EVALUACION DEL SISTEMA EXISTENTE DE AGUA POTABLE DE LA CIUDAD DE

3.3.1

Introduction

The samples were packaged and transported to Massey University in New Zealand for total gold analysis and microscope imaging. Prior to analysis, samples were prepared by initial air-drying at room temperature, homogenisation using a mortar and pestle, with subsequent sieving of the homogenised samples into six size fractions:

1. > 1 mm, 2. > 0.5 – 1 mm, 3. > 0.25-0.5 mm, 4. > 0.15-0.25 mm, 5. > 0.075-0.15 mm 6. < 0.075-0.075 mm

The two main geochemical techniques used to analyse the various grain sizes included Au concentration quantification using GFAAS analyst 600 (Perkin Elmer Precisley) for the whole and sieved fraction samples, and the use of FEI Quanta 200 scanning electron microscope with EDAX module to investigate mineralogy, elemental composition, gold grain size and distribution. The graphite furnace spectrometer is located in the analytical chemistry laboratory at the School of Agriculture and Environment, Massey University, Manawatu campus whilst the SEM is under the auspice of the Manawatu Microscopy Imaging Centre (MMIC).

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Samples from the first trip were all collected from one mine site and were thus, assayed individually. The homogenised and sieved samples from the eight mine sites visited in the second field trip comprised various ore types and were composited in order to attain a representative ore blend grade for each mine site. Time and resource limitations precluded analysis of each ore sample individually as there were 80 samples in total from the second field trip. The compositing of samples was achieved through subsampling 10 g of each sample to form a 100 g representative mixture of each mine site.

3.3.2

Sample Preparation for Total Gold Analysis

The preparation of samples for analysis of total gold concentration by GFAAS followed the method of Anderson, Moreno, and Meech (2005), and involved the sequence of sample digestion in aqua regia (3:1 hydrochloric acid to nitric acid), extraction of gold into methyl isobutyl ketone (MIBK), and then standardised quantification of the gold concentration in solution.

A subsample (1 g) of each sample was weighed in triplicate into 50 mL borosilicate digestion tubes. Aqua regia solution was then prepared using industrial standard hydrochloric acid (HCl) and nitric acid (HNO3) at a 3:1 ratio. Freshly prepared aqua

regia (10 mL) solution was added to each subsample and the preparations allowed to pre-digest overnight for approximately 16 hours. The following morning, each sample was digested on a heating block for 6-8 hours with the temperature progressively increased from 50 to 150°C to reduce the gold acid solution to a minimal amount (avoiding complete evaporation and sample drying).

During pre-digestion, the nitrate ion from the nitric acid oxidizes the gold metal to Au3+, which then combines with the Cl-released from the HCl acid to form stable AuCl4- ions

(C. Anderson, personal communication, 11th August 2015). Excess nitric acid is evaporated during heat digestion whilst the gold-chloride residue is left behind along with solid silicates. Each sample was then diluted to 50 mL using deionized water. The samples were then mixed on the vortex mixer and subsequently filtered into P35 (35 mL) containers using the size 42 Whitman filter papers to remove the silicates. A 5 mL aliquot of the filtered solution was next extracted into P10 (10 mL) containers using a calibrated pipette. The organic solvent MIBK (2 mL) was added to the filtered solution and shaken in order to extract gold into the organic phase. This organic fraction was then extracted into smaller cups for gold analysis on the GFAAS.

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3.3.3

Graphite Furnace Atomic Absorption Spectrometer for Au Assay

The GFAAS‟ limit of detection for soluble gold is 0.01 ppm. Five (5) standard gold solution samples (with Au concentrations of 0.2, 0.4, 0.6, 0.8 and 1 ppm) were prepared from the 1000 mg L-1 standard stock solution (Merck CertiPUR) along with a blank solution (0 ppm Au) to produce linear (absorbance vs concentration)calibration curves for the spectrophotometer. A quality assurance and quality control (QAQC) method using the standard reference material (SRM) PTM 1a (3.3 ppm Au) and blanks (0 ppm Au) was implemented to verify the analysis (Appendix 2). Results from analysis of the SRM were acceptable when compared to previous analysis of this material at Massey University. The gold assay results of the samples are given in ppm or ppb by the

GFAAS software and are presented in mg/kg and ppm interchangeably in this thesis. In conventional mining, Au concentration is presented in g/t.

3.3.4

Sample preparation for Microscopic Analysis

A scanning electron microscope was used in the microscopic imaging and elemental analysis of selected samples. Sample preparation for SEM analysis involvedmounting the sieved fractions on a sticky carbon pad covering a metallic specimen stub and

coating these with a thin layer of carbon. The samples were thus prepared as unpolished grain mounts and not as polished sections. The photomicrographs and EDAX elemental analysis were both acquired at an accelerating voltage of 20 kV using a beam current of 5 nA in a high vacuum mode at a working distance of 10.2 mm.

3.3.5

Scanning Electron Microscopy

The SEM is a powerful instrument used for studying materials by generating high- resolution microscopic imagery and elemental spectra. The SEM at Massey University is a FEI Quanta 200 scanning electron microscope with energy dispersive x-ray (EDAX) spectroscopy for elemental analysis. Its use was essential in the identification of gold, gold morphology, gold grain size, ore and gangue mineralogy, and associated elements of the mine samples. SEM elemental analysis is qualitative and semi-quantitative as opposed to the electron microprobe, which attains accurate and precise elemental data. The size distributions of the gold particles from Wau were determined in two ways: (1) by using the measuring tool in the SEM software and (2) by measuring with a ruler

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using the scale from the saved SEM photomicrographs. Approximately 130 gold grains were measured along their long and short axes in the grain mounts and statistically analysed to understand the predominant gold grain size range. The size of gold grains are presented in this thesis as either mm or µm.

The SEM produces two images: the backscatter electron (BSE) image and the secondary electron (SE) image (N. Minards, personal communication, May 2017). It does this by scanning a high-energy electron beam on a sample, which results in three types of responses: (1) Backscatter electrons reflected from the surface of the sample, (2) Secondary electrons and (3) X-rays emitted from the sample as the electron beam penetrates its surface to a few microns. The SEM contains three main detectors to sense these responses and converts them accordingly to electron imagery and x-ray elemental spectra. The Everhart-Thornley detector (ETD) detects the secondary electron imagery in conventional high vacuum mode. It is ideal for producing images with excellent definition of the surface topography and morphology. The backscatter detector (BSD) senses backscatter electrons and produces a basic image of dark grey to bright white colours depending on the elemental composition of the material. Non-metallic elements (with lower atomic numbers) produce a dull grey colour whilst metallic elements with higher atomic numbers produce a brighter white colour. Gold has a distinctive bright white appearance in backscatter mode. As a result, the backscatter image is useful for differentiating between metals and non-metals. For x-ray detection, the energy EDAX spectrometer detects the characteristic x-rays emitted by specific elements and produces a spectrum depending on the elemental composition and the abundance of each element. The interaction between the electron beam and the sample results in the removal of an inner shell electron. This causes a higher energy electron to fill in the vacancy, thus, releasing characteristic x-ray energy.

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4

Chapter Four

GEOLOGY, MINING & ECONOMIC VALUE CHAINS

SYSTEM ANALYSIS

4.1 Introduction

This chapter presents the field research data in the following structure: a) Mine site geology of the selected ASGM sites,

b) Mining techniques, mercury use and waste disposal in these mines, and economic value chains system analysis.