Pyrite oxidation and the associated geochemical processes in tailings in the Atacama desert / Chile: the influence of man controlled water input after disuse

Proceedings Chapter

Reference

Pyrite oxidation and the associated geochemical processes in tailings in the Atacama desert / Chile: the influence of man controlled water

input after disuse

DOLD, Bernhard Stefan, EPPINGER, K.J., KÖLLING, M.

Abstract

This paper reports the results of a study of two tailings, under desert conditions in the III.

Region of Chile. Both tailings received significant amounts of man controlled water from the hill side above, after operation had ceased. Sampling was undertaken with a soil sampling equipment up to a depth of 8 metre. The samples were analyzed by x-ray diffraction and ICP-MS. Sampling observation in the field showed a zonation inverse, known in humid climates zones, in which the oxidation zone lies above an accumulation zone and a basal primary zone. In the studied tailings the stratigraphic column changes from a homogenous primary zone at the top, to an inhomogenous zone with intercalations of oxidized layers, to a very homogenous oxidation zone at 5-8 metre depth. The pH changes from 7-8 to 4 and the grain size from fine sandy to clayey between the primary and the oxidation zone. These zones are directly related to the water level in the tailings. X-ray diffraction analysis has confirmed that sulphides such as pyrite are only present in the primary zone; whereas gypsum and jarosite are present in the oxidation zone. The [...]

DOLD, Bernhard Stefan, EPPINGER, K.J., KÖLLING, M. Pyrite oxidation and the associated

geochemical processes in tailings in the Atacama desert / Chile: the influence of man controlled

water input after disuse. In: Sánchez, Mario A.; Vergara, Froilán & Castro, Sergio H. Clean

Technology for the mining industry . 1996. p. 1-10

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CLCAN ·fECHNULUUY FOR Tiffi MINING INDUSTRY (M.A. Sanchez, F. Vergara, and S.H. · Castro, Eds.), University of Concepci6n, Concepci6n-Chile, 1996

PYRITE OXIDATION AND THE ASSOCIATED GEOCHEMICAL PROCESSES IN TAILINGS IN THE ATACAMA DESERT / CHILE:

THE INFLUENCE OF MAN CONTROLLED WATER INPUT AFTER DISUSE.

B. DOLol), K.J. EPPINGER2) & M. KoLLJNG3)

1) Universite de Geneve, Department de Mineralogie, Rue des Maraichers 13, 1211 Geneve, Switzerland.

2) Universidad de Atacama, IDICTEC • UDAmbiental, CC 240, Copiap6, Chile.

3) Universitat Bremen, Fachbereich Geowissenschaften, Postfach.330 440, 28334 Bremen, Germany.

ABSTRACT

This paper reports the results of a study of two tailings, under desert conditions in the III.

Region of Chile. Both tailings received significant amounts of man controlled water from the bill side above, after operation had ceased. Sampling was undertaken with a soil sampling equipment up to a depth of 8 metre. The samples were analyzed by x-ray diffraction and ICP-MS. Sampling observation in the field showed a zonation inverse, known in humid climates zones, in which the oxidation zone lies above an accumulation zone and a basal primary zone. In the studied tailings the stratigraphic column changes from a homogenous primary zone at the top, to an inhomogenous zone with intercalations of oxidized layers, to a very homogenous oxidation zone at 5-8 metre depth_ The pH changes from 7-8 to 4 and the grain size from fine sandy to clayey between the primary and the oxidation zone. These zones are directly related to the water level in the tailings. X-ray diffraction analysis has confirmed that sulphides such as pyrite are only present in the primary zone; whereas gypsum and jarosite are present in the oxidation zone. The latter indicate the influence of sulphate rich acid solutions, resulting from the oxidation of pyrite (known as acid mine drainage - AMD). The element distribution indicates also a sulfide oxidation zone. The distribution of environmentally unhealthy heavy metals shows an accumulation at the uppermost parts of the oxidation zone, as a result of "unspecified" adsorption (surface) on the Fe(IIl)hydroxides and/ or of sulphide precipitation.

Below in the homogenous oxidation zone, heavy metal distribution is also homogenous and the metal contents are much higher than in the primary zone, which indicates a "specified" adsorption (incorporation in the crystal system, stable at low pH) and a geochemical equilibrium. The results show clearly that the geochemical processes in the studied tailings are directly related to the man controlled water input. The detected processes are related to pyrite oxidation and show a strong mobilization of all types of elements. The distribution of elements is interpreted to indicate an upwards migrating oxidation zone, characterized by the precipitation of Fe(IIl)hydroxides from Fe(II), metal rich solutions, and a water flow confined to the top of the oxidation zone.

