Investigation of the potential for mineral carbonation of PGM tailings in South Africa

Minerals Engineering 24 (2011) 1348–1356

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Investigation of the potential for mineral carbonation of PGM tailings in South Africa J. Vogeli a,⇑, D.L. Reid b, M. Becker a, J. Broadhurst a, J.-P. Franzidis a a b

Minerals to Metals Initiative, Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa

a r t i c l e

i n f o

Article history: Available online 6 August 2011 Keywords: Global warming Mineral carbonation PGM tailings

a b s t r a c t Increasing atmospheric CO2 concentration is currently of considerable concern in terms of global warming. A possible technology that can contribute to the reduction of CO2 emissions is its sequestration by mineral carbonation. In this study, tailings from several different platinum mines in South Africa will be mineralogically characterised and their potential for mineral carbonation reviewed. Mg and Ca-rich minerals (plagioclase, olivine, orthopyroxene, clinopyroxene) present in the tailings are good candidates for mineral carbonation, which mimics natural weathering processes in which these minerals react with gaseous CO2 to form Ca or Mg carbonates. Since the reaction is influenced by particle surface area, the ultra fine grained nature of the PGM tailings provides another reason for the promise of PGM tailings for mineral carbonation. A preliminary ranking of the tailings samples and their efficacy for mineral carbonation has been developed according to whether the samples showed harzburgtic (e.g. Northam Platinum mine), pyroxenetic (e.g. BRPM) or noritic mineral assemblages. This information and understanding will assist in identifying opportunities and guiding the development of engineered facilities for the sequestration of CO2 by means of mineral carbonation. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Rock and mineral tailings produced by the platinum mining industry are composed of silicates and oxides that have a potential to be recycled in a variety of innovative schemes designed to reduce their environmental footprint. Incorporation into building materials, road aggregate, agricultural applications, landfills, manufactured fillers and many other schemes have been proposed, all of which raise issues of technical viability and cost effectiveness. In this paper we focus on the suitability of tailings as a mineral storage reservoir of anthropogenic CO2 that is being added to the atmosphere at an ever increasing rate, mainly through the combustion of fossil fuels. South Africa generates 93% of its electricity from coal combustion and thus is a coal-based, energy-intensive economy which generates about 420 million metric tons (Mt) of CO2 (Fig. 1) and very substantial volumes of industrial wastes per annum (Surridge and Cloete, 2009). Strong national and international incentives exist to reduce the carbon footprint of energy-generating systems. Water vapour is the major contributor to the greenhouse effect, however is vital for maintaining life. CO2 ranks second, and drives a positive feedback mechanism, governed by the Clausius–Clapyeron relation, which establishes that air can hold more water vapour per unit volume when it warms (Kiehl and Trenberth, 1997). ⇑ Corresponding author. Tel.: +27 21 650 2912. E-mail address: [email protected] (J. Vogeli). 0892-6875/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2011.07.005

Sequestration of CO2 is based on its reactions with the silicates that are common in platinum mine tailings, such as olivine, pyroxene and plagioclase, to form carbonate analogues that are relatively inert and benign in surface or buried reservoirs. Replacement of the silicate radical by carbonate can be modelled in terms of thermodynamic equations that would define the maximum possible CO2 that could be taken up, providing a reference with which mineral reservoirs can be compared. These reservoirs could be provisionally ranked to enable selection for further evaluation in terms of reaction kinetics, cost–benefit analysis and overall suitability as an industrial process. 1.1. Natural carbonation A natural example of mineral carbonation has been reported in studies by Ngwagwe (2009) and Mathivha (2010), who described dolerite dykes intruding coal seams in the Highveld and Witbank coalfields of South Africa. The high temperature magmatic intrusions were capable of igniting coal and presumably generating significant CO2 in what is essentially a natural fossil fuel combustion process. While it cannot be discounted that some of this CO2 could have escaped, the extensive carbonate alteration of the adjacent dolerite clearly demonstrates a form of internal self-sequestration. The above studies reported features such as in situ volume for volume replacement of the igneous rock-forming minerals by carbonates, secondary silica and clays. Replacement was complete in narrow dykes (<1 m) and partial in thicker intrusions, where the

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Fig. 1. Carbon dioxide emission breakdown (adapted from Koonin, 2008) Total emissions for South Africa in the year 2008 equalled 45 giga metric tonnes (Gt) CO2.

