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Journal of Applied Sciences and Environmental Management
World Bank assisted National Agricultural Research Project (NARP) - University of Port Harcourt
ISSN: 1119-8362
Vol. 8, Num. 1, 2004, pp. 37-44

Journal of Applied Sciences & Environmental Management, Vol. 8, No. 1, June, 2004, pp. 37-44

Mineralogy of Tailings Dump around Selebi Phikwe Nickel-Copper Plant, Botswana

1*EKOSSE, G; 2VAN DEN HEEVER, D J; 3DE JAGER, L; 4TOTOLO, O

1*Department of Geology, University of Botswana, P/Bag 0022 Gaborone, Botswana
2VDH Industrial Hygiene CC.PO. Box 26792 Langenhoven Park, 9330, Free State, South Africa
3Department of Environmental Health, Technikon Free State, P/Bag X20539 Bloemfontein, Free State, South Africa
4Department of Environmental Science, University of Botswana, P/Bag 0022 Gaborone, Botswana

Code Number: ja04008

ABSTRACT:

This study aimed at mineralogically characterizing the tailings dump emanating from the mining and smelting of nickel-copper (Ni-Cu) at Selebi Phikwe, Botswana, Southern Africa. Samples of tailings dump around the Selebi Phikwe Ni-Cu plant were studied using petrographic microscopy and X-ray Powder Diffraction (XRPD) technique. The tailings dump comprised of the following minerals: nickelblodite, pyrrhotite, quartz, pentlandite, malachite, chalcopyrite, actinolite, cristobalite, tremolite, kaolinite, mica and albite. Nickelblodite and malachite were mineralized from the primary ore minerals in the tailings dump. The sulfur-rich minerals contained in the tailings dump serve as principal promoting sources for acid mine drainage in the immediate surrounding environment. Consequently, aspects of pollution management such as phytoremediation and phytomining, and environmental desulfurization could be introduced as pollution control measures at Selebi Phikwe. @JASEM

Botswana is one of Africa's top three mineral producers by value, including South Africa and the Democratic Republic of Congo. Its mining industry provides employment for 10 000 Batswana (National Census, 1991). Diamond, soda ash, gold and Ni-Cu remain the mainstay of the mining industry in Botswana. However, recent growth in mining activities in the country has generated a corresponding increase in tailings dump and other contaminants that are considered as environmental pollutants. The Selebi Phikwe Ni-Cu mine area which is of concern is located geographically between longitudes 27o 47’ East and 27o 53’ East, and latitude 22o 55’ South 22o 00’ South (Figure 1). Mining and smelting activities may pose there as sources of environmental contamination. Previous studies conducted at Selebi Phikwe were directed at understanding the petrogenesis (Brown, 1987), structural geology (Gallon, 1986), and mineralogy (Nkoma and Ekosse, 1998; 1999; 2000) of the orebodies. X-ray powder diffraction studies of Ni-Cu orebodies (Nkoma and Ekosse, 1999; 2000) and soils (Ekosse et al., 2003), and particulate air matter (Ekosse et al., 2004) have also been conducted. Over the past years, there has been growing concern on some possible environmental implications resulting from the mining and extraction activities. In this study, an attempt was made to mineralogically by XRPD characterize the tailings dump and determine any possible implications to the environment.

MATERIALS AND METHODS

Samples And Sampling: Sampling was done twice monthly from January to July 2000. Tailings dump samples were collected from the tailings dump mount close to the concentrator/smelter plant. Three representative samples were chosen from the tailings dump at each sampling period. Two of the three samples were collected close to the sides and one was taken from the centre of the tailings dump, giving a total of 42 tailings dump samples collected for analyses. A hand shovel and a trowel were used to collect the samples. Sampling was done at about 5 cm from the surface of the tailings dump mount, and at the sides taking into consideration the altitude of the dump, which was close to 6 m. Each sample was placed in a 14 cm X 12 cm Hubco Sentry environmental sample bag, and transported to Gaborone. The samples were then placed overnight in a drier (60°C) to eliminate surface moisture.

Field observation: A Wards 10X hand lens was used to examine hand specimens of tailings dump. The shape and size of the tailings dump mount were visually observed as well as the color of the mineral crystals. All the observations were carried out at the sampling sites on the same day and time that samples were obtained for laboratory analyses.

Descriptive mineralogy: The optical microscopy technique was used for descriptive analyses of samples of tailings dump. The process involved testing for hardness, cleavage, fracture, color, streak, luster and crystal appearance. A Leitz Ortholux II Pol-BK petrographic microscope was used for descriptive mineralogy of the samples. 

X-ray powder diffraction analyses: The mineral phases in the tailings dump were identified using XRPD techniques. A Philips PW 3710 XRPD system with a Cu-Ka cathode radiation source was used at 40kV and 45 mA. Samples were scanned from 2o to 70o 2q.

RESULTS

Field observations: The tailings dump mount is continuously built from waste products of the mining and processing of the ore. The surface of the tailings dump mount was exposed to wind erosion. Small, dark and brassy yellow pseudo-tetragonal crystals of chalcopyrite and acircular green films of malachite could visibly be detected as constituents of the tailings dump. Hand specimens of the tailings dump samples, observed by using a hand lens, showed sub-angular quartz grains and crystals of greyish green amphiboles.

