METHODS

1.3.1 Sampling and field methods

The field work was realized in collaboration with the "Instituto de Investigaciones Cientificas y Technologicas" (IDICTEC) University of Atacama, Copiap6, the Geological Department of the University of Chile, Santiago, and the "Servicio Nacional de Geologfa y Minerfa" (SERNAGEOMIN), Santiago, Chile, which have collaborated with logistic support during the field work in 1996 and 1997. Percussion drilling, which was used to reach depths of 10 rn and surface sampling were applied to obtain solid samples. Water samples were collected from effluents.

Chapter 1: introduction

More lhan 370 samples were collected from 6 tailings impoundments from the mines La Andina, El Teniente, and El Salvador, and from the impoundments of the flotation plants Ojancos and P. Cerda which treated ore from the Punta del Cobre belt.

Samples and geochemical data from a previously conducted study of the Ojancos tailings impoundment No. 2 (Dold, 1994; Dold et al., 1996) are also included.

The solid samples were sealed in plastic bags and stored in an ice-packed cool box.

Previously, the description of mineralogical characteristics, color and grain size estimation, and pH measurement was noted (paste pH according to MEND, 1991 using a WTW® pH-meter; in the 1997 field campaign a pH-electrode which was originally developed for quality control in meat, was successfuliy used for in situ pH-measurement in the moist tailings sediment). The samples were transported immediately to local mine laboratories for drying (< 35oC) and water content determination. The dry samples were homogenized and packed into polyethylene (PET) containers for storage at room temperature.

Water samples were filtered with 0.45 J.lm sodium acetate filters. Temperature, conductivity, pH, and dissolved oxygen were measured in the field. Every water sample was separated into two aliquots, one untreated for anion analysis, and the other conserved at pH 2 (obtained by addition of suprapure HN03) for cation analysis. The water samples were refrigerated until analysis.

1.3.2 Analytical methods

The wt.% of moisture of ali tailings samples was measured using sample weight be fore and after drying in the mine laboratories. The particle size distribution of selected samples were measured by a Coulter® and a Fritsch Analysette® laser particle size analyzer at the Institute F.-A. Forel, University of Geneva and the Mineralogical and Petrography Department of the University of Lausanne, respectively.

Polished and polished thin sections were prepared from bulk samples and undisturbed sediment samples to study the ore mineralogy. Ali samples were analyzed as bulk sample by X-ray diffraction (XRD), using a Philips® 3020 diffractometer at the Mineralogical Department of the University of Geneva. Oriented samples of the fraction < 2J.Lm for clay minerais determination were analyzed by a Rigaku® Rotaflex diffractometer at the Mineralogical and Petrography Department of the University of Lausanne. The poorly crystalline Fe(III) hydroxides such as ferrihydrite and schwertmannite were detected by differentiai X-ray diffraction (DXRD). Trace elements associated with schwertmannite and biotite were studied by a CAMECA SX50 electron microprobe at the University of Lausanne. The mineral morphology and the qualitative element composition of acid mine drainage precipitates was studied by a scanning electron microscope (SEM-EDS) JOEL® JSM 6400 at the Geological Department, University of Geneva.

The dissolution kinetics of schwertmannite and ferrihydrite in 0.2M NH4-oxalate, pH 3, under exclusion of light, were performed in the "Centre d'Analyse Minérale" (CAM), University of Lausanne. The extracted solutions were analyzed for Fe and S by ICP-AES Perkin-Elmer Plasma-2000 at the Soil Science Institute, EPFL, Lausanne.

Chapter 1: Introduction

The sequential extractions with multi-element analyses by ICP-ES were performed by the X-Ray Assay Laboratories (XRAL) of Toronto, Canada. Total sulfur necessary for acid base accounting was measured, using a Leco® furnace. For measurement of the sulfate sulfur the 0.2 M oxalic acid hot 2h leach (step 4 in sequence B, chapter 3) was used. Sulfur was determined by ICP-ES, performed at XRAL. Total and mineral carbon has been analyzed by coulometric titration (Strohlein CS 702®) at the Centre d'Analyse Minérale, University of Lausanne.

For the microbiological study a separate aliquot of each sample was taken and maintained at a temperature of 5°C untreated in ice-packed coolers. Ten samples were selected and delivered 1996 shortly after sampling to the laboratory of the biometallurgical group of the Chemical Engineering Department of the University of Chile, Santiago de Chile. The samples were analyzed for total number of cells (direct microscopie counting; Phyroff-Hauser counting chamber), Thiobacillus ferrooxidans cultivation (plate counting), and oxidizing activity (Fe(II) oxidation rate).

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CHAPTER2

2 Basic concepts in environmental geochemistry of sulfide mine-waste

2.1 Mining and the environment

As minerais, which are essential to industrial economies, are presently not in short supply, nor do they seem to be for the next severa! generations, mining and mineral processing can no longer be presumed to be the best of all possible uses for land; it must compete with compelling demands for alternative uses. Environmental protection and rehabilitation are fast becoming high priorities throughout the world, no longer confined to industrialized countries.

Environmental regulations in the developed countries are one of the main reasons for the departure of metal mining companies to less developed nations in the last few decades. Low labor costs, exploration potential, and lax or no existing environmental policies, reinforced this process (Hodges, 1995). While industrialized countries started to formulate environmental reports and to implement environmental framework laws in the 1970's (e.g., USA, Central Europe, Japan), developing countries started this process only recently in the 1990's (e.g., Chile, Peru, Korea, Nigeria), as reported in Janicke and Weidner (1997). A main task for the future will be to build a body of local experts in these countries, which will be able to implement the environmentallaws. Increasing world population together with economie growth in developing countries will increase the demand for minerais in the near future and the associated environmental assessment.