INTRODUCTION

The problem of pyrite oxidation and the formation of acid mine drainage was the focus of investigations in the last 30 years (BRYNER et al. 1967, SINGER & STUMM 1970, NORDSTROM 1977, NODRSTROM et al. 1979 & 1982). The solubility and mobility of heavy metal ions by acid, formed by sulphide oxidation, is well known (BLOWES et al. 1994, HAKANSSON et al.1994, PTACEK & BLOWES 1994, WEBSTER et al. 1994). The sorption processes at the interface solid/water by iron hydroxides and other minerals received slrong interest for research (DAV1S &

LECKIE 1978, LECKIE et al. 1980, GERTH & BRiiMMER 1981 & 1983, DONNERT et al. 1990, DAVIS et al. 1986, HSIA et al.1992, RIMSTIDT et al. 1994). The environmental impacts associated to mining industry, especially the tailings are well studied in humid climates (FUGE et al. 1994, BOYLE & SMITH 1994, PEDERSEN et al. 1994). Data from arid climates for the conduct of tailings are rare (HAMMAN & VERNON 1987, RAMPE & RUNNELLS 1989).

The increasing sensibility of Chilean people for the environmenl, makes the environmental impacts of mining industry to a real problem in this country. In Chile most of the mining industry took place in the north of Chile under extreme arid conditions since Inca times. Toe increasing mining activity mostly in the last century was the reason for some heavy environmental impacts. An example is the beach of Cbafiaral in which since 50 years tailing material from the "Potrerillos·El Salvador" was deposited. As a result, the beach of Chaiiaral resulled biologically death. In the Ill.Region of Chile alone 264 tailings are known.

THE STUDIED TAILINGS

The two studied tailings are situated in the Atacama desert in the III. region of Chile. Both represent similar history: tailing No. 1 has been about out of operation for 30 years, tailing No. 2 about 20 years. The mill company bought material from different stratabound sulphide copper mines in the Copiap6 mining district. The tailings show a similar design of construction (Fig. I).

A small valley was closed by a dam composal of leached material. Behind this dam the flotation tailings were deposited by cyclone separation. After the disuse of the tailings the company started the deposition of material at the hillside above the tailings. At tailing No.1 site the deposition at the hillside was stopped for years, while above the tailing No.2 site, deposition is still going on.

Together with the material, there are also deposited about 4000 m3 / month of water above of tailing No.2. As a result of the seepage of this water into the tailing, it has a high water content (Fig. 2 water level). The dam is constructed by leached material, while the tailing material represents the fine fraction after the separation by cyclones. Toe recent deposited material on the hillside is the coarse fraction of recent processing.

CLEAN 1ECHNOLOGY FOR THE MINING INDUSTRY

recent deposition and water input

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deposition

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Fig.1: The sketch shows the situation at tailing No.2. Tailing No.I is situated in the next valley to the left and shows the same situation but with out recent deposition. The tailing has a surface area of ea. 17000 m2 and the dam has a length of ea. 150 m and is ea. 30 m high.

SAMPLING METHODS

The samples are taken by soil sampling equipment up to a depth of 8 metres (The depth of the tailings is about 30 m). This simple coring method was very successful. Because of the fine grain size the drill bole is very stable and the contamination of the sample is low, indicated by good correlation of horizons in all depths. The samples, after the lithological identificaLion and the measurement of pH, were sealed in a plastic bag. The samples were transported immediately to the laboratory for drying and water content dete1JDination. On tailing No.2, ten drill boles to 8 m depth were made. One drill core on tailing No.1 up to 7 m depth was taken to compare the two tailings. Every metre depth one sample was taken for examination. Because of the similar stratigraphy in all drillings, only S2, S7 and S8 were analyzed.

ANALYZING METHODS

The dry and homogenized samples were analyzed for mineralogical analysis by X-ray diffraction. For geochemical element analysis it was used an HF/HN03/HCl04 pressure digestion and ICP-MS analysis method. Mercury was determined by cold vapor spectrometry. Stotal was determined by coulometric titration and S04 was determined turbidometrically. The difference between the two gave the sulphide sulfur content

RESULTS

Sampling observation in the field showed a zonation inverse known in humid climatic zones, in which the oxidation zone lies above an accumulation zone and a basal primary zone. In the studied tailings the stratigraphic column changes from a homogenous primary zone of dark gray greenish color at the top, to an inhomogenous zone with intercalations of oxidized layers of ochre to reddish brown color, to a very homogenous oxidation zone at 5-8 metre depth of reddish brown color. The pH changes from 7-8 to 4 and the grain size from fine sandy to clayey between the primary and the oxidation zone.

