

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

199

Blesa, M.A., Maroto, A.J.G., 1986. Dissolution of metal oxides. J. Chem. Phys. 83, 757. Blesa, M.A., Borghi, E.B., Maroto, A.J.G., Regazzoni, A.E., 1984. Adsorption of EDTA and iron–EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98, 2. Bonney, C., F., 1994. Removal of iron from kaolin and quartz: dissolution with organic acids. In: Hydrometallurgy ’94. Chapman and Hall, London, pp. 313–323. Borghi, E.B., Regazzoni, A.E., Maroto, A.J.G., Blesa, M.A., 1989. Reductive dissolution of magnetite by solutions containing EDTA and FeII. J. Colloid Interface Sci. 130, 2. Chiarizia, R., Hortwiz, E.P., 1991. New formulations for iron oxides dissolution. Hydrometallurgy 27, 339–360. Conley, R.F., Lloyd, M.K., 1970. Improvement of iron leaching in clays: optimizing processing parameters in sodium dithionite reduction. Ind. Eng. Chem. Process. Des. Develop. 9 (4), 595–601. Davies, O.L., Box, G.E.P., Connor, L.R., Cousins, W.R., Himsworth, F.R., Sillitto, G.P., 1979. The design and analysis of industrial experiments. In: Davies O.L. (Ed.). Longman, New York. De Wolff, P.M., Visser, J.W., 1964. Absolute Intensities — Outline of Recommended Practice. Technique for obtaining absolute intensities with internal standard, Report 641.109 Technisch Physische Dienst TNO ENT. H, Reprinted Pow. Diffr. 3, pp. 202–204. Dos Santos Afonso, M., Morando, P., Blesa, M., Banwart, S., Stumm, W., 1990. The reductive dissolution of oxides by ascorbate. The role of carboxylate anions in accelerating reductive dissolution. J. Colloid Interface Sci. 138, 1. Dudeney, A.W.L., Narayanan, A., Tarasova, I.I., Teer, J.E., Leak, D.J., 1995. Treatment of iron oxalate leachates in anaerobic sludge blanket systems. In: Vargas, T., Toledo, H., Wiertz, J.V., Jerez, C.A. (Eds.), Biohydrometallurgical Processing. Univ. of Chile, Santiago, Vol. 2, pp. 277–286. Groudev, S.N., Genchev, F.N., Gaidarjiev, S.S., 1978. Method of biocatalytic removal of iron from mineral raw materials. Bulgarian Patent Nr. 29063. Guimares, S.J.F., De Olivera, N., De Salles, F.L., 1987. Purification of registro kaolin by magnetic separation. Min. Metall. 51 (485), 13–17. Litter, M.I., Blesa, M.A., 1988. Photodissolution of iron oxides. J. Colloid Interface Sci. 125, 2. Marabini, A.M., Falbo, A., Passariello, B., Esposito, M.A., Barbaro, M., 1993. Chemical leaching of iron from industrial minerals. XVIII Int. Miner. Process. Congr., Sydney, 23–28 May, 1993, pp. 1253–1259. Martell, A., Smith, R.M., 1974. Critical stability constants. In: Martell, A., Smith, R.M. (Eds.). Plenum Press, New York, Vol. 3. Montgomery, D.C., 1991. Design and analysis of experiments. In: Montgomery, D.C. (Ed.). Wiley, New York. Passariello, B., 1989. Processo per la deferrizzazione di caolino, sabbie quarzifere, carica per carta, pigmento pomice e materiali per l’elettronica. Italian Patent Nr. C 04 B33=04. Patermarakis, G., Paspaliaris, Y., 1989. The leaching of iron oxides in boehmitic bauxite by hydrochloric acid. Hydrometallurgy 23, 77–90. Stone, A.T., Morgan, J.J., 1987. Reductive dissolution of metal oxides. In: Stumm, W. (Ed.), Aquatic Surface Chemistry. Wiley, New York. Toro, L., Paponetti, B., Marabini, A., Passariello, B., 1990. Processo per la rimozione del ferro da concentrati di caolino. Quarzo ed altri minerali di interesse industriale. Italian Patent Nr. 21707A=90. Toro, L., Paponetti, B., Veglio´, F., Marabini, A., 1992. Removal of iron kaolin from ores using different microorganisms: the role of the iron reductase and the organic acid. Part. Sci. Technol. 10, 201–208. Torres, R., Blesa, M., Matijevic, E., 1990. Interaction of metal hydrous oxides with chelating agents. J. Colloid Interface Sci. 134, 2. Ubaldini, S., Piga, L., Fornari, P., Massidda, R., 1996. Removal of iron from quartz: a study by column leaching using a complete factorial design. Hydrometallurgy 40 (3), 369–379. Veglio´, F., Toro, L., 1993. Factorial experiments for the development of a kaolin bleaching process. Int. J. Miner. Process. 39, 87–99. Veglio´, F., Toro, L., 1994. Process development of kaolin bleaching using carbohydtrates in acid media. Int. J. Miner. Process. 41, 239–255. Veglio´, F., Passariello, B., Toro, L., Marabini, A.M., 1996. Development of a bleaching process for a kaolin

200

F. Veglio´ et al. / Int. J. Miner. Process. 54 (1998) 183–200

of industrial interest by oxalic, ascorbic and sulphuric acids: preliminary study using statistical methods of experimental design. Ind. Eng. Chem. Res. 35, 1680–1687. Waite, D., Morel, F.M.M., 1984. Photoreductive dissolution of colloidal iron oxide: effect of citrate. J. Colloid Interface Sci. 102 (1), 121–137. Yan, L.G., Yu, Y.J., Song, S.S., Nan, H.L., Cheng, Y.L., Li, X.M., Kong, Q.W., Dai, Y.M., 1978. Development of superconducting high gradient magnetic separator for beneficiation of kaolin clay. Proc. 2nd World Congr. Non-Metallic Minerals, Beijing, pp. 17–20.