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MICROWAVE – ASSISTED HYDROMETALLURGICAL BENEFICIATION OF A CASSITERITE ORE IN KURU, NIGERIA
ABSTRACT
Microwave pre-treatment of cassiterite ore prior to leaching was carried out in an attempt to obtain high grade of SnO2 and to conserve the total energy expended during processing. Microwave study was selected due to the presence of associated minerals which behaves differently to microwave energy. Studies in this work showed that microwave pretreatment of cassiterite ore can be applied to increase the recovery of tin oxide in 5 minutes. The morphological and elemental characterization were performed by the combination SEM, XRD and ICP-OES. The mineral phases present in the cassiterite ore are; cassiterite (SnO2), rutile (TiO2), manganocolumbite (Fe, Mn, Nb2O5), zircon (ZrO2), quartz (SiO2) and monazite (Ce, La, Th, Ca, Y) PO4.. The mineralogy of the ore components was affected by microwave irradiation at optimum conditions of 950 W and 2.45 KHz. Microwave treatment of the ore resulted in mineral liberation at all particle sizes. The oxides of Sn, Fe and Ti were separated from the major impurities, resulting in 98% of SnO2 at pH 7. The dissolution kinetics of the cassiterite in alkali medium using the shrinkage core model (1-(1-X)2/3 = kt) showed that the leaching of the microwaved and untreated ores are controlled by surface chemical reaction due to the activation energies (10kJmol -1 and 105kJmol -1 respectively). The model for the extraction of SnO2 from associated minerals was derived. The model was found to depend on the amount of tin oxide extracted, and which varies with time. The validity of the model is rooted on the expression; Amod = 0.1898 t1.53 where both sides on each of the expressions are equal and t is the time taken for the leaching. The graphical and sum square deviational (SSQ) techniques were used to ascertain the validity. The deviation of the model – predicted amount of extracted SnO2 from the corresponding experimental value is 8%, which is quite within the acceptable range of deviation limits of experimental results, impacting a 92% confidence on the model. The energies utilized for both processes were analysed. It showed that the energy consumed for the microwave assisted beneficiation of the ore is 12.3 times less than the energy utilized for the conventional leaching of the cassiterite ore. This impact more economic viability on the microwave assisted process.
(Keywords: Cassiterite, hydrometallurgy, tin oxide, microwave, modeling, SEM, XRD)
TABLE OF CONTENTS
Pages:
Title page i
Certification – – – – – – – – ii
Dedication – – – – – – – – – iii Acknowledgements – – – – – – – iv
Abstract – – – – – – – – – vi
Table of Contents – – – – – – – vii
List of Tables – – – – – – – – xii
List of Figures – – – – – – – – xiv
CHAPTER ONE
Introduction 1
1.1 Background – – – – – – – 1
1.1.1 Ores and Minerals – – – – – – 1
1.1.2 Complex ores – – – – – – – 2
1.1.3 Ore Composition – – – – – – – 3
1.1.4 The Nature of Complex Ores – – – – 5
1.1.5 Chemical Characteristics – – – – – 6
1.1.6 Multi-Stage Processes – – – – – – 6
1.1.7 Cassiterite – – – – – – – – 8
1.1.8 Cassiterite Occurences in Nigeria – – – – – – – – – – – – – – – 9
1.1.9 Associated Minerals – – – – – – – – – – 10
1.3 Justification – – – – – – – – – – 17
1.4 Notable Clarification – – – – – – 18
1.5 The Research Question – – – – – – 19
1.6 The Problems Statement – – – – – 19
1.7 Hypotheses – – – – – – – 20
1.8 Objectives of the Study – – – – – 20
1.9 Relevance of the study – – – – – 21
CHAPTER TWO
Literature Review
2.1 Tin mining in Nigeria – – – – – – 23
2.1.1 Tin – – – – – – – – – 24
2.1.2 Properties of Tin – – – – – – – 25
2.1.3 Extraction of Tin – – – – – – – 29
2.2 Current trends in mineral processing- – – – 29
2.2.1 Hydrometallurgy of Cassiterite- – – – – – 30
2.2.2 History and Application of Microwave – – – 34
2.2.3 History and Application of Microwave:- – – – – 34
2.2.4 Current Views on Microwave and Hydrometallurgy – — 34
2.2.5 Microwave Heating – – – – – – 37
2.2.6 Theoretical concepts of Microwave Heating – – – – 38
2.2.7 Electic Field Strenghtr and Dielectric Permitivity – -42
2.2.8 Microwave Treatment of Oxides and other Minerals – – – – – 43
2.2.9 Microwave assisted mineral Leaching – – – – 56
2.3 Chemical modeling – – – – – – – – – 31 CHAPTER THREE
Materials and Methods
| 3.0 Experimental – – – – – – – – | 65 |
| 3.1 Reagents Used – – – – – – – – | 66 |