These zones are directly related to the water level in the tailings (Fig2).

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Fig.2: The figure shows a profile of the mineral distribution from for drillings, indicating the stratigraphic zonation and the minerals, which show differences in the distribution. It shows that primary sulphides are only stable in the first 2-4 metres over the water level. In the inhomogenous zone the stability of calcite goes down in favor of gypsum; and jarosite appears as a typical secondary mineral of the pyrite oxidation. In the oxidation zone there is no more calcite stable. Gypsum and jarosite dominate. The zonation is directly associated to the water level.

CLEAN 1ECHNOLOGY FOR TIIE MINING INDUSTRY

The X-ray diffraction analysis confirmed the field observations, that sulphides such as pyrite are only present in the primary zone at the top of the stratgraphic column; whereas jarosite as a typical secondary mineral of the pyrite oxidation (NORDSTROM 1982) is only present in the oxidation zone.

The stability of calcite and gypsum show the change from primary calcite in the primary zone to the secondary product gypsum in the oxidation zone (Fig.2). Minerals which are present in all zones are:

quartz, albite, kalifeldspate, hematite, magnetite, clinoclor, amphibole, sericite, kaolonite and montmorillonite. Minerals which are only stable in the primary zone, are: edenite, ferro-pargasite, pyroxene, hedenbergite, grossular, hydrogrossular and pyrite. It was not possible to detect the hydroxides of iron by X-ray diffraction, because of their low crystallization degree, but their presence was visible in lhe field observation because of their ochre, reddish-brown color.

The results of the geochemical element analysis show a distribution which also indicates the influence of pyrite oxidation. The iron content show decreasing contents in the inhomogeneous zone, while in the oxidation zone a homogene consolidation on a lower level than in the primary zone is visible (Fig.3). Sulphide sulfur shows a significant decrease from the primary zone to the oxidation zone. At the top of the oxidation zone the content shows a peak before decreasing to zero in the oxidation zone.

The sulphate content shows an increasing tendency to the oxidation zone (Fig.3).

The heavy metals (Cr, Pb, Cu, Cd, Zn, As, Ag, Ba, Hg) show a similar bebavior (Fig.4) They show relatively constant levels in the primary and oxidation zone, while a peak on the top of the oxidation zone show an accumulation in this area. Some metals (Pb, As, Ag, Ba, Zn) are accumulated in the oxidation zone on higher level than in the primary zone. The peak near the input (S8) by seepage is much higher than in some distance (S7).

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on a lower level than in the primary zone is visible. Sulphide sulfur shows a significant decreasing from the primary zone to the oxidation zone. At the top of the oxidation zone the content shows a peak before decreasing to zero in the oxidation zone. The sulphate content shows an increasing tendency to the oxidation zone.

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Fig.4: The diagrams show some representative distribution of the heavy metals Pb, Zn and As in two cores. They show relatively constant levels in the primary and oxidation zone, while a peak on the top of the oxidation zone show an accumulation in this area. Some metals (Pb, As, Ag, Ba, Zn) are accumulated in the oxidation zone on higher level than in the primary zone. The peak near the input (S8) by seepage is much higher than in some distance (S7). This distribution show all heavy metals in a similar way.

CLEAN 'IECHNOLOGY FOR THE MINING INDUSTRY

DISCUSSION

The investigated tailings are situated in special environmental conditions:

- arid climate -> very low precipitation (4 mm/a) -> high evaporation

- man controlled input of water (4000 m3/month) - the output is not detectable

- the depositions history of the tailings is not to reconstruct, so that horizons can be a product of deposiling circumstances.

- only the upper 8 m of the tailing could be sampled (tailings depth 30 m)

The zonation already visible during sampling may have different reasons: 1. The zonation is a result of deposition of different material on the tailing or 2. The zonation is a result of geochemical processes associated with the water budget of the tailing. The presence of macroscopic pyrite only in the uppermost zone and the change from fine sandy to clayey reddish brown horizon could be the result of deposited material change. The reddish, yellowish to brown color indicates the presence of iron oxides and hydroxides in the horizons and the basal zone. The decreasing grain size may be a product of precipitation ofFe(III)hydroxides. Change of pH from 7-8 in primary zone to 4 in the oxidation zone is the principal indicator of field observation for changing of the geochemical situation. The zonation is direcUy associated with the water level (Fig.2), so that the stratigraphic zonalion is interpreted as an inverse oxidation zone known in humid climates.

