1. Introduction

The aim of this paper is to investigate some of the effects of the presence of goethite on the surface chemistry of Iron Duke hematite, a high grade deposit in the southern of . A high grade synthetic hematite sample was used as a reference for surface characteristics. The surface chemistry of these hematite samples was investigated using conventional microelectrophoresis techniques as well as a modified form of potentiometric titration that provided a measurement of the zero point of charge (zpc) for the three samples investigated.

2. Literature Survey

2.1. Surface Chemistry of Oxides

When a relatively insoluble oxide e.g. hematite is placed in an aqueous electrolyte solution, rearrangement of ionic species at the solid-liquid interface occurs and an electrical double layer forms. This double layer consists of one layer of surface charge and another layer of counter ions. The net surface charge may be positive, negative or zero, and since the system as a whole must remain electrically neutral, the medium surrounding the oxide must contain an equivalent number of counter ions with opposite charges. These will be attracted to the charged surface sites and adsorbed at the agitation, this adsorbed layer extends as a diffuse layer over a finite distance from the particle surface.

The electrical double layer that develops at the hematite/solution interface is the result of hydrolysis of the surface species, followed by pH-dependent dissociation of surface hydroxyls. A general equation for this process is:

(1)

For oxides, protons (H+) and hydroxyl ions (OH-) are considered to be surface potential determining ions (PDI).

The double layer potential is zero when the surface charge is zero. This condition is known as the zero point of charge (zpc). The surface potential, ψ₀ of these systems is given by:

(2)

Where T = temperature (K), R is the gas constant (cal/K-mole), and F = Faraday constant (96,500 C/mole).

Z₊ and Z_- are valencies of PDI in solution inclusive of sign.

a⁺ is the activity of positive PDI

a^- is the activity of negative PDI

a₀⁺ is the activity of positive PDI with zero surface charge

a₀^- is the activity of negative PDI with zero surface charge.

For oxides, a⁺ and a₀⁺ will be the activities of H⁺ ions in the solution considered and at the point of zero charge respectively, and a^- and a₀^- will be the activities of hydroxyl ions under corresponding conditions. Thus, oxides will carry a positive charge in solutions that are more acidic than the zpc and a negative charge in solutions that are more alkaline.

The surface charge, σ_s, on a solid in an electrolyte solution is determined by the adsorption density of the PDI on the solid surface. In the case of a univalent salt, σ_s is given by equation (3).

(3)

where Γ_a+ is the adsorption density (in moles/cm²) of the potential determining cation (proton) and Γ_a- is the adsorption density of the potential determining anion (hydroxyl ion).

Surface characteristics can also be investigated using electrokinetic phenomena, which involve the interrelation between electrical and mechanical effects at a moving interface. The electrokinetic results obtained in this study are expressed in terms of the electrophoretic mobility (μ) which is the ratio of the velocity of a particle in an aqueous environment to the applied potential gradient. In this study, for the timing of particles (t seconds) over one graticle square (50 μm) under a known voltage (V volts) being applied over the effective length of the cell, (10 cm.) μ is given by equation (4):

(4)

Hence,

(5)

The electrophoretic mobility can be converted to a zeta potential using a correlation dependent on the value of κa where κ is the reciprocal “thickness” of the diffuse double layer and a is the radius of the particle. The changes in zeta potential reflect changes in solution characteristics and adsorption of various species at the double layer.

The isoelectric point (iep) refers to the conditions under which the zeta potential, as determined by electrophoretic measurements, is zero. The surface potential, Ψ₀, need not be zero when the zeta potential is zero, particularly in the case of specific adsorption, hence the values of iep and zpc need not be the same.

Using these two techniques i.e. the determination of zero point of charge and the zeta potential (in mV) as a function of pH, it was hoped to characterise the samples of hematite. These hematites can be compared directly by comparing the values of zpc and iep determined using these two techniques.

2.1. Surface Chemistry of Hematite and Goethite

Since we are dealing with two samples of natural iron ore minerals, it is necessary to be aware of impurites associated with them. The usual impurity associated with naturally occurring iron oxides is silica, and a brief review of the effect of silica on the surface characteristics of hematite are given below.

