Effect of convectional and locally sourced reagents on the floatation of iperindo gold bearing ore

Masterarbeit , 2021 , 99 Seiten , Note: 4.5

Geowissenschaften / Geographie - Geologie, Mineralogie, Bodenkunde

This research is aimed at studying the amenability of lperindo lode gold deposit to froth flotation using convectional and locally-sourced frother and collector, leading to the comparison of their recovery efficiency. The high demand for metals have resulted in the need for large throughput plants to deliver increased productivity, increased utilization and reduced operational cost. The 315 Exploration Licenses issued in 2016, in respect of the priority minerals identified by Nigeria government, gold top the list. It has been observed that the gross domestic products of many countries of the world are directly related to the quantum of metal products produced or consumed by those nations and Nigeria deserves to be among gold producing nations of the world with enhanced economy because Nigeria economy cannot longer rely on petroleum. However, Nigeria will benefit immensely from the mining and processing of its gold deposits because Iperindo lode gold ore, being one of Nigeria’s gold deposits can affect our economy positively if value is added to it by way of mining and processing it efficiently.

The scope of this research encompasses sourcing for Iperindo lode gold ore from its deposit, chemical characterization of the sourced gold ore by using atomic absorption spectrometry (AAS), ascertaining the mineralogical assemblage and elemental composition of the gold ore using X-ray diffractometer (XRD) and X-ray fluorescence (XRF) while the optical microscopy analysis using scanning electron microscope (SEM/EDX). Comminution and particle size analysis of the gold ore was carried out using the following sieve sizes: +500, -500+355, -355+250, -250+180, -180+125, -125+90, -90+63, -63+45 and -45 µm. Froth flotation process using the locally sourced frother and collector follow by the characterization of the concentrate and tailing to determine the grade of gold recovered at each stages and finally, evaluation of the results.

TABLE OF CONTENTS

Content Page

Title Page

Acknowledgements

Abstract

List of Tables

List of Figures

List of Appendices

CHAPTER ONE
1 INTRODUCTION
1.1 Orientation of the Proposed Study
1.1.1 World production of gold
1.1.2 Properties of gold
1.1.3 Uses of gold
1.2 Statement of Research Problem
1.3 Aim of Research
1.4 Objective of the Research
1.5 Justification
1.6 Scope of the Research

CHAPTER TWO
2 LITERATURE REVIEW
2.1 Geology of Gold Ore Deposits
2.1.1 Africa's gold deposits
2.1.2 Nigeria's gold deposits
2.2 Mineralogy of gold ore
2.2.1 Atomic absorption spectrometry (AAS)
2.2.2 X-ray fluorescence spectroscopy (XRF)
2.2.3 X-ray diffraction (XRD)
2.2.4 Electron microscopy
2.2.5 Scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDS)
2.2.6 Transmission electron microscopy (TEM)
2.2.7 Scanning-transmission electron microscopy (STEM)
2.2.8 Ore microscopy
2.3 Mining
2.4 Mineral Processing
2.4.1 Comminution
2.4.2 Particle size analysis
2.5 Concentration Techniques
2.5.1 Gravity concentration
2.5.2 Froth flotation
2.6 Flotation Reagents
2.6.1 Collectors
2.6.2 Oxyhydrly collectors
2.6.3 Sulfhydrly collectors
2.6.4 Locally-Sourced Reagents
2.6.5 Convectional Reagents
2.7 Frothers
2.7.1 Groundnut oil
2.7.2 Methyl iso-butyl carbinol
2.8 Modifier

CHAPTER THREE
3 MATERIALS AND METHOD
3.1 Materials
3.2 Method
3.2.1 Sample collection and preparation
3.2.2 Chemical analysis of Iperindo lode gold ore sample
3.2.3 Mineralogical analysis of Iperindo lode gold ore
3.2.4 Preparation of collector
3.2.5 Froth flotation experiment for locally-sourced reagent
3.2.6 Froth flotation experiment for convectional reagent

CHAPTER FOUR
4 RESULTS AND DISCUSSION
4.1 Chemical Analysis of Iperindo Lode Gold Ore
4.2 Mineralogical Analysis of Iperindo Lode Ore
4.4 The Effect of Pulp Density on the Recovery of Gold Concentrate
4.5 The Effect of Collector Concentration on the Recovery of Iperindo Lode Gold Concentrate
4.6 The Effect of Pulp pH on the Recovery of Iperindo Lode Gold Concentrate
4.7 The Effect of Impeller Speed on the Recovery of Iperindo Lode Gold Concentrate
4.8 The Optimum Recovery of Gold from Iperindo Lode Deposit at Vary Pulp Density Concentration of Collector, Pulp pH and Application of Impeller Speed

CHAPTER FIVE
5 CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion
5.2 Recommendation
5.3 Contribution to Knowledge