Descriptive mineralogy: Mineral constituents of hand specimens of the tailings dump were viewed under the microscope. Actinolite, albite, cristobalite, chalcopyrite, malachite, pyrite, pyrrhotite, tremolite and pentlandite were identified and have been in Table 1.

Table 1: Descriptive petrography of the minerals occurring in the tailings dump from the Ni-Cu orebodies.

Mineral

Hardness

(Mohr scale)

Cleavage

Fracture

Color

Streak

Luster

Crystal appearance

Actinolite

5 - 6

Good

Uneven to subconchoidal

Light to blackish green

White

Vitreous

Monoclinic

Albite

6 - 6.5

Distinct

Uneven

White to colorless

White

Vitreous to pearly

Triclinic

Cristobalite

7

None

Conchoidal

Colorless to gray

White

Vitreous

Tetragonal

Chalcopyrite

3.5 - 4

Poor

Uneven to conchoidal

Brassy yellow

Green to black

Metallic

Tetragonal

Malachite

3.5 - 4

Good

Uneven

Emerald green

Light green

Vitreous to silky

Monoclinic

Pyrite

6 - 6.5

None

distinct

Conchoidal to uneven

Pale yellow

Green to black

Metallic

Cubic

Pyrrhotite

3.5 - 4.5

None

Uneven to subconchoidal

Bronze yellow

Dark gray to black

Metallic

Hexagonal

Tremolite

5 - 6

Good

Uneven to subconchoidal

Colorless white

White

Vitreous

Monoclinic

Pentlandite

3.5 - 4

None

Uneven to conchoidal

Light bronze yellow

Bronze to brown

Metallic

Isometric

X-ray powder diffraction analyses: The following minerals were found to be unevenly distributed in the samples of the tailings dump: nickelblodite, pyrrhotite, quartz, pentlandite, malachite, chalcopyrite, actinolite, cristobalite, tremolite, kaolinite, mica and albite (figures 2, 3 and 4). Samples from the older part of the tailings dump were deprived of pyrrhotite, pentlandite and chalcopyrite. The samples contained mainly malachite, cristobalite and nickelblodite. The XRPD analyses revealed the following mineral phases in the mineral content of the more recent part of the tailings dump: pyrrhotite, pentlandite, actinolite, malachite and chalcopyrite. Samples of the tailings dump obtained from the sides of the tailings mount were found to contain traces of phyllosilicate minerals such as kaolinite and mica, as well as tremolite, albite and actinolite.

DISCUSSION

Mineralogical aspects: Figure 5 depicts the distribution pattern of minerals contained in the tailings dump mount. The tailings dump contained metals as well as pyrite and other sulfur minerals. These could, when associated with other weathering products, dissolve and wash away both vertically and horizontally into soils. Researchers at other mining sites (Al and Blowes, 1999; Blowes et al., 1995) have observed similar mineralogical trends. The slopes of the tailings dump serves as migratory pathways for ions of SO42-, CO32-, Cl-, NO3-, PO42-, Cd, Co, Cr, Cu, Fe, Ni, Se, and Zn to have been leached through labile corridors to the subsurface environments where applicable (Ekosse et. al., 2003). In a related study, the pH of soils around the Selebi Phikwe area was determined and found to be in the range of 3.5 to 6 (Ekosse et. al., 2003). The soils close to the tailings dump and smelter/concentrator plant had very high pH (between 3.5 and 4.5), compared to soils from other areas (between 5 and 6), around Selebi Phikwe (Ekosse et. al., 2003). The high acidity in soils was due to the leaching of sulfur-rich minerals contained in tailings dump (Ekosse et. al., 2003). High acidity in the mine drainage water could be attributed to bacterially catalyzed oxidation of pyrite (FeS2) and chalcopyrite (CuFeS2), leading to the dissolution of many other sulfide minerals (Bain et. al., 1995).  Dissolved heavy metals have the tendency of contaminating the surrounding soils mainly by oxidation Singh et. al., 1999; Buseck and Posfai, 1999; Kozák et. al., 1995).  Oxidizable heavy metals are associated with mineral sulfides. Sulfide oxidation reactions from pyrrhotite oxidation are as follows, according to Blowes et al., (1995): 

Fe(1-x)S + 2O2 → (1-x) Fe2+ + SO42-                      (1)

The Fe2+ shown in equation 2 is further oxidized to Fe3+ as follows

4Fe2+ + O2 + 4H+  4Fe3+ + 2H2O               (2)

Further oxidation and hydrolysis of pyrrhotite results in goethite precipitation as indicated in the equations of reaction below:

Fe(1-x)S + (8-2x) Fe3+ + 4H2O (9-3x) Fe2+ + SO42- + 8H+  (3)

 Fe(1-x)S + (2-2x) Fe3+  (3-3x) Fe2+ + S       (4)

4Fe3+ + O2 + 6 H2O  4FeOOH + 8H+        (5)