Kulkarni and Somasundaran[1] determined the iep of a natural hematite sample to be 3.0, whereas the zpc of the same hematite, using titration techniques, was determined as 7.1. The explanation for these results was mineral heterogeneity i.e. the presence of fine silica in the hematite exerted its influence on the surface properties measured by electrophoresis. Lyklema[2] suggested that the low iep values recorded for some hematites were due to thin layers of silicates covering their surfaces. In contrast to this, Joy, Watson and Cropton[3] measured the iep of a Brazilian hematite as 5.4 and the zpc at 5.7. Smith and Salman[4] reported a zpc of 8.68 and an iep of 8.5 for a sample of natural hematite. More recently, Mwaba[5] measured the iep and zpc of a Swedish hematite at 4.2.

Pugh and Lundstrom[6] used surface chemical techniques to characterise hematite fines. They identified two distinct types of hematite particles, a black, “coarser crystallite” fraction coarser than 5 μm and a red “colloidal” fraction of approximately 1 μm in size. Using ESCA it was postulated that the red hematite particles were more hydroxylated than the black hematite, possibly due to the smaller particles having freshly formed stressed surfaces with higher surface energy which is more easily hydrated on dispersing in water.

Kulkarni and Somasundaran[1] also showed that the measured iep of a heterogeneous mineral surface is between the iep values of the two mineral components. Values of the iep for naturally occurring hematites are usually in the acid to neutral pH region (e.g. 3.5-6.7 (Joy and Watson,[7]; 2.2-6.9 (Parks,[8]; 5.4-6.7 (Fuerstenau,[9]; 6.2 (Winer and Wright,[10] and 8.68 (Smith and Salman,[11]). For synthetic hematites, these values are usually in the alkaline region (e.g. 8.7-9.04 (Parks,[8]; 9.04 and 6.95 (Mular and Roberts,[12]).

Montes et al[13] examined the differences in surface chemical behaviour between two size ranges of hematite, particles less than 10 μm and those coarser than 10 μm. The Fe₂O₃ contents were 86.6% and 91.1% respectively. The zeta potentials for the fine particles were determined using electrophoresis and the coarser particles were examined using streaming potential. The isoelectric point for the fine particles was determined as 3.3 and for the coarse particles it was 6.0. The lower value was explained by the presence of quartz. Desliming the particles with acid and washing with water returned a value of 8.8 using potentiometric titration. This technique also revealed a very low value of zeta potential of less than 1 mV over the pH range 5 to 9. This would have consequences when this mineral is subjected to flotation.

Kosmulski[14] and Kosmulski et al[15] has tabulated many values of the iep and zpc of natural and synthetic samples of hematite and goethite. The zpc values reported for synthetic hematites range from 3.2 to 9.5 with an average of 7.8. For natural hematites the range was 5.3 to 7.8 with an average of 6.0. The zpc values of synthetic goethite range from 5.6 to 10 with an average of 8.4. Recently Madigan et al[16] reported an isoelectric point of synthetic goethite at 6.2. For all other goethites the reported range was 5.8 to 9.2 with an average of 7.5. Ramos and McBride[17] measured the zero point of charge of a synthetic goethite sample as between pH 7.0 and 7.7 using light transmittance. They made the comment that it was lower than that reported by potentiometric titration because it was shifted lower by the presence of adsorbed bicarbonate ion. Carlson and Kawatra[18] showed that sparging a hematite slurry with carbon dioxide caused a reduction in pH and a surface modification of the hematite surface. Sparging with carbon dioxide did not change the value of the isoelectric point much from its initial value of approximately 2, however subsequent purging the slurry with distilled water for 24 hours returned a value of the isoelectric point of approximately 4. The hematite sample only contained 81% hematite.

For mineral goethite Parks[8] reported values of the isoelectric point between 3.2 and 6.7. Values for the iep of silica are usually in the strongly acidic region (e.g. 2.5[19]; 1.3-3.7[9] and 1.5-2.8[8]).

Smith and Trivedi[19] reported that the iep of hematite progressed from an initial value of 2.5 to a final value of 7.0 after 60 days ageing in water in plastic phials. (The change in iep was substantially complete within 20 days). The reason given for this change related to the dissolution of disturbed, possibly amorphous layers on the surface of the mineral ultimately exposing inner, undisturbed layers, accompanied by readsorption of complex hydroxy species back onto the surface of the mineral. Chuanyao and Yongxin[20] reported a value for the iep of hematite at 2.0.

From this brief literature review it can be seen that the surface chemistry of naturally occurring iron oxide samples can be strongly influenced by the presence of impurities, primarily silica, and other physical pretreatments conducted on the samples.