REFERENCES

APPENDICES

ACKNOWLEDGEMENTS

I give all the glory to God who through thick and thin gave me a safe landing all through my stay in the university. Indeed, He has been my helper, sustainer, energizer, shepherded; always guiding in the path of profit. A big appreciation goes to my supervisors, Prof. Ajayi J.A and Dr. Alabi O.O. It's a privilege to be supervised by men of deep knowledge. I sincerely appreciate the lessons I learnt from them, hard work and right attitude to whatever you find yourself doing. I appreciate the Head of Department of Metallurgical and Materials Engineering, Prof. A.O Oyetuji, you are a father indeed. My sincere appreciation goes to Dr. Oladele, Dr. Daramola, Prof. Adewuyi, Dr. Bodunrin, and Mr. Akin for their contributions during my project. I acknowledge the immense contribution of Dr. Mrs Ola-Omole, Mrs. Akinlo and Marindoti Ayobami for their support and encouragement and several authors and researchers from whose works I have extracted information or quoted, and several other, I owe you all my immense debt of gratitude. I say a big thank you to my brothers, Dr. David Adetula, Richard Adetula and my lovely wife; Engr. Damilola Yomi-Adetula for their invaluable moral and financial support all through to this stage. Sure you will receive your reward. I also appreciate my highly beloved son; Joshua Yomi-Adetula, my pastor and friends for their diverse contributions. I owe you all my love and gratitude.

DEDICATION

This thesis is dedicated to Almighty God, our Lord and Creator.

ABSTRACT

The effect of convectional and locally-sourced frother and collector on floatability of Iperindo lode gold ore was investigated. Samples of the mineral were sourced from Iperindo deposit regions in Ilesha Local Government Area, Osun State, Nigeria. The sourced samples were crushed, homogenized, and 20 kg was weighed for further research. Chemical and mineralogical characterization was carried out, followed by fractional sieve analysis of the crude sample sieve range of 500 to -45 u rn to determine particle size distribution and liberation size of the ore. Three kilogram of the crude was randomly sampled out and pulverized to 100 % passing -63+45 u rn. Amenability of the mineral was assessed via froth flotation using convectional (xanthate and methyl iso-butyl carbinol) and locally-sourced (potassium salt of groundnut oil and groundnut oil) as flotation reagents. Flotation parameter used were pH value of 8.0, 8.5, 9.0, 9.5 and 10.0, at a pulp density of 50, 100, 150, 200 and 250 g/cm 3, collector concentrations of 0.2, 0.3, 0.4, 0.5 and 0.6 g/dm 3 and agitation speed of 1200, 1250, 1300, 1350 and 1400 rpm respectively. Composition analysis of the crude sample via Atomic Absorption Spectrometer (AAS) and Energy Dispersive X- ray Fluorescence Spectrometry (ED-XRF) revealed that the crude sample contain 4.10 ppm Au while 32.08 % Si, 21.28 % Fe, 36.07 % Ba and 2.21 % S are the major elemental constituents of the ore matrix reveal by the EDS analysis. Mineralogical analysis revealed the mineral phases present in the crude sample are Quartz (73% SiO 2), Dolomite (19% CaMg(CO 3) 2), Annite (13% KFe 3 2 AlSi 3 O 1 (OH) 2) with other associated mineral such as Sylrite (12% KCl). SEM imaging revealed an embedded gold grain in the quartz and spotted interlocked in feldspar within a microscopic size level while a fracture type of surface morphology was observed in quartz-feldspar veins texture. Chemical analysis of the concentrate samples after froth flotation, further revealed that the crude assaying 103 ppm at -63+45 u rn has been successfully upgraded to 630 ppm Au at a recovery of 97.9 % via a pulp density of 50 g/cm 3, concentration of 0.6 g/dm 3, pH of 9 and agitation speed of 1350 rpm when potassium salts of groundnut oil was used as collector as against the use of potassium amyl xanthate (convectional reagent) as collector. Hence, this is the optimum froth flotation condition suitable for the recovery of gold from Iperindo lode ore.

LIST OF TABLES

Table 1.1: Gold ore types and gold occurrences

Table 3.1: Equipment/facilities and location

Table 4.1: Elemental analysis of iperindo lode gold ore using AAS

LIST OF FIGURES

Figure 1.1: Geological Map of Nigeria showing the Major Areas of Gold Mineralization and Location of the Proposed Study Area

Figure 1.2: Geological Map of Nigeria showing the Proposed Study Area

Figure 2.1: Classification of collectors

Figure 2.2: Chemcal structure of hydrogenated groundnut oil

Figure 2.3: Chemical structure of Methyl Iso-bulty Carbinol

Figure 3.1: Processing flow sheet for Iperindo lode gold deposit at Ilesha

Figure 3.2: Process flow-sheet of the froth flotation process of Iperindo lode ore

Figure 4.1: Chemical analysis of Iperindo lode gold ore using ED-XRF

Figure 4.2: Photomicrograph of the ore sample as view by an Optical microscope

Figure 4.3: Diffractograph of Iperindo lode ore viewed by an X-ray Diffractometry