degree of alteration dropped towards their interiors. Carbonation was moreover restricted to where the intrusions cut the coal seams, and fresh unaffected dolerite still containing the primary magmatic mineralogy could be found both above and below the carbonaceous horizons within the Karoo sequence. While significant, the results of these past studies need to be augmented with quantitative modelling of the CO2 in the Karoo coal measures as a consequence of the magmatic event. An important conclusion that can be drawn from these studies is that the process of natural carbonation of dolerite dykes inside the coal seams may be explained by abundant fluid, magmatic temperature and geologic time. The question that needs to be addressed is whether these controls could have proxies which are attainable in industrial processes. Natural carbonation of peridotite in the Samail ophiolite, an uplifted slice of oceanic crust and upper mantle in Oman, has been noted by Kelemen and Matter (2008). Carbonate veins in the mantle peridotite in Oman have an average 14C age of 26,000 years and show that 104 to 105 tons per year of atmospheric CO2 are converted to solid carbonate minerals via peridotite weathering (Kelemen and Matter, 2008). Carbonation of mine tailings as a product of natural weathering processes has been reported by Wilson et al. (2009a,b) who investigated closed chrysotile asbestos mines at Clinton Creek and Cassiar in Canada. Accelerated weathering and effective CO2 sequestration was enhanced by the high surface area of the tailings particles, produced by the fine milling of the ore. X-ray diffraction and stable-isotope analyses revealed the presence of new carbonate species and their formation at low temperatures typical of the tailings deposits. Over a period of 26 years since the Clinton Creek mine ceased production, the uptake of atmospheric CO2 during surface weathering of the tailings was estimated to be 164 metric kilo tons (kt) with an average of 6.3 kt CO2/year.

promising concept that may play an important role in any management strategy as a CO2 mitigation option in the South African context of climate change. Waste products from mining and power generation could represent a valuable resource in this initiative. A summary of the various engineered mineral carbonation studies reviewed here is presented in Table 1. The high reactivity of wollastonite (a rare member of the pyroxene group of silicates) with CO2 has been noted by Daval et al. (2009). Batch experiments carried out at T = 90 °C and pCO2 = 25 MPa showed that the maximum extent of the reaction was 100% over a period of 5 days. The presence of local mines containing this mineral in their waste dumps provides another opportunity to assess natural carbonation in South Africa. In the study of Koukouzas et al. (2009), dunite, harzburgite and pyroxenite samples collected from the mountain of Vourinos, in Western Macedonia, Greece, were experimentally carbonated through an aqueous carbonation scheme. The Mg-rich silicates only showed low (10% of the stoichiometrically possible amount) carbonation into MgCO3. The unfavourable results may be attributed to insufficient reaction time, the particle size, or improper choice of reaction conditions and must be taken into account during future carbonation reactions (Koukouzas et al., 2009). Other examples of engineered carbonation include the red mud wastes generated through the Bayer process for alumina manufacture in Australia which have the capability to sequester CO2 through the reaction of red mud slurry and liquid CO2. A 100 g red mud slurry (45% solids) can effectively sequester 2.3 g of CO2 at 298 K and 6.7–10 MPa. The reaction reaches completion within 5–15 min. The value of the process lies in the fact that not only does it sequester CO2, but it also neutralises the high alkalinity of the red mud from pH 12.5 to 9.0 (Shi et al., 2001). Mineral carbonation of fly-ash, a waste product of coal-fired power plants, and steel slags is currently an area under investigation locally by the Council for Geoscience in South Africa (Doucet, 2010; Mlambo et al., 2011a). For instance, the Council for Geoscience conceptualized an approach whereby coal-combustion fly ash could be injected as a slurry into deep saline formations to

Table 1 Summation of mineral carbonation techniques reviewed in the text. Route Mineral carbonation technique

Author

Direct

Daval et al., 2009a

1.2. ‘‘Waste’’ mineral carbonation and opportunities The basic concept behind mineral CO2 sequestration is the mimicking of natural weathering processes in which Ca–Mg–Fe containing minerals (usually silicates) react with CO2 to form carbonates:

ðCa; Mg; FeÞSiO3 ðsÞ þ CO2 ðgÞ > ðCa; Mg; FeÞCO3 ðsÞ þ SiO2 ðsÞ

ð1Þ

Advantages of this process over geosequestration include limited reversibility in near surface and short term conditions. The products are benign in nature, contributing further to its attractiveness. Mineral carbonation of alkaline solid wastes with CO2 is an increasingly