This interpretation is confirmed by the X-ray diffraction results. They show clearly that pyrite and some other minerals are stable only in the uppermost primary zone. If those minerals get come into contact with man controlled water input in the unsaturated inhomogeneous zone, pyrite starts oxidizing by forming sulphate and acid. The Fe2+ ions in solution became oxidized by Thiobacilli and / or oxygen to Fe3+ which precipitate as Fe(III)hydroxides, amorphous or as Ferrillydrite or Goethite (NORDSTROM 1982). The secondary products of iron oxidation make detection by x-ray diffraction a problem, but field observation indicates their presence. The sulphate is buffered by calcite changing to the secondary gypsum. The presence of Jarosile indicates high sulphate contents and low pH (NORDSTROM 1982). So that the mineralogical results clearly indicate that the zonation is a producl of pyrite oxidation. In this case producing an inverse column, because of the absence of natural precipitation and presence of the man controlled input of additional water.

Element analysis indicates a strong mobilization of all elements. Fctotal, Ssulphide and S04, the main parameters of pyrite oxidation show the typical distributions. Iron, is in the inhomogeneous zone a strongly leached, while the values in the oxidation zone are very stable. The leaching in the zone between primary and oxidation zone indicates, that this is the zone in which the water flow takes place.

The homogene values of iron in the oxidation zone are interpreted as the result of an precipitation equilibrium of the Fe(IIl) hydroxides.

Sulphide sulfur distribution is conform to the pyrite oxidation. From high values in the primary zone, a strong decrease in the inhomogeneous zone is observable as a result of pyrite oxidation. On the top of the oxidation zone a peak of sulphide sulfur indicates perhaps secondary sulphide precipitation. In the oxidation zone no others sulphides are stable. The distribution of sulphate is opposite to sulphide distribution. A constant increase in the oxidation zone is to remark, which reflects the change of calcite / gypsum equilibrium.

Heavy metal distribution is marked generally by a accumulation peak on Che top of Che oxidation zone and homogene distribution in the primary and oxidation zone. This accumulation is in relation to sorption and precipitation's processes. The adsorption of heavy metals on amorphous iron oxide is a function of time. So it is possible to differentiate between a fast adsorption at the surface ("unspecified adsorption") and a slow migration of metal ions into the crystal system ("specified adsorption") (GERTH & BROMMER 1981 &1983, HSIA et al. 1992). The adsorption at iron hydroxides is also a function of pH which is not totally reversible (DZOMBAK & MOREL 1990). The distribution of environmentally unhealthy heavy metals shows an accumulation at the uppermost parts of the oxidation zone, as a result of "unspecified" adsorption (surface) on the Fe(III)hydroxides and/ or of sulphide precipitation. Below, in the homogenous oxidation zone, heavy metal distribution is also homogenous and the metal contents are much higher than in the primary zone, which indicates a

"specified" adsorption (incorporation in the crystal system, stable at low pH) and a geochemical equilibrium.

CONCLUSION

The only source of water in the tailing is the input at the hillside above the tailing. The flotation mud leaves the process with a pH of 11. Once deposited it is in contact with atmospheric oxygen and the pyrite oxidation begins (While still in aqueous solution the solute oxygen content limits the oxidation rate). On the seepage from the hillside to the tailing the acid and sulphate rich solutions leaches heavy metals from the primary material. This metal rich solution, by reaching Che tailings, get buffered by calcite and H+ adsorption at clay minerals, so that Fe(III)hydroxides precipitate. By precipitation ofFe(Ill)hydroxides heavy melals became adsorbed and/ or precipitate as secondary sulphides. The precipitated Fe(III)hydroxides are in the pore spaces and so produce an water impermeable horizon, so that water flow takes place on the top of the oxidation zone. This way the oxidation zone migrates upwards.

The reactant and medium of transport - the water - plays an important part in arid conditions, and is the controlling factor in pyrite oxidation. The results of this study shows, that especially in extremely arid regions the man controlled water input in abandoned tailing has major effects in the pyrite oxidation and the associated processes.

Nevertheless, the fact that in the oxidation zone the heavy metal content is much higher than in the primary zone, shows that the secondary precipitation products of pyrite oxidation are able to fix additional contents of heavy metals, stable at low pH. For the exact determination of sorption capacity more data are necessary. But it could be a possibility for tailings treatment not to provide pyrite oxidation but to accelerate this geochemical processes and to control the secondary fixing of heavy metals.

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CLEAN 'IECHNOLOGY FOR THE MINING INDUSTRY

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