Figure 4.4: SEM micrograph of Iperindo lode gold ore (a)

Figure 4.4: SEM micrograph of Iperindo lode gold ore (b)

Figure 4.4: SEM micrograph of Iperindo lode gold ore (c)

Figure 4.5: Graph of the chemical analysis using AAS (a)

Figure 4.5: Graph of sieve size against cumulative % retained and passing (b)

Figure 4.6: Ilustrate the effect of pulp density variation on gold concentrate recovery and grade

Figure 4.7: Ilustrate the effect of collector concentration variation on gold concentrate recovery and grade

Figure 4.8: Ilustrate the effect of pulp pH variation on gold concentrate recovery and grade

Figure 4.9: Ilustrate the effect of pulp Ph variation on gold concentrate recovery and grade

Figure 5: Froth flotation parameters for the optimum recovery of Iperindo lode gold concentration using pottasuim salt

LIST OF APPENDICES

APPENDIX I

Plate 1: Sample of Iperindo lode gold ore

Plate 2: Sample of crushed gold ore with sledge hammer

Plate 3: Pulverized gold ore sample

Plate 4: Use of Ball mill to ground the gold ore sample

Plate 5: Sample of ground gold ore in pan

Plate 6: Use of Sieve for particle size analysis

Plate 7: Reagent used for froth flotation

Plate 8: Froth flotation cell used for floating the gold ore

Plate 9: Stable froth for local-sourced reagent

Plate 10: Stable froth for convectional reagent

Plate 11: Oven used in drying concentrate and tailings

Plate 12: Sample of dried concentrate and tailing for analysis

APPENDIX II

Table 1: Particle size analysis of Iperindo gold ore fed to the Ball mill

Table 2: Particle size analysis of Iperindo lode gold ore product from the Ball mill

Table 3: Result of compositional analysis of the particles size fraction of Iperindo

Lode gold using AAS

APPENDIX III

Result of recovery of gold concentrate using convectional reagent

Result of recovery of gold using locally-sourced reagent

CHAPTER ONE

1 INTRODUCTION

1.1 Orientation of the Proposed Study

The solid minerals industry is one of the strongholds that contribute to any nation's gross domestic product (Alabi et al., 2016). It constitutes a wide range of natural resources like galena, gold, sphalerite, columbite, tantalite, among others that provide bulk of raw materials for the industry (Ajayi, 2004). Industrial raw materials are either mining or agricultural products; the former is depletable while the latter is replenishable. It behooves any nation therefore, to judiciously explore, exploit, process, extract and utilize her solid mineral resources. The sector has great potentials in contributing greatly to the economy and it can stand as a source of job creation and by this, eradicate poverty (Olumide et al., 2013).

Nigeria is highly mineralized, she is relatively backward technologically and industrially. This is partially attributed to the fact that value is hardly added to the nation's mineral resources. The mining sector in Nigeria is currently dominated by small-scale enterprises, divided into artisanal operations that rely on manual labour and simple tools; and small scale operations with high volume outputs and small degrees of mechanization. In most cases, mining operations are not only exploited below their full potentials, they are uneconomic, toxic, hazardous and environmentally unsafe (Ajayi and Awe, 2010).

In Nigeria, solid mineral occurrences are classified into five mining leases classes A - E: Class A (metalliferous base and alluvial minerals such as cassiterite and gold ores), class B (Base and precious lode minerals such as azurite, malachite, sphalerite, gold and silver lode ores), class C (mineral fuels such as coal and bitumen), class D (gemstone like emerald, ruby and diamond) and class E (industrial/non-metal minerals such as granite, clay, shale, limestone and gypsum (Anon,2015). However, there are occurrences of gold deposit mostly mined and processed by artisanal/illegal miners in Nigeria, yet there is no official documentation of Nigeria among comity of gold producing nations of the world. There are occurrences of gold deposits in various parts of Nigeria which are predominant found in three regions which, for convenience, classified as Zamfara-Malenjo in Zamafara state, Lapai- Birnin Gwari in Kaduna state and Ife-Ilesa area in Osun state. Gold occurs in two forms: As disseminated gold in quartz veins occurring in quartz gneiss in Iperindo and alluvial gold associated with amphibolite complex (Anon, 1981; Fletcher and Clarke, 1996).

Gold concentrates are processed by chemical methods in order to recover up to > 99% gold, depending on type and efficiency of the processes (Manning and Kappes, 2016). In order to develop Ife-Ilesha gold field, there have been research work on extraction methods such as percolation leaching and amalgamation of the placer gold deposits at Igun (Ajayi, 2004). The conventional methods of cyanidation and amalgamation are cost effective but environmentally unfriendly (Oluwabunmi, et al 2014). The health and well-being of miners and nearby communities is often adversely affected (WHO, 2016) due to the chemical exposure in gold mining when mercury is used to amalgamate the gold and cyanide used to extract gold, (WHO, 2013).