Indirect

a

Aqueous

Reagents/ reaction conditions

Wollastonite, T = 90 °C, pCO2 = 25 MPa, t = 5d Aqueous– Koukouzas Dunite/ et al., additive Harzburgite enhanced 2009a reacted with 1 M NaCl, 0.64 M NaHCO3, pCO2 = 158 bar, T = 155 °C, t = 2 h Mlambo Fly-ash, brine et al., 2010 solution, pCO2 = 90 bar, T = 90 °C, t = 2 h Shi et al., Red mud slurry 2001 (45% solids), T = 25 °C, pCO2 = 6.7– 10 MPa, t = 5– 15 min Other Doucet, Steel slag-leached solvents 2010 with 0.5 M HNO3, T = 22 °C, t = 1 h

Indicates a mineral feedstock.

Extent of reaction/ conclusions 100% conversion to carbonates

10% Conversion to carbonates

Low conversionsuitable for in situ carbonation 100% ConversionNeutralises the high alkalinity of the red mud from pH 12.5 to 9.0 Theoretical CO2 capacity of 253 ktpa

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Fig. 2. Map of PGM operations in the Bushveld Igneous Complex in South Africa. The positions of BRPM and Northam platinum mines are highlighted with an asterisk, as well as the position of Secunda, the location of the coal-to-liquid fuel plant emitting the stream of concentrated (95%) CO2 directly into the atmosphere (Adapted from Northam, 2010 annual report).

improve the integrity of the reservoirs via in situ mineral carbonation (Mlambo et al., 2010), and demonstrated that fly ash can react with supercritical CO2 (Mlambo et al., 2011a,b), a primary requirement for the theoretical concept to be conceivable. Slags generated at steel mills in South Africa could effectively sequester 253 metric kilotons per annum (ktpa) of CO2 in mineral storage. The analysis of carbonated mineral reservoirs is designed to better understand the factors controlling natural carbonation, with a view to industrial applications. A potential source of suitable material, and one virtually unique to South Africa, is that of mine tailings from the primary beneficiation of Platinum Group Element (PGE) ore bodies (Fig. 2). This material is known to contain significant quantities of Ca–Mg–Fe bearing silicates, already finely milled and available in large quantities. In South Africa, the synthetic fuels sector captures about 32 million metric tons per annum CO2, at an estimated purity of 95%. This provides further opportunities as concentrated CO2 is readily available and the cost associated with purification of the CO2 is lowered for mineral sequestration purposes (Surridge and Cloete, 2009). 1.3. Characteristics of South African PGE mine tailings PGE ores are traditionally regarded as low grade, high tonnage materials that contain on average 2–5 g of Pt–Pd per metric ton (g/t), so that the published figures of millage closely approximate the tailings produced. The amount of ore milled has increased over time as existing mines ramp up production and new mines come on stream. Economic minerals probably do still remain in the tailings, and while their abundance is not always reported in detail, it is quite probable that older tailings contain higher quantities as the recovery technology has improved over time. The host gangue mineralogy (silicates and oxides) obviously dominates the tailings but non-economic sulphides such as pyrite and pyrrhotite may be present in variable but small quantities.

Factors controlling the mineral content of PGE mine tailings follow the composition of the ores, which can be subdivided into three broad types: (1) Merensky Reef: Silicate dominated (pyroxene, feldspar, olivine, chromite) (2) UG2 Chromitite: Oxide dominated (chromite, pyroxene, feldspar) (3) Platreef: Silicate dominated (pyroxene, feldspar, amphibole, carbonate) Historically, tailings from Merensky ores dominate stock piles, but in terms of current and future production the UG2 and Platreef types will probably become more significant. In view of the inherent greater perceived viability of silicate dominated assemblages to react with CO2 and the desire to recycle existing dumps, we have started this survey by focussing on the Merensky tailings. Some recovery plants have in the past processed Merensky ores blended with small quantities of UG2, but modern practice is to process them separately and dump the respective tailings at common sites after thickening. Part of this study is to provide a contribution to the debate regarding this latter practice, should recycling initiatives be better served by keeping the tailings separate. 1.3.1. Merensky reef types Frequently regarded as a uniform reef type, the Merensky Reef shows large variations in reef thickness, composition along strike, as well as the position of the mineralization. The Merensky reef is well-known as a major source of PGE. Considerable debate over definition, classification and practical application of reef characteristics has preoccupied workers throughout the PGE mining community. Even the use of the term ‘‘reef’’ has limited consensus, although many workers would agree that it should refer to the mineralized interval, irrespective of lithological variation, and therefore represent that which is mined preferentially (e.g. Viljoen, 1999; Viring