| 3.2 Apparatus – – – – – – – – – | 66 |
| 3.3 Equipment – – – – – – – – – | 66 |
| 3.4 Chemical Analysis – – – – – – – | 67 |
| 3.4.1 Scanning Electron Microscopy (SEM) – – – – | 67 |
| 3.4.2 X –Ray Powder Diffraction (XRD) – – – – – | 69 |
| 3.4.3 Inductive Coupled Plasma – Optical Emission Spectrometry | 72 |
| 3.5 Mineral Liberation Studies – – – – – – | 72 |
| 3.6 Crushing and Sieving – – – – – – – | 74 |
| 3.7 Separation by Shaking Tables – – – – – | 74 |
| 3.8 Magnetic Separation – – – – – – – | 75 |
| 3.9 Extraction Experiment – – – – – – | 76 |
| 3.9.1 Conventional leaching – – – – – – | 76 |
| 3.9.2 Leaching of Microwave treated Cassiterite – – – | 77 |
| 3.10 Precipitation of SnO2 from Alkaline liquor – – – | 78 |
| 3.11 Model formulation – – – – – – – | 78 |
3.1.2 Economic Analysis – – – – – – – – – – – – – – 79
CHAPTER FOUR
Results and Discussion
4.1 Mineralogical characterization – – – – – – 80
4.1.1 Scanning electron microscopy (SEM) – – – – 80
4.1.2 X – ray diffraction – – – – – – – 94
4.1.3 Inductive Coupled Plasma – Optical Emission
Spectrometry (ICP-OES) – – – – – – 94
4.2 Mineral Liberation Studies – – – – – – 95
4.3 Physical Separation – – – – – – – 95
4.4 Leaching Investigation on Cassiterite Ore – – – 97
4.4.1 Effect of Stirring Speed – – – – – – – – – – 97
4.4.2 Effect of Concentration of Fe, Ti and Sn extraction – – 98
4.4.3 Effect of Particle Size – – – – – – – 99
4.4.4 Effect of Temperature – – – – – – – 100
4.5 Microwave Treatment of Ore – – – – – – 101
4.5.1 Effect of Microwave Power – – – – – – 101
4.5.2 Effect of Microwave Irradiation on Ore Morphology – – 103
4.5.3 Effect of Microwave Irradiation Time – – – – 104
4.5.4 Leaching of Microwave Treated Cassiterite Ore – – 105
4.5.5 Effect of Particle Size on Microwave Treated – – –
Cassiterite – – – – – – – – 108
4.6 Separation of SnO2 from the Liquor – – – – 110
4.7 Analysis of Waste Solution – – – – – – 113
4.8 Model formulation – – – – – – – 115
4.9 Graphical validation of the model – – – – – 118
4.10 Deviational methods of model validation – – – – 120
4.11 Kinetic Analysis – – – – – – – – – 121
4.12 Economic Analysis of the Process – – – – – 125 CHAPTER FIVE
Conclusion and Recommendations
5.1 Conclusion – – – – – – – – – 126
5.1 Recommendations – – – – – – – 127
REFERENCES – – – – – – – – – 128
CHAPTER ONE
INTRODUCTION
1.1 Background
1.1.1 Ores and Minerals
An ore is a complex material or rock that contains minerals with important elements, including metals (Onyedika et al., 2012). The ores are extracted through mining; these are then refined to extract the valuable element(s). The grade or concentrate of an ore mineral or metal as well as its form of occurrence will directly affect the cost associated with mining the ore. Ores have no definite chemical composition except the native ores.
A definition which has been current in capitalist economics for nearly a century runs as follows; ore is a metaliferous mineral, or an aggregate of metaliferous minerals, which from the stand point of the miner can be won at a profit. There are many similar definitions which all emphasize (a) that it is a material from which we extract a metal, and (b) that this operation must be a profit making one.
A mineral is sometimes defined as any rock or substance that occur naturally in the earth crust and is of economic value, exclusive of metallic ore, mineral fuels and gemstones. For ore mineral, it should have a definite chemical composition.
1.1.2 Complex ores
Nearly all metals are derived from ores, which are concentrates of minerals situated accessibly at or near the earth’s crust. Hence ores are complex composite materials bearing minerals of several metals. The two important exceptions perhaps are magnesium, which may also be worn from sea water, and plutonium, which is produced in atomic reactors. Ores therefore, from their own very nature are complex, the simplicity of their chemical formula notwithstanding. As the complexity of these ores increased, various technologies have been developed, effecting their laboratory and industrial processes and utilizations (Ogwuegbu et al., 1997, 2000; Onyedika et al; 2012). Nonetheless, these utilization processes, including those still in the laboratory scales and desks, have created an avalanche of problems, in terms of capital cost with respect to engineering equipment, research and development, energy requirements, reagent consumption and waste management (Habashi, 1979a,b; Rybakov et al., 1999).