Most of the previous works from different authors focused on the process of design for Ilesa placer gold deposit, either by gravity concentration, amalgamation and cyanidation (Adekola et al., 1999). However, Cyanide leaching has been used to leach gold from ores for more than a century because of its simple process, high efficiency and low cost (Zhonglin et al, 2017). Earlier researchers have reported studies on the location, mineralogical and processing of gold and currently, prospecting and exploration activities for the mineral are ongoing. Figure 1 shows the geological map of Ilesha schist belt, in southwestern Nigeria.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1.1: Geological Map of Nigeria showing the Major Areas of Gold Mineralization and Location of the Proposed Study Area (Kankara and Darma, 2016)

Iperindo lode gold is one of the few primary gold deposits known in Nigeria. The mineralized lodes generally comprise highly silicified fine-grained foliated biotite gneiss typically intruded by both discordant and concordant pegmatitic quartz-feldspar veins. The veins are mainly as discrete particles, up to 100 pm in size, at grain boundaries between quartz and carbonates and the most prominent and dominant sulphide in Iperindo lode gold deposit is pyrite (Oyinloye and steed, 1996). Similarly, the lode gold deposit lies in amphibolite-facies biotite granite-gneisses of Proterozoic age in the Ilesha schist belt and some 4 km to the east of a major crustal 'break' known locally as the Ifewara-Zungeru fault (Oyinloye, 2006). The mineral resources deposit based on the estimated grades in the block model spatially constrained by geological and statistical parameters was 4,580,863 tonnes at a grade of 3.8 (Segiola, 2016). However, the appropriate technique to be adopted for processing a particular mineral deposit depends on the mineralogy of the deposit, grade of ore, nature and concentration of the impurities associated with the ore (Hagni, 1978).

Figure 1.2 show the map of Nigeria showing the proposed study area.

Abbildung in dieser Leseprobe nicht enthalten

1.1.1 World production of gold

Gold is a noble metal, an indispensable, social, political significance and non-substitutable strategic resource due to its broad applications in industries as well as national economy (Tong et al., 2013). In 2014, the world gold production increased from 2800 to 2860 metric tonnes, an increase of 2.1%. Gold is one of the rarest elements in the world, making up roughly 0.003 parts per million of the earth's crust. China is the number one producer of gold in the world in 2017, according to the GFMS Gold Survey 2018, extracting almost 131 tonnes more than second-place Australia. That's about 13% of global mine production. In 2017, global gold mine production was reported 3,247 tonnes. Australia is the world's second-largest producer of gold, Australia produces around 295.6 tonnes, followed by Russia 270.7 tonnes, and the United States 230.0 tonnes.

In 2018, China was the world top gold producer for the twelfth consecutive year since 2006 with production of 440 tonnes of gold. Similarly, Australia produced 300 tonnes of gold which is the second largest gold producer while Russia was the third largest gold producing country with 255 tonnes. In the same vein, United States of America produced 245 tonnes of gold to be ranked the world's forth producer of gold, South Africa was the seventh largest gold producer with gold production averaging 245 tonnes and Ghana ranked twelfth position with production of 90 tonnes of gold among others (USGS,2018). Other gold producing countries in Africa are Burkina Faso, Kenya, Zambia, Zimbabwe, Ethiopia, Liberia, Gabon, the Central Africa Republic, Burundi and Madagascar (Anon, 2015).

1.1.2 Properties of gold

Gold, a metallic chemical element, one of the transition elements with chemical symbol Au and a tomic number 79, is characterized by this physical properties: bright yellow colour, conductor of heat and electricity and is non-magnetic, it has a high malleability (one ounce can be drawn into a sheet of 9m 2), and very soft metal rating 2 on Mohs' scale. It has a melting point of 1,063 o C and a boiling point of 2160 o C. It is a very heavy metal having a specific gravity ranging 15.5 - 19.3 g/c m 3 depending on the associated minerals content and resistant to change but often confused with pyrite (fool's gold), chalcopyrite and weathered biotite.

Among the chemical characteristics, it is said that gold, in its natural form, is not attacked by either sulphur or oxygen in the presence of air or water but fuses easily to a malleable globule and is resistant to weathering, as it tends to accumulate in placer deposits. The unique role that g old plays in society is to a large extent related to the fact that it is the most noble of all metals: it is the least reactive metal towards atoms or molecules at the interface with a gas or a liquid. The inertness of gold does not reflect a general inability to form chemical bonds, however gold forms very stable alloys with many other metals. The metal dissolves in the presence of haloges or complexing agents such as cyanide solutions as well as oxidizing agents, such as aqua regia (a mixture of nitric and hydrochloric acids which liberates chlorine). Finally, gold has a good alloyability with other precious metals, such as silver and mercury to increase its strength (Bardaj^ and Laguna, 1999).