J. Vogeli et al. / Minerals Engineering 24 (2011) 1348–1356

and Cowell, 1999). Others have advocated that the reef is restricted to an individual specialized lithology (e.g. pegmatoid horizon bounded by thin chromitites; Lee, 1996) which makes up a significant proportion of the mining width and the contained metal grade. The typical Merensky Reef comprises a feldspathic pyroxenite zone under- and overlain by thin (5–15 mm) layers of chromite concentrations. The nature of Merensky reef along the western limb of the Bushveld changes, in a broader sense, from a harzburgitic rock-type (Northam Platinum mine) to a pyroxenetic rock-type (BRPM Platinum mine) and, finally, a noritic rock-type (Viljoen, 1999), located further south east along the western limb of the Bushveld Complex (Fig. 2). The fact remains that the run-of-mine consists of a range of lithologies that contribute only two dominant minerals: Mg–Fe pyroxene (60%) and Ca–Na feldspar (20%). Other minerals that might be locally important but with levels still <15% include olivine (1–5%), chromite and a Ca–Mg–Fe pyroxene (15%). Secondary minerals produced from in situ alteration of the primary phases include serpentine, talc, chlorite, carbonate and quartz; these may be locally important, but do not appear to reach above 1–5% in runof-mine fresh tailings. 1.3.2. Platinum group metals (PGM) processing The process of extracting the precious metals from the ore (Merensky, UG2 or Platreef) is categorised by liberation and separation by flotation. Liberation is the process of crushing and grinding the run-of-mine ore so as to expose the PGMs contained within. This is typically achieved through a series of crushers and tumbling mills, which operate autogenously or with varying loads of steel balls. In the flotation circuit which is broken down into streams through a series of flash floats, roughers, cleaners, re-cleaners and scavengers, reagents are added to render the surfaces of the PGM-bearing particles sufficiently hydrophobic to enable the valuable minerals to be separated from the waste material by froth flotation. The PGM that are not successfully extracted are left in the tail stream and are pumped out to tailings deposition facilities with average tail grades for UG2 currently 0.75 g/t (Fourie, 2008). Due to the very fine grained nature of the PGM which are typically less than 30 lm in these ores (Schouwstra and Kinloch, 2004), ultrafine grinding is used to liberate the valuable minerals. The implementation of IsaMills in ultra-fine grinding circuits has significantly improved the liberation and recovery of the PGMs, producing a finer average grain size in the tailings (Rule, 2009). This property of the tailings is favourable to the carbonation process, as a smaller average grain size results in a higher mineral surface area per unit mass, which promotes reactivity. Overall, the aims of this paper are; to mineralogically characterise Merensky reef tailings from the Northam and BRPM PGM operations for the purposes of mineral carbonation. Secondly, to establish a ranking scheme, whereby PGM operations may be graded in their suitability for mineral carbonation in South Africa. Finally, to estimate the maximum theoretical carbonation capacity of the PGM industry in South Africa annually. The aim of this paper is not to establish or design an engineered mineral carbonation process, but rather to evaluate the feasibility of mineral carbonation of PGM tailings in South Africa. It is anticipated that the results of this study will provide a platform for future engineering research into the creation of the appropriate engineered facilities. 2. Samples and analytical methodology 2.1. Samples Bulk Merensky tailings samples were obtained from BRPM and Northam Platinum Mines. It is noted that the BRPM tailings were