3. Materials Examined

Two samples of high grade iron ores from Iron Duke in the Middleback Range area were investigated in this study. These samples were received as lump ore from BHP Long Products Division, Whyalla, South Australia. According to Kirk[21], these represent end-members of material at the Iron Duke deposit. Sample # 1 was described as well bedded blue hematite, typically specular with well-defined bedding planes. The crystals are elongated parallel to the bedding planes and mechanical breakage was listed as ‘easy”. Sample # 2 is massive brown goethitic hematite, very hard and showing a good preservation of lump. Mechanical breakage was listed as “difficult”. X-ray Diffraction suggests that sample # 2 contained approximately 25 % goethite and 75 % hematite. Chemical analysis gave acid insolubles of 0.1 % for sample # 1 and 1.9 % for sample # 2 (for method of analysis see Section 4.1 below).

The synthetic hematite sample of greater than 99 % purity was obtained from Merck Pty. Ltd. Chemical analysis gave less than 0.1 % acid insolubles.

4. Procedure

4.1. Characterisation of Minerals

The Iron Duke samples were received as large, hand-picked lumps. They were crushed and pulverised in a ring mill until they were all finer than 150 µm prior to analysis and testing.

Iron assays were determined using the method of Kolthoff et al[22] which involved digestion using stannous chloride and hydrochloric acid followed by titration with potassium permanganate solution. The amount of acid insolubles (primarily silica) was determined gravimetrically on the residue remaining by filtration and drying followed by combustion of the ashless filter paper after acid digestion.

Specific Gravity (S.G.) was measured by displacement of nitrogen in a Quantachrome Stereopycnometer and the surface areas of the pulverised samples were measured using a Fisher Sub-sieve Sizer model 95 as per ASTM B330-88[23].

4.2. Measurement of Zeta Potential

Suspensions of 1 g of hematite in 50 ml of distilled water, with the pH adjusted using dilute solutions of either hydrochloric acid or sodium hydroxide were centrifuged for 30 seconds at 2000 rpm. Substituting the centrifugal force acting in Stoke’s Law equation for free settling reveals that the largest hematite particle remaining in suspension after centrifuging was 1.5 μm, and the largest quartz particle was 2.3 μm. The supernatant liquid containing sub-micron particles was transferred to the flat cell of a Rank Bros. Microelectrophoresis Apparatus Mark II.

Particles were timed in moving a distance of 50 μm under a known potential gradient that was chosen to give timings between 5 and 10 seconds. At least 10 separate particles were timed, followed by another 10 after the electrode polarity had been reversed. The average of the 20 timings was used in the calculation of mobility as given in equation (5). Zeta potentials were calculated from mobility data using the rationalised Smoluchowski equation which is applicable for this system. Under normal operating conditions, zeta potential (ζ) in mV is given by equation (6).

(6)

where μ is the mobility in μm.cm.(sec.) ^-1.(volt) ^-1 as calculated in equation (5).

The results for samples tested using this technique are given in Figure 1.

4.3. Measurement of Zero Point of Charge (zpc)

The method of Mular and Roberts[12] was used to determine the values of zpc for the various hematite samples. Suspensions of 2 g of hematite in 50 ml of 10^-2 M potassium nitrate (KNO₃) (in distilled water) were prepared and the pH adjusted using either potassium hydroxide or nitric acid as required. Only one pH regulator was used, as ionic strength is an important consideration in this method. Increasing the concentration of the electrolyte will increase the magnitude of the surface charge through the corresponding compression of the double layer. Increasing the magnitude of the surface charge will cause a rearrangement of the surface ions except at the zpc. Effectively, the added cations and anions from the salt compete for sites in the electrical double layer occupied by H⁺ or . Protons and hydroxyl ions can be adsorbed or released resulting in a change in pH, which is measured. At the zpc, increasing the electrolyte concentration will have no effect on either the surface charge or the pH, hence the zpc can be identified by the pH where the change in surface charge is zero[24].

Figure 1. Zeta potential characteristics of Iron Duke ores compared to synthetic hematite

Figure 2. Mular and Roberts plots for Iron Duke ores compared to synthetic hematite

When the pH was stable at the initial value, pH (i), sufficient solid KNO₃ was added to increase the KNO₃ concentration to 10^-1 M. When the pH again stabilised (which usually took from 2 to 5 minutes), this value was designated the final pH, pH (f). The change in pH, ΔpH, was determined by the difference (inclusive of sign) between the final pH and the initial pH i.e. ΔpH = pH (f) – pH (i). The results for these tests are shown in Figure 2.

5. Results

Some physical characteristics of pulverised hematite samples are given in Table 1.