1.1.3 Uses of gold

Gold played a central role in metallurgy; it was used and valued by the ancient people who formed it into a variety of objects (Pricker, 1996). Gold is more useful than any of the minerals mined from the earth. Its usefulness is derived from a diversity of special properties which makes it useful in every sphere of modern life in some way, shape and form. The following are the common uses of gold in the world today.

i. Jewelry: Approximately 80% of annual gold mined is consumed for jewelry production. Jewelry is the most common way gold reaches consumers and has been a primary use for the metal in various cultures. Gold jewelry is an adornment that is both ethereal and cherished because of its beautiful and durable properties (Zhang et al., 2015).
ii. Finances and Investing: Early transactions were done using pieces of gold or pieces of silver. Because gold is highly valued and in very limited supply, it has long been used as a medium of exchange or money. The rarity, usefulness, and desirability of gold make it a substance of long­term value. Gold works well for this purpose because it has a high value, is durable, portable, and easily divisible (Glasner, 2017).
iii. Electronics and Computers: The most important industrial use of gold is in the manufacture of electronics. Solid-state electronic devices use very low voltages and currents which are easily interrupted by corrosion or tarnish at the contact points. Gold is a highly efficient conductor that is able to carry tiny electrical charges, this property makes it useful in almost all electronic devices, including cell phones, televisions, GPS units and more. Due to its efficient conductor of electrical charges, it is also often found in desktop and laptop computers to transfer information quickly (Lansaker et al., 2013).
iv. Dentistry and Medicine: Gold makes for the best fillings, crowns, bridges and orthodontic appliances because the metal is chemically inert, easy to insert and non-allergenic. Gold will probably continue to be the best option for replacing broken or missing teeth because of its superior performance and aesthetic appeal (Alexander et al., 1996).
v. Aerospace: Gold plays a very important role in the aerospace industry where reliable and effective technologies are key to survival, gold plays an essential role. Gold is used to lubricate mechanical parts, conduct electricity and coat the insides of space vehicles to protect people inside from infrared radiation and heat (Lino Alves et al., 2016).
vi. Medals and Awards: Gold makes a natural appearance in crowns, awards, and religious statues. It is used for academy awards and Olympic medals because of its beautiful appearance and rarity. Gold is recognized for its admirable qualities and it holds a permanent place of value in humanity's eyes (Fernie, 2013).

1.2 Statement of Problem

The preliminary mineralogical study carried out on Iperindo lode deposit showed that gold occurs in finely disseminated quartz veins (Anon,1981 and Oyinloye, 1996). This revealed that the ore is not amenable to gravity concentration whereas cyanidation and amalgamation method of processing gold are effective but environmentally unfriendly. This constraint leads to exploration of alternative method of Iperindo lode gold processing and one of the favored alternatives is froth flotation.

Froth flotation is a widely used mineral concentration technique which utilizes different surface properties of particles and their wetting behaviour. This technique has a great commercial benefits owing to its high separation efficiency, low costs and the process is potentially used for the recovery of fine gold as well as other tailings from gravity concentration and leaching. It is very effective in the processing of the material in the size range 850 to 100 p.m (Mitchell et al., 1997).

However, the challenge with froth floatation is the conventional floatation reagents, particularly collectors and frothers are generally scarce and can be expensive (Ajayi, 2004). Thus, there is the need to source for local froth floatation reagents to process lode gold, reducing production cost and assuring safe and environmentally friendly operations. In current economic environment, the mining industry is focused on reducing costs and improving productivity (Mark et al., 2016).

1.3 Aim

This research is aimed at studying the amenability of lperindo lode gold deposit to froth flotation using convectional and locally-sourced frother and collector, leading to the comparison of their recovery efficiency.

1.4 Objectives

The specific objectives are to;

(i) determine the chemical and mineralogical characteristics of Iperindo crude gold ore;
(ii) beneficiate the ore by froth flotation using convectional and locally-sourced reagent and;
(iii) compare the recovery efficiencies of gold by convectional and locally-sourced frother and collector.

1.5 Research Justification

The high demand for metals have resulted in the need for large throughput plants to deliver increased productivity, increased utilization and reduced operational cost (Tinashe, 2017). The 315 Exploration Licenses issued in 2016, in respect of the priority minerals identified by Nigeria government, gold top the list (NEITI, 2018). It has been observed that the gross domestic products of many countries of the world are directly related to the quantum of metal products produced or consumed by those nations and Nigeria deserves to be among gold producing nations of the world with enhanced economy because Nigeria economy cannot longer rely on petroleum (Ajayi, and Awe, 2010). However, Nigeria will benefit immensely from the mining and processing of its gold deposits because Iperindo lode gold ore, being one of Nigeria's gold deposits can affect our economy positively if value is added to it by way of mining and processing it efficiently. The economic impact of gold mining and processing is also critical to the creation of jobs in countries, with South Africa creating the most number of jobs (Anon, 2015).