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sourced from a regrind circuit, and may not be entirely representative of the run of mine tailings for BRPM, but are still considered suitable to evaluate the potential of PGE tailings as a whole. Prior to the analytical measurements, the unwashed tailings samples were dried at room temperature, rolled and split into 1 kg representative samples. The 1 kg representative samples were then split into 100 g sub-samples for further analysis. 2.2. Analytical methodology The particle size distribution of the tailing samples was determined using a Malvern Mastersizer 2000. The refractive index for the analyses was 1.620. This was calculated through the factoring of the modal mineralogy, and the average density of each mineral phase. A Tristar was used for the BET surface area analysis. X-ray Fluorescence Spectroscopy (XRF) was used to determine the bulk chemical composition of the PGM tailings. A series of fusion discs and pellets were manufactured for analysis on a Phillips PW 1480 X-Unique X-ray fluorescence spectrometer. Quantitative X-ray Powder Diffraction (QXRD) was used to determine the relative proportion of the crystalline phases present in the PGM tailings. A 3.5 g representative sample from each of the mine tailings was micronized prior to analysis. A Bruker D8 diffractometer with a Vantec detector and Bruker Topas Rietveld refinement software was used for phase quantification. The bulk mineralogy of the samples was measured at the University of Cape Town using a QEMSCAN based on a LEO platform and equipped with two state of the art Bruker SDD detectors. The BMA (Bulk modal analysis; Goodall et al., 2005) routine was used. Tailings samples were wet screened and sized into five size fractions (+106, +75, +38, +10 and 10 lm), split and prepared into ore mounts for QEMSCAN analysis. Samples were analysed with pixel spacing between 2 and 5 lm depending on the size fraction. The results from the BMA were validated with those from the QXRD and XRF. 3. Key results 3.1. Particle size distribution and surface area The particle size distributions of both the Northam and BRPM tailings samples are illustrated in Fig. 3, which shows the fine grained nature of the tailings samples. The BRPM (dv, 90 = 138 lm) tailings sample is more fine-grained than the Northam (dv, 90 = 238 lm) sample. Similarly, the BRPM surface area (1.488 m2/g) is greater than for Northam (1.301 m2/g). As the average grain size of particles decreases so there is an increase in the exposed surface area in the tailing dump, promoting its reactivity with the CO2. 3.2. Mineralogy and chemical composition The major phases in both BRPM and Northam Merensky tails are represented in Table 2 and shown on a size by size basis in Fig. 4a and b. The QXRD was used to confirm the mineralogy of the tailings samples. The dominant minerals constituting the Northam tailings samples as shown in Table 2 are; plagioclase (13.6%), orthopyroxene (41.9%) and the alteration minerals, namely talc (12.4%) and serpentine (3.6%). It is clearly seen in Fig. 4a that olivine is relatively abundant throughout all the size fractions (6.2–9.6 wt.%), highlighting the near hartzburgitic nature of this reef (Viljoen, 1999). Enstatite (41.9%), the Mg end-member in the pyroxene group, is more dominant than the Ca pyroxene end-member, diopside (1.81%). Other than being the most abundant mineral phase, orth-

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J. Vogeli et al. / Minerals Engineering 24 (2011) 1348–1356 100 90

BRPM Northam

80

cumulative volume %

70 60 50 40 30 20 10 0 0.1

1

10

100

1000

10000

Particle Size Distribution (µm) Fig. 3. Particle size distribution of the BRPM (d50 <22 lm) and Northam (d50 <66 lm) mine tailings.

Table 2 Bulk mineralogy of the Northam and BRPM tailings samples, as determined by QEMSCAN. The asterisk indicates minerals suitable for carbonation. Mining operation Modal abundance (%)

Northam Merensky tailings

BRPM Merensky tailings

Pyrrhotite Pentlandite Pyrite Chalcopyrite Other sulphides Quartz Plagioclase feldspar⁄ Alkali feldspar⁄ Mica Chlorite⁄ Olivine⁄ Diopside⁄ Enstatite⁄ Amphibole Epidote Other silicates Calcite Oxides/hydroxides Chromite Others Talc⁄ Serpentine⁄ Total sequesterable Total non-sequesterable

<0.1 <0.1 0.3 <0.1 <0.1 0.9 13.6 0.1 0.4 0.8 8.1 1.8 41.9 3.9 0.8 <0.1 0.1 0.6 5.6 4.9 12.4 3.7 82.3 17.7

0.1 <0.1 0.4 <0.1 <0.1 1.3 32.2 <0.1 0.5 1.7 2.4 2.1 30.6 5.5 2.1 0.1 0.1 0.4 1.9 6.8 11.1 0.7 80.8 19.2

opyroxene tends to be more common in the coarser size fractions, and similarly for plagioclase. The alteration minerals, predominately chlorite, serpentine and talc, however, are more common in the intermediate to fine size fractions. Amphibole is restricted to the very fine size fractions. Quantitative mineralogical results of the BRPM tailings are given in Fig. 4 b and Table 2, and indicate that there is a distinctive increase in the modal abundance of plagioclase (32.2%) and a decrease in the modal abundance of olivine (1.8–2.7 wt.%) compared to the Northam tailings. This is expected, due to the variation of the modal mineralogy along strike of the Merensky reef, where the reef mineralogy matches more of a pyroxenetic rock-type (Viljoen, 1999). Once again, enstatite (30.6%) is more abundant than diopside (2.1%), and is more common in the coarser size fraction.