Table 1. Physical and chemical characteristics of pulverised samples of hematites. Sample Colour %Fe S.G. Surface Area(m2/g) Iron Duke# 1 Black 70.0 5.20 0.109 Iron Duke# 2 Brown 59.1 4.82 0.320 Synthetic Red-brown 70.0 4.84 >2

6. Discussion

The selection of two samples of relatively pure, naturally-occurring hematites from the same general area but containing different amounts of goethite should give an indication of the sensitivities of the various techniques used to study their surface properties in respect to changes in mineralogy. The inclusion of a synthetic A.R. grade sample in the suite should also highlight any differences between the two types of hematite.

The zeta potential characteristics of the hematites investigated show a range of behaviour (see Figure 1). One method of comparison is to compare the values of the iep of the various samples (from Figure 1) to each other and to the values of zpc (from Figure 2). These are given in Table 2.

Table 2. Summary of surface characteristics of hematites Hematite IsoelectricPoint Zero point ofCharge Iron Duke # 1 2.4 6.4 Iron Duke # 2 6.2 6.4 Synthetic 2.8 6.4

From Table 2 there is a good correlation between iep and zpc values for Iron Duke # 2 whereas the values for iep and zpc for the two other hematites are very different even though their values of iep are very similar. The values of iep vary, but the values of zpc are identical. (As a matter of interest, Termes and Wilfong[25] also determined the iep value of a natural goethite sample at 6.2 and Nebo et al[26] at 6.4.) In contrast to this, Schuylenborgh and Sanger[27] measured the iep of a natural goethite at 3.2. As mentioned in the Literature Survey, the values of zpc and iep may not necessarily be coincident as they can be measuring different surface characteristics. Knowing that fine silica is present in the ore samples (see Materials Examined) and that the coarse particles were centrifuged out prior to testing in the microelectrophoresis apparatus (see Procedure) suggests that a significant portion of the material in the flat cell could be silica. (The coarse particles were centrifuged out because the settling of these particles under gravity would disrupt the horizontal velocity of the particles under the influence of the applied electric field). This effect has been observed by the author for hematite from the Iron Prince deposit, also in the area of (Quast,[28]).

The isoelectric point of sample # 2 is very similar to the zero point of charge, suggesting that, for this sample, the main species under test in the microelectrophoresis apparatus is goethite/hematite, not silica. From the surface area measurements (see Table 1) it is obvious that sample #2 is finer than sample # 1, but not as fine as the synthetic sample. The low isoelectric point of the high purity, synthetic sample is problematical. The synthetic sample does have a low specific gravity in comparison to pure hematite, which may give a clue to its uncharacteristic behaviour. Pugh and Lundstrom[6] reported that the iep of their colloidal red hematite measured using microelectrophoresis was 3, but the zpc using potentiometric titration was 6.3 (almost identical to that of all the samples used in this study).

Chuanyao and Yongxin[20] have published one of the few articles that compare the zeta potential of hematite, magnetite and martite. Comparing the data of Chuanyao and Yongxin[20] to that reported in Figure 2 shows that the curve for the Iron Duke # 1 follows their hematite curve closely, whereas there are no other matches, suggesting little contribution from any magnetite or martite that may be present in these samples. Joy and Watson[7] also found that the iep of natural hematites varied with colour, pretreatment, size and degree of hydration.

Although the method of Mular and Roberts[12] is very simple, it does have some limitations. One of these is the very small changes in pH (of the order of 0.01 units) which must be accurately measured both to determine the exact value of the zpc and to accurately plot the curves at high and low values of pH. Other conditions for the use of this technique are:

1. That the supporting electrolyte does not specifically interact with the oxide surface. (Gibb and Koopal[29] have reported some adsorption of KNO₃ on high surface area synthetic hematite).

2. That the addition of the electrolyte in the absence of the oxide sample under test does not change the pH, although this situation can be overcome using “blank” runs.

When comparative tests were done on the three hematite samples (see Figure 2), the curves cut the ΔpH = 0 axis at 6.4, and approach the axis at regions of high and low pH. The shape of these curves is very similar to that reported by Mular and Roberts[12]. It was obvious while conducting the tests that at very low ΔpH values in the pH region 6 – 7, the oxide settled rapidly out of the suspension leaving a clear supernatant liquid. This is supporting evidence for the zpc of the hematite samples being in this range, as the zpc coincides with the maximum settling rate (Fuerstanau,[9], Sadowski and Laskowski,[30]).Kanungo and Mahapatra[31] used this salt titration technique to establish the zero point of charge of a synthetic hydrated iron oxide (FeOOH-2.75 H₂O) at 6.95).