Furthermore, the process design of the Ilesha goldfield would ensure sustainable solid mineral development which entails exploration, exploitation, processing, extraction and utilization of the depletable resources and the evolving of the process design will ensure safe and economic extraction of gold from Ilesha goldfield under environmentally friendly condition. Most of the previous works from different authors focuses on the process routes for Ilesha placer deposit, either by gravity concentration, amalgamation or cyanidation (Ajayi, 2003). A process routes for Iperindo lode gold deposit has not been reported in literature. However, sustainable development is a major focus of worldwide research in the gold mining industry therefore an alternative gold processing method which is more environmentally friendly than cyanidation and amalgamation is therefore imperative.

1.6 Scope of the Research

The scope of this research encompasses sourcing for Iperindo lode gold ore from its deposit, chemical characterization of the sourced gold ore by using atomic absorption spectrometry (AAS), ascertaining the mineralogical assemblage and elemental composition of the gold ore using X-ray diffractometer (XRD) and X-ray fluorescence (XRF) while the optical microscopy analysis using scanning electron microscope (SEM/EDX). Comminution and particle size analysis of the gold ore was carried out using the following sieve sizes: +500, -500+355, -355+250, -250+180, - 180+125, -125+90, -90+63, -63+45 and -45 |im. Froth flotation process using the locally sourced frother and collector follow by the characterization of the concentrate and tailing to determine the grade of gold recovered at each stages and finally, evaluation of the results.

CHAPTER TWO

2 LITERATURE REVIEW

2.1 Geology of Gold Ore Deposits

Native gold occurs as very small to microscopic particles embedded in rock, often together with quartz or sulfide minerals such as "Fool's Gold", which is a pyrite (Nguyen et al., 2018). These are called lode deposits. Laniece (2009) also observed that the metal in a native state is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits called placer deposits. Such free gold is always richer at the surface of gold- bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets (Fernie, 2013).

Some metallurgical implications to these gold ore types are summarised below: placers, quartz vein gold ores and oxidized ores: Generally, placers, quartz vein gold ores and oxidized ores are free-milling and gold can be recovered by gravity and/or direct cyanide leaching. Some epithermal deposits may be free-milling (such as the oxidized portion) but more commonly contain significant amounts of sulphides in which gold occurs as tiny inclusions or sub microscopic gold and are therefore refractory. Based on the mineralogical characteristics and mineral processing techniques required, gold ores can be classified into 10 types (Table 1)

Table 1: Gold Ore Types and Gold Occurrence

Abbildung in dieser Leseprobe nicht enthalten

Sources: Joe et al, 2004 and modified

2.1.1 Africa's gold deposits

Ghana is Africa's second-largest gold producer after South Africa and occupy tenth in the world's gold output with production of 90 tonnes in 2014. Geologically, Ghana is auspicious with large areas overlaid with a Pre-Cambrian greenstone, known as Birimian, which hosts vein, lode, conglomerate and placer deposits. Ghana's goldfield includes Tarkwa, Tebereble and Damang mines. Other gold producing countries in Africa are Mali (Africa's third gold producer), Burkina Faso (Africa's fourth gold producing), Kenya, Tanzania, Zambia, Zimbabwe, Ethiopia, Guinea, Liberia, Nigeria, Gabon, the Central Africa Republic, Burundi, Mpumalanga provenience and Madagascar (Taiwo and Awomeso, 2017).

2.1.2 Nigeria's gold deposits

In Nigeria, gold is predominantly found in three regions which are Sokoto, BirninGwari-Minna, and Ife- Ilesha goldfields (Adekoya, 1998). The Sokoto goldfield is now better called Zamfara- Kebbi goldfield because almost all the gold deposits are located in Zamfara and Kebbi states with only one deposit known at Gunmi in Sokoto State (Ajayi, 2003). Gold occurs primarily as fine dusts or grains (a few microns to 100 microns in size) in small quartz veins, quartz stringers, tourmaline-quartz veins and aplitic dykes which are either conformable with or cross-cutting diverse rocks types including amphibolites, schists, phyllites, quartzites, gneisses, pegmatites, and granites. However, gold nuggets, which have been found occasionally in the goldfields, are probably products of near-surface neo-mineralization, resulting in the formation of large gold grains. Gold occurrence in economic quality has been reported in Ife-Ilesha area of Southern Nigeria. Ilesha gold field stretches from Ilesha through Iperindo, Osu, Itagunmodi to the vicinity of Igun (De Swardt, 1947).

2.2 Mineralogy of Gold Ore

The mineralogy of an ore determines the recovery process and this is a branch of geology that studies the chemistry, crystal structure and physical properties of minerals. However, minerals are substances having a definite chemical composition and atomic structure which are formed by an inorganic process of nature (Klein and Dutrow, 2007). Gold occurs in a number of minerals (Harris, 1990) and the most important of these is metallic gold and the gold metal alloys (Chryssoulis and Cabri, 1990). Ores in which gold occurs in chemical composition with other elements are comparatively rare, some of which are calaverite, sylvanite, petzite, nagyagite, and krennerite (Wills, 2006).