Similarly, plagioclase is also more common in the coarser size fractions. Once again, the alteration minerals and amphibole (5.5%) show a reverse in this trend, and are more common in the fine to intermediate size fractions, most likely due to the fine grained nature of these alteration minerals. XRF data were used to validate the results from the QEMSCAN analysis. The XRF results confirmed the relative abundance of the major oxides present in BRPM and Northam PGM mine tailings (Table 3). Northam and BRPM show relatively similar SiO2 wt.%, however Northam shows a greater Al2O3 wt.% relative to that of BRPM, at 7.87 and 14.4 wt.% respectively. This is attributed to the higher plagioclase content in the BRPM tailings. This increased abundance of plagioclase confirms the greater CaO wt.% seen in BRPM (8.73%) when compared to Northam (4.97%). The higher MgO content of the Northam tailings (7.3% higher than BRPM), is attributed to the increased abundance of enstatite (Mg end-member of the pyroxene group) and forsterite (Mg end-member of the olivine group).

3.3. Maximum theoretical CO2 specific sequestration capacity The theoretical CO2 specific sequestration capacities for common gangue minerals present in PGM mine tailings have been calculated and are shown in Table 4 to indicate the mass of CO2 (g) sequestered per 100 g of gangue mineral. A balanced direct-carbonation reaction for each mineral end-member is presented, as well as the products formed. The table may be used as an indicator as to which specific gangue minerals are ideal targets for CO2 specific sequestrative purposes. Note that these theoretical capacities are independent of the kinetics of the mineral carbonation reaction. Forsterite has the greatest capacity for CO2 sequestration, and theoretically captures 62.5 g CO2/100 g of forsterite, followed by enstatite at 43.8 g CO2/100 g enstatite. Plagioclase, a common silicate mineral in the tailings from Northam and BRPM, has the lowest capacity at 15.8 g CO2/100 g anorthite. A forsterite and enstatite-rich rock (harzurgite), therefore, has the greatest theoretical CO2 specific sequestration capacity, and a plagioclase-rich rock (norite) has the lowest theoretical CO2 specific capacity. It is important to note that talc and chlorite will partake in the mineral carbonation reaction, however only at temperatures above 650 °C, which may not be favourable for the purpose of industrial mineral carbonation, due to the increased energy input.

J. Vogeli et al. / Minerals Engineering 24 (2011) 1348–1356

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Fig. 4. Size by size modal abundance of mineral suitable for mineral carbonation in the (a) Northam and (b) BRPM mine tailings samples. Alteration minerals represent talc, chlorite and serpentine.

Table 3 Chemical composition (in weight %) of the Northam and BRPM tailings. Major oxide

SiO2 TiO2 Al2O3 Fe2O3 MgO CaO Na2O Cr2O3 NiO

Mining operation Northam Merensky tailings

BRPM Merensky tailings

47.8 0.29 7.87 13.1 21.9 4.97 0.75 2.25 0.13

49.9 0.24 14.4 8.85 14.6 8.73 1.20 0.87 0.08

4. Discussion 4.1. Development of a ranking scheme to evaluate the potential of PGE tailings In order to evaluate the potential of different PGE tailings for mineral carbonation in South Africa, some consideration needs to be given as to the criteria which may make tailings from one deposit more suitable than from another. This is manifested in a preliminary ranking given in Table 5 of the effectiveness of tailings

from the BRPM and Northam PGM operations based on their capacity, relative reactivity and distance from a sequestered CO2 source. At Secunda in South Africa, 32 mega metric tons (Mt) of 95% pure CO2 stream is available annually (Surridge and Cloete, 2009), enhancing its suitability for the mineral carbonation reaction. Calculations are based on an ‘‘as the crow flies’’ distance of the PGE tailings from Secunda. This plays a particularly important role in the evaluation of the feasibility of a PGM operation for mineral carbonation, as costs of CO2 transportation, whether it be via truck or pipe, increase with distance. Furthermore, the carbon capture capacity and storage (CCS) is dependent on two properties of a PGM operation, the tonnages of tailings disposed of per annum, as well as the mineralogy of the tailings. The particle size distribution and surface area, in turn, have an effect on the rate of mineral carbonation. Larger capacities and increased reactivities are favourable for the purpose of industrial mineral carbonation, as they reduce costs of the procedure. Important factors recognised in the literature of recent kinetic studies of the mineral carbonation reaction include (1) mineral species (Koukouzas et al., 2009), (2) mineral compositions (Daval et al., 2009), (3) mineral structure, (4) mineral proportions (Koukouzas et al., 2009), (5) grain size distribution (O’Connor et al., 2000), (6) morphology and (7) surface texture. Characterisation of these mineralogical properties can provide an initial guide to the relative reactivity of mineral tailings, after which selected