Appel et al[32] compared the determination of the surface chemistry characteristics of goethite using three techniques. These were potentiometric titration in solutions of varying ionic strength (point of zero salt effect-the same as the Mular and Roberts[12] technique, direct assessment of surface charges via non-specific ion adsorption as a function of pH (point of net zero charge and electroaccoustic mobility of reversible particles with pH-the isoelectric point). For synthetic goethite, all the techniques gave a value of 7.4-8.2 with the electroaccoustic technique giving values between 8.1 and 8.2.

On the basis of these tests it is argued that the values of zpc reported in Table 2 represent those of the iron oxide surface, since the whole surface is in contact with the electrolyte, not just the fine component which may be high surface area fines of different surface characteristics to the bulk material.

The approach to the ΔpH = 0 axis at high and low values of pH has been discussed by Korpi (cited in Joy and Watson,[7]). Korpi pointed out that an oxide surface has a finite number of adsorption sites capable of reacting with hydrogen and/or hydroxyl ions. When this point is reached any further addition of acid or base will alter the pH of the suspension strictly according to the amount added, and no further adsorption will occur. An alternative hypothesis has been reported by Kanungo and Mahapatra[31] who suggest that the approach to the axis at pH 2 is due to the formation of neutral polycations of Fe (OH)₃ on the surface of the goethite sample that they were testing.

From the data reported here it should be possible to characterise these samples of hematite. From Table 1 there is a gradation in colour from black (Iron Duke # 1) to brown (Iron Duke # 2) to red-brown (synthetic sample). Joy and Watson[7] were able to distinguish various surface characteristics relating to the colour of the hematite samples.

An important consideration is hematite purity where the % Fe gives a good indication of how much hematite is present in the sample. Pure hematite contains 70 % Fe and pure goethite 62.8 % Fe. Hematite samples containing >69 % Fe should be fairly pure, although any fine silica can dramatically affect the zeta potential characteristics

The specific gravity tends to follow the purity of the hematite samples as would be expected, as impurities generally have lower values of S.G. than pure hematite (S.G. 5.24). The specific gravity of goethite is given as 4.37 (Hurlbut,[33]). According to Kosmulski[14] the S.G. of hematite can vary between 5.2 and 5.3 and that for goethite can range between 3.8 and 4.4. If the Iron Duke # 2 sample contains only hematite and goethite in the ratio 3:1, it would have a specific gravity of 5.04. As seen in Table 1, the measured value is 4.82, suggesting the presence of other lower density components. This is supported by the 1.9 % acid insolubles measured for this sample but the S.G. is still in the range reported by Kosmulski[14]. The low value of specific gravity measured for the synthetic sample may indicate the presence of amorphous material.

The surface area gives an indication of the degree of fineness to which these samples were ground. The only equipment available to the author at the time this project was done was a Fisher Sub-sieve sizer Model 95. Winer and Wright[10] cite a practical upper limit for accurate operation of the Fisher instrument of 1.2 m²/g. Up to surface areas of 1 m²/g (see Table 1), the Fisher could be expected to yield surface area data of reasonable accuracy for regular-shaped powders. It does mean, however, that the surface area of the synthetic sample could not be determined using this technique.

As discussed above, zeta potential is not a reliable method of characterisation due to the presence of fine silica which can dominate the species under test in the microelectrophoresis apparatus.

The Mular and Roberts[12] plots give a good estimate of the zpc of the hematites, since now the total surface is taking part in what is essentially a potentiometric titration. The advantage of the Mular and Roberts plots is that they allow a good method for the determination of the zero point of charge.

7. Conclusions

Different samples of iron oxides exhibit different surface characteristics depending on such factors as mineralogy, colour, purity (and S.G.) and surface area. The natural hematite samples have similar mineralogical characteristics, being formed under similar processes. The hematite samples had high iron contents, but the iron content of sample # 2 was much lower suggesting different mineralogy.

Surface chemistry was investigated using two techniques: microelectrophoresis and a modified form of potentiometric titration. These techniques allowed the determination of the isoelectric point and zero point of charge respectively. Isoelectric points for the samples examined showed a range of values due to the presence of fine silica and possible amorphous material, and, in the case of Iron Duke # 2, goethite in the ore strongly influencing the values of iep for the samples. Zero points of charge were identical for all the samples tested in this study.

ACKNOWLEDGEMENTS

The author is grateful to BHP Long Products Division (now Onesteel), Whyalla for providing the Iron Duke samples investigated in this study.