However, gold ores containing carbonaceous materials are difficult to treat because the gold is tied up with the carbon present in the ores and therefore reports in the tailings (Abaka-Wood et al., 2019). In order to characterize ore, some certain analysis should be carried out so as to determine the nature of the ore, metallic content of the ore and the mineralogical assemblage of the ore. Some of the equipment used for the analysis include Atomic absorption spectroscopy (AAS) analysis, X-ray fluorescence (XRF) analysis, X-ray diffraction (XRD) analysis and Scanning electron microscopy with energy dispersive X-ray (SEM/EDX) analysis and petrography study.

2.2.1 Atomic absorption spectrometry (AAS)

This technique makes use of the absorption spectrometry to assess the concentration of an analyte in a sample. It requires standards with known analyze content to establish the relation between the measured absorbance and the analyze concentration and relies on the Beer-Lambert law. The electrons of atoms in the atomizer can be promoted to higher orbitals (excited state) for a short period of time (nanoseconds) by absorbing a defined quantity of energy (radiation of a given wavelength). This amount of energy, that is, wavelength, is specific to a particular element. In general, each wavelength corresponds to only one element, and the width of an absorption line is only of the order of a few picometers (pm), which gives the technique its elemental selectivity. The radiation flux without a sample and with a sample in the atomizer is measured using a detector, and the ratio between the two values (the absorbance) is converted to analyte concentration using Beer-Lambert law (Ramesh et al., 2000).

2.2.2 X-ray fluorescence spectroscopy (XRF)

The energy dispersive X-ray fluorescence spectroscopy (XRF) is a method used for the qualitative and quantitative determination of the elemental composition of a material sample. The specimen is excited with the primary X-radiation. In the process electrons from the inner electron shells are knocked. Electrons from outer electron shells fill the resultant voids emitting a fluorescence radiation that is characteristic in its energy distribution for a particular material. The energy dispersive detector measures the energy distribution of the fluorescence radiation. A multistage electronics circuit processes the measurement signals. The measured spectrum shows lines or peaks that are characteristic of the chemical elements in the sample (Ricketts et al., 2013).

2.2.3 X-ray diffraction (XRD)

X-ray diffraction analysis is a unique method of determination of crystallinity of a compound or substance. It is based on constructive interference of monochromatic X-rays and a crystalline sample: the X-rays are generated by a cathode ray tube, filtered to produce monochromatic radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident ray with the sample produces constructive interference (and a diffracted ray) when conditions satisfy Bragg's law (if. = 2d sin0). This law relates the wavelength of electromagnetic radiation to the diffraction angle and the lattice spacing in a crystalline sample.

The characteristic x-ray diffraction pattern generated in a typical XRD analysis provides a unique “fingerprint” of the crystals present in the sample. When properly interpreted, by comparison with standard reference patterns and measurements, this fingerprint allows identification of the crystalline form (Chauhan and Chauhan, 2014).

2.2.4 Electron microscopy

An electron microscope is a type of microscope that uses electrons to illuminate a specimen and create an enlarged image. Electron microscopes have much greater resolving power than light microscopes and can obtain much higher magnifications. Some electron microscopes can magnify specimens up to 2 million times, while the best light microscopes are limited to magnifications of 2000 times. Both electron and light microscopes have resolution limitations, imposed by their wavelength. The greater resolution and magnification of the electron microscope is due to the wavelength of an electron, its de Broglie wavelength, being much smaller than that of a light photon, electromagnetic radiation.

The electron microscope uses electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it on a specific plane relative to the specimen in a manner similar to how a light microscope uses glass lenses to focus light on or through a specimen to form an image. Electron microscopy can be done in transmission or in back-scatter mode. The back­scatter mode is used in scanning electron microscopy (SEM). The transmission mode is used in transmission electron microscopy (TEM) (Zandbergen, 1997).

2.2.5 Scanning electron microscopy with energy-dispersive x-ray spectroscopy (SEM/EDS)

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron sample interaction reveal information about the sample including external morphology (texture), chemical composition, crystalline structure and orientation of materials making up the sample. The signals used by a scanning electron microscope to produce an image result from interactions of the electron beam with atoms at various depths within the sample. In secondary electron imaging (SEI), the secondary electrons are emitted from very close to the specimen surface. Consequently, SEI can produce very high-resolution images of a sample surface, revealing details less than 1 nm in size. Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. They emerge from deeper locations within the specimen and, consequently, the resolution of BSE images is less than SE images.

However, BSE is often used in analytical SEM, along with the spectra made from the characteristic X-rays, because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen. BSE images can provide information about the distribution, but not the identity, of different elements in the sample (Mignot, 2018). An EDS detector contains a crystal that absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element (Ying, 2003).