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Table 4 Theoretical mass of CO2 sequestered per 100 g of mineral end-member. Mineral

End member (100 g)

Direct carbonation reaction

Products formed

Olivine

Fayalite Fe2SiO4 forsterite Mg2SiO4 Enstatite⁄ Mg2Si2O6 Ferrosilite⁄ Fe2Si2O6 Diopside⁄ aMgSi2O6 Hedenbergite⁄ CaFeSi2O6 Serpentine 2Mg3Si2O5(OH)4 Magnetite Fe3O4 Anorthite (An70) CaAl2Si2O8

Fe2SiO4 + 2CO2 ? 2FeCO3 + SiO2 Mg2SiO4 + 2CO2 ? 2MgCO3 + SiO2

2FeCO3 2MgCO3

SiO2 SiO2

– –

21.6 62.5

Mg2Si2O6 + 2CO2 ? 2MgCO3 + 2SiO2

2MgCO3

2SiO2

–

43.8

Fe2Si2O6 + 2CO2 ? 2FeCO3 + 2SiO2

2FeCO3

2SiO2

–

33.3

CaMgSiO3 + CO2 ? (Ca,Mg)CO3 + SiO2

(Ca,Mg)CO3

SiO2

–

37.9(Ca), 43.8(Mg)a

CaFeSiO3 + CO2 ? (Ca,Fe)CO3 + SiO2

(Ca, Fe)CO3

SiO2

–

37.9(Ca), 33.3(Fe)a

4Mg3Si2O5(OH)4 + 6CO2 ? 6MgCO3 + Mg6Si8O20(OH)4 + 6H2O

6MgCO3

6H2O

Mg6Si8O20(OH)4

23.8

Fe3O4 + CO2 + H2O ? FeCO3 + 2FeOOH

FeCO3

2FeOOH

–

19

CaAl2Si2O8 + CO2 ? CaCO3 + Al2O3 + 2SiO2

CaCO3

Al2O3

2SiO2

15.8

Pyroxenes

Serpentine Magnetite Plagioclase a

Mass of CO2 sequestered (per 100 g starting material)

CO2 g sequestered according to the relevant mineral end-member enrichment.

Table 5 Summary table highlighting the essential factors necessary to rank the potential of PGE tailings for mineral carbonation. Mining operation Capacity

Reactivitya Distance from CO2 source a

Tonnage of Merensky tailings Mineralogy Carbonation capacity kt CO2 pa PSD BET surface area Distance from secunda

Northam

BRPM

1002.2 kt Olivine–orthopyroxenite 387.56 50 vol.% <66 lm 1.301 m2/g 300 km

1049.0 kt Pyroxenite 269.72 50 vol.% <22 lm 1.488 m2/g 280 km

Note that particle size distribution and surface area are used as a proxy for reactivity.

candidates can be subjected to experimental carbonation. In addition to the above mineral reservoir properties, the efficacy of carbonation will also be strongly controlled by the manner in which CO2 is reacted with mineral assemblages, including temperature, pressure (including pCO2), as well as the permitted time duration. In terms of the ranking scheme given in Table 5, the relative capacity for mineral carbonation is the first important factor. The relative capacities of the two tailings samples have been calculated based on their proportion of sequesterable minerals (see Section 1.2) and milled tonnages. The encouraging outcome from the mineralogical analyses of these tailings shows that over 80% of the mineralogy should react during the mineral carbonation reaction. Theoretically, Northam should have a greater CCS capacity than BRPM due to the favourable mineralogy. However, it is important to note that the tonnage of Merensky tailings produced annually has a large impact on the CCS capacity of each operation. As shown in Table 5, Northam produced 1002 kt of Merensky tailings as opposed to 1049 kt from BRPM in the year June 2009 – June 2010 (Northam, 2010; Anglo American plc. Annual Report, 2010). Based on the mineralogy from Table 2, and the tonnages produced by each PGM operation, it has been calculated that Northam has a CCS capacity of 388 kt and BRPM 270 kt CO2 pa for the Merensky tailings. However, the CO2 capacity of the PGM operations is not sufficient to determine the effectiveness of an operation for mineral carbonation, since the reactivity of the tailings must be included in the ranking criteria. In this study, particle size distribution and surface area are used as proxies for reactivity: The smaller the grain size, the larger the surface area, the greater the reactivity during a direct mineral carbonation reaction. Therefore, one may infer that BRPM tailings may be more reactive than the Northam tailings. Coupled with the fact that BRPM is 20 km closer to the CO2 source, and the tailings may be more reactive during mineral carbonation, BRPM