 

Environmental implications: Tailings dump from Ni-Cu mining and processing is known to promote acid mine drainage (AMD) (Blowes et al., 1995). Groundwater is consequently contaminated and the fauna and flora of the environment are affected (Kozák et. al., 1995; Blowes et. al., 1992; De Vos et. al., 1995. AMD may be defined as drainage occurring as a result of sulfide oxidation in rock exposed to air and water (Blowe et. al., 1994; Blowes et al., 1995; Benner et. al., 1997). The rocks become acidic in a low pH environment, leading to precipitation of ions including those of heavy metals. Pyrrhotite and pyrite in tailings dump are oxidized on exposure to water to form sulfate and sulfuric acid as indicated in equations 7 and 8 below (Blowes et al., 1995a; Benner et. al., 1997; Al et. al., 1994;  Al et. al., 1994a):

2FeS2 + 16H2O ® 2FeSO4 + 2H2SO4 + 14H+(6)

4FeSO4 + 10H2O +O2 ® 4Fe(OH)3 +4H2SO4          (7 )

The acidification reactions could be accelerated by the presence of the bacterium Thiobacillus ferroxidans, the absence of neutralizing minerals such as calcite (CaCO3), and release of heavy metals into their ionic state (Blowes et. al., 1998). Piles of tailings dump around the concentrator/smelter plant remain for years without proper disposal. During stockpiling, ions of heavy metals leach into the soils and water bodies, and could eventually be absorbed by plants. The grains of tailings dump observed at Selebi Phikwe were subangular, depicting limited transportation from source and also indicative of less aggressive leaching activities taking place as observed in Sundbury and Danodar river (Bain et. al., 1995; Singh et. al., 1999). Mineral orebodies and tailings dump release particles into the atmosphere through convectional forces with wind energy as a vector agent (Buseck and Posfai, 1999). More significant, however, are wastes released from tailings dump into the soils, which eventually influence the concentrations of heavy metals (Kozák et. al., 1995), contained in living organisms. Finer particles from the tailings dump could be windblown to distant environments, and eventually affect the ecosystem. Tailings dump from Ni-Cu mines and smelter/concentrator plants leach out ions of heavy metals that could contaminate soils and cause the development of dead zones; depleted of vegetative growth.

Aspects of pollution management: Degradation of water quality in surface and ground waters in contact with an acid producing tailings dump poses as a major environmental problem associated with AMD. The water pH is lowered by the alteration of sulfide minerals and the release of heavy ions such as Cd, Co, Cr, Cu, Fe, Ni, Se and Zn. The residual sulfides contained in tailings dumps have to be addressed in order to alleviate possible contamination of surface and ground water bodies (Mayer et. al., 1999). The discharge run-offs from S-rich tailings dumps are unsuitable for irrigation, stock weathering, drinking water, or recreational use.

Implementing phytoremediation could reduce contaminant levels of heavy metals in soils and plants at Selebi Phikwe and phytomining techniques as explained by Chaney et al. (1997). Phytoremediation as investigated and discussed by Salt et al. (1996), Cunningham et al. (1995), and Chaney et al. (1995), is a potential practical and more cost effective technology. Phytoremediation does have advantages which include non destruction and non destabilization of site, low environmental impact, favorable esthetics, continuous in situ generation of biomass, ability of living plant cells to supplement passive sorption of metals, and metabolic mechanism of metal uptake and detoxification (Nedelkoska and Doran, 2000). Sulfides at Selebi Phikwe are oxidized with the aid of Thiobacilli ferroxidans under favorable geochemical conditions, thereby promoting acidified environments (Chai et. al., 2000). In an effort to reduce growing environmental concerns of AMD at Selebi Phikwe, desulfurization is suggested. The desulfurized tailings can be used as cover material, thereby reducing rehabilitation costs by a factor of between 10 % and 35 % (Bussiere et. al., 1997; Bussiere et. al., 1998).  However an effective surface water drainage system as discussed by Gatzweiler et al. (2001), should be part of the remediation design. Furthermore, chemical stabilization (Eusden et. al., 2001), could also be considered as a possible complimentary clean-up technique at Selebi Phikwe.

Conclusions: This study focused on mineralogically characterizing the tailings dump at the Selebi Phikwe Ni-Cu mine area in an attempt of understanding the influence of its mineralogy on the immediate surrounding environment.  Minerals identified in the tailings dump included nickelblodite, pyrrhotite, quartz, pentlandite, malachite, chalcopyrite, actinolite, cristobalite, tremolite, kaolinite, mica and albite. Nickelblodite and malachite not occurring in the orebodies, were mineralized from the primary ore minerals in the tailings dump. The S-rich minerals in tailings dump could be the principal factor responsible for AMD in the area. Due to the fact that tailings dumps are main sources of AMD (Galan et. al., 1999), adequate control measures are necessary in limiting the spread of contaminants in the surrounding environments. The steps obviously taken by the BCL authorities in containing their tailings dump are in conformity to basic principles of environmental management of tailings dump as referred to Galan et al. (1999), and Jambor and Blowes (1998).

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