2.2.6 Transmission electron microscopy (TEM)

Transmission electron microscopy is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. The use of an optical microscope limited the ability to resolve an object because of its wavelength. TEM is a technique developed to obtain magnification and hence details of a specimen, to a much better level than the conventional optical microscopes. In TEM a beam of electrons is passed through an ultra-thin specimen interacting with the specimen as it passes through. When electrons are accelerated up to high energy levels (few hundred keV) and focused on a material, they can scatter or backscatter elastically or inelastically, or produce many interactions, source of different signals such as X-rays, Auger electrons or light. However, the transmission of an electron beam is highly dependent on the properties of the material being examined (Ma et al., 2006).

2.2.7 Scanning-transmission electron microscopy (STEM)

A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing through a sufficiently thin specimen. However, unlike CTEM, in STEM the electron beam is focused to a fine spot (with the typical spot size 0.05 - 0.2 nm) which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis. A typical STEM is a conventional transmission electron microscope equipped with additional scanning coils, detectors and necessary circuitry, which allows it to switch between operating as a STEM, or a CTEM. Scanning transmission electron microscopy (STEM) combines the principles of transmission electron microscopy and scanning electron microscopy and can be performed on either type of instrument. One of its principal advantages over TEM is in enabling the use of other signals that cannot be spatially correlated in TEM, including secondary electrons, scattered beam electrons, characteristic X-rays, and electron energy loss (Nellist, 2011) . Its primary advantage over conventional SEM imaging is the improvement in spatial resolution (Arturo et al., 2012).

2.2.8 Ore microscopy

The ore microscopy is the basic instrument used for the petrographic examination of an ore. It is the study of opaque minerals or synthetic solids in polished section by means of a polarizing reflected-light microscope for the identification and characterization of the opaque phases in a sample and the textural relationships between them. The surface of the mineral is prepared for study by grinding and polishing to a flat surface, it is then mounted so the polished surface is parallel to the microscope stage, and the microscope is focused on the surface to be studied (Edgar et al., 2016). Gold is detected in some polished samples that appear as very fine bright specks enclosed within sulphides and iron oxides. Mostly, it is associated with pyrite, arsenopyrite, chalcopyrite, covellite, and sphalerite. Fine-grained disseminated and fine specks of gold, enclosed within quartz veins are also detected (Ramadan et al., 2010).

2.3 Mining

Gold mining is the process of mining of gold or gold ore from the ground. There are several techniques and processing by which gold may be extracted from the earth. These techniques include placer mining, panning, sluicing, dredging, rocker box, and hard rock mining. All gold deposits started as hard rock formations and stayed that way until there were earthquakes, rivers, glaciers, tidal waves and a lot of earth movement which eroded mountains and ground down the rock into its smallest form; dust. Trapped in all this rock were veins of gold, typically in quartz. These veins were also pulverized down into small pieces in nugget, flake and flour size. Because gold is so heavy, it settles to the bottom of our rivers, streams and any deep depression or crevice. This is where placer mining comes in. The quickest way to reach this heavy mineral is to go deep and that is the hardest thing we could possibly do. Most placer gold is found within 50 feet of the surface of the ground (Oke et al., 2014).

It is all the piles of gravels and sand we have to move first before we can reach the heavy nuggets. All this unwanted material is called "overburden" and it is the overburden that will make or mar a mine. Gold mining in the Ilesha-Ife schist belt began to play a modest role in the twentieth century, although the deposits were known in the Middle Ages and recovery from gold deposit has been a major pre-occupation since 1936 in the area, but they were illegally mined. The mining technique used to extract gold depends upon the type and nature of the deposit. Once mined, the gold is processed with some of these common processes such as flotation, amalgamation, cyanidation, or carbon in pulp, thus each process relies on the initial dissemination of the gold ore and prior to smelting of gold ore since the gold is still not pure (Hilson and Monhemius, 2006).

2.4 Mineral Processing

Mineral processing is a process of physically separating the grains of valuable minerals from the gangue minerals to get the enriched portion known as the concentrates - containing most of the valuable minerals and discard called the tailings - containing predominantly the gangue minerals (Obassi et al., 2015). It is the interface between mining and extractive metallurgy which make use of difference in physical, physico-chemical, or sometimes chemical properties of mineral particles to separate them (Wills and Napier-Munn, 2006).

2.4.1 Comminution

Comminution can be defined as the process that involves the size reduction of ore in lode deposit form in other to access and liberate the valuable trapped minerals from the gangue (Obassi et al., 2015). It is the progressive liberation of run-of-mine ores through crushing and grinding process. In mineral processing plant or mill comminution takes place as a sequence of crushing and grinding processes (Wills and Napier-Munn, 2006). Most minerals are finely disseminated and intimately associated with gangue; they must be intimately unlocked or liberated before separation can be undertaken and establishing the liberation or release of the ore from the associated gangue minerals at the possible particle size is achieved by comminution (Wills, 2006).

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• Note: 4.5
• Autor: Yomi Adetula (Autor:in)
• Erscheinungsjahr: 2021
• Seiten: 99
• Katalognummer: V1060138
• ISBN (eBook): 9783346495129
• ISBN (Buch): 9783346495136
• Sprache: Englisch
• Schlagworte: effect