Merensky tailings should be a more viable option for the mineral carbonation strategy. The differential in distance is small, however when comparing operations with greater distances, it may play a more important role. Even though the capacity of Northam is 117.84 kt greater than that of BRPM, the costs associated with transportation of the gas and the energy required to achieve mineral carbonation is slightly reduced for BRPM. 4.2. Maximum theoretical CO2 specific sequestration capacity of the PGM industry in SA One of the major properties of the PGM industry in South Africa which makes it attractive for the mineral carbonation process is the vast volumes of Ca and Mg-rich tailings produced annually. During the period June 2009 to June 2010, the major ‘‘players’’ in the PGM industry (Anglo American plc. Annual Report, 2010; Implats, 2010; Northam, 2010; Lonmin, 2010) produced 77,505 thousand tons of Merensky, UG2 and Platreef tailings (Anglo American plc. Annual Report, 2010; Implats, 2010; Northam, 2010; Lonmin, 2010; Goodall, 2010; personal communication). In order to determine the theoretical CCS capacity of tailings for the entire PGM industry in South Africa in one year, a generic Merensky, UG2 and Platreef mineralogy was used (Solomon, 2010). The calculated CCS capacity of the PGM industry in South Africa is 13.937 million metric tons of CO2 per annum, and is represented in Fig. 5. This accounts for both the direct and indirect CO2 emissions of the Impala, Anglo and Northam PGM operations during the past year (Anglo American plc. Annual Report, 2010; Northam, 2010; Implats, 2010). Note that Fig. 5 does not include the minor PGM operations within the Bushveld Complex, and therefore means that the results shown here are an underestimate of the real potential. More importantly though, it shows that mineral

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Fig. 5. Estimated theoretical carbonation capacity per annum of the major players of the PGM industry in South Africa. The annual milled tonnages as well as CO2 emissions are also shown.

carbonation of PGE tailings can theoretically account for 43.6% of the 95% pure CO2 produced by the synthetic fuels industry annually in South Africa.

5. Conclusions and future opportunities The aim of this paper was to evaluate the potential of PGE tailings for mineral carbonation in South Africa. This was done using two Merensky tailings samples, and accomplished by developing a preliminary ranking system for the application of a mineral carbonation technique to Merensky tailings produced by the PGM industry. Two operations, BRPM and Northam, were used to determine a ranking scheme based upon three key properties: CCS capacity, relative reactivity and distance from CO2 source. Using the mineralogical characteristics of the tailings, we established that a finer-grained, more olivine-rich, plagioclase poor, tailing would be the best candidate for mineral carbonation. The distance from the CO2 source, in turn, would have an influence on the cost of the operation. Secondly, we estimated the CCS capacity of the entire PGM industry, and found that mineral carbonation of the tailings produced by the PGM industry could account for both the direct and indirect CO2 emissions of the PGM industry, annually. More importantly, the CCS capacity from PGM tailings may account for 43.6% of the 32 million tons of 95% pure CO2 produced by the synthetic fuels industry annually. Finally, apart from addressing the challenges associated with the availability and preparation of suitable CO2 sequestration feedstock, mineral carbonation may also have a stabilizing effect on the tailings and mineral reservoirs – thereby reducing environmental risks associated with their land disposal. The products of mineral carbonation, Mg-rich carbonates, may have particular value to the cement industry, thus reducing CO2 emissions, reducing material wastes and producing a product to facilitate development within South Africa.

Acknowledgements The authors would like to acknowledge Dr Kirsten Corin, of the Department of Chemical Engineering at UCT, for carrying out the QXRD analyses. Financial support from the NRF SARChI Chair in Mineral Beneficiation, THRIP and Inkaba yeAfrica is also gratefully acknowledged.

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