Some of these ores more often than not consist of many metals, some highly entangled and others intricately locked up in interior matrices of the ore body that in almost all cases enhanced and sophisticated technologies and multi–stage processes are sine qua non to separation and recovery of minerals or metals of interest contained in them (Murthy, 1990; Onyedika et al; 2012). Some may involve pyro-metallurgy, in their utilization, some requiring hydrometallurgy, while others may need both processes, and even some eventually culminate in electrometallurgical route for the recovery and refining of the valuable metals and others of secondary value.
1.1.3 Ore Composition
Metals occur in the earth’s crust in the following chemical forms: (i) oxides in which the metal exists in the form of oxide; these occur as primary or secondary oxides. (ii) The sulphides in which the metal exists as a sulphide. Minerals of this kind are associated in families, and with iron almost always present (Gilchrist, 1980). For example, copper, iron and nickel are often found together just as silver, zinc and lead occur together; (iii) Oxy–salts – they include the silicates, sulphates and carbonates, zircon, titanates etc. Apart from these ores, there are those found in the localities, also called “native ores”, which exist in the elemental (uncombined) state. Examples are the precious metals and a little of copper. The most important groups with respect to complexity, quantity and occurrence are the oxide and sulphide ores.
Chemically, metal ores contain three categories of mineral: (i) valuable minerals of the metal being sought, and are the concentrations of the metals of economic value; (ii) compounds of associated metals, which may be of secondary value, e.g. niobites, tantalites and wolframite commonly associated with cassiteirte; and (iii) gangue minerals of no value, usually associated with (i) and (ii).
In order to appreciate the problems that can be envisaged in the utilization of complex ores, it may become necessary to look at the compositions of some major complex mineral ores as shown in table 1. 1
Table 1: Composition of some complex ores
Ore Position Composition Remark
Arsenopyrite Source of As and Au
| Autunite | CaO.2UO3.P2O5.8H2O | Hydrated calcium | ||
| uranyl phosphate. | ||||
| Alumina | Al2O3.xH2O | A mixture of diaspore, boehmite | ||
| (Al2O3.H2O), and gibbsite, | ||||
| Al2O3.3H2O). Major source of Al. |
Carnotite K2O.2UO3.V 2O5.3H2O Hydrated potassiumuranyl
vanadate. A source of V and one of
the sources of radium.
Chalcopyrite CuFeS2 Major source of Cu
Cassiterite SnO2 Major Source of Sn
Chromite FeCr2O4 or FeO.Cr2O3 Only mineral of Cr.
Ilmenite FeO. TiO2 Major Source of Ti
Kaolinite Al2O3. 2SiO2.2H2O
Monazite Sand (Th,La,Ce,Y,Zr,Ti, Fe,) PO4 Major source of Th
and rare earths.
Pitchblende U3O8 Major Source of U
containing Th, rare earths,
sulphides of base metals yUO2.xUO3.
xUO2.yUO3
Pyrochlore (Na,Ca)(Nb,Ta)2O6(O,OH,F) A major source of Ta and Nb.
| Tantaloniobates (Fe,Mn)O.(Nb,Ta)2O5 | A major source of Ta and Nb. |
| Titania TiO2 | A major source of Ti |
| Wolframite (Fe, Mn)WO4 | Major source of W. |
Molybdenite Fe2O3.3MoO3.7H2O Mixed oxides of Fe and Mo
Source: Read, 1972; Wills, 1981; Onyedika et al., 2012.
1.1.4 The Nature of Complex Ores
The separation of one or two component phases from an ore body depends on the structural complexities of the host phases, and the crystal field stabilization energies (CFSEs) of the component elements (Vezina, 1970). In essence, the effective utilization of such ores will have strong bearing with the internal structures, of not only the ores but also the minerals and the elemental assemblages. An example is geothite (α-FeOOH), which is the most frequently observed Fe bearing mineral present in Mn nodules is intimately intermixed with other phases or minerals giving rise to a complex internal structure (Murray, 1979; Thijs et al., 1981). It is difficult, if not impossible to extract significant amount of any particular phase. Some new processes have been developed to separate tin from its impurities by concentration. Separation of zircon, tin, tantalum and columbite from tin gravity concentrate has been accomplished by direct flotation using a mixture of sulphosuccinate collectors modified with fatty alcohol ester sulphate, and depression of impurities using organic acids and sodium silicate. When using this method, more than 90% of the impurities were separated from the gravity concentrate. The imprisonment of some of the minerals/ element in the crystal structure of the ores may necessitate consumption of excessive reagents, hence increasing the cost of utilization of such ores. For instance, some Au ores are associated with the minerals, chalcopyrite and arsenopyrite. These cannot be treated directly by the conventional methods because of the various reasons outlined above
(Murray, 1979).
1.1.5 Chemical Characteristics
The chemical characteristics of ores influence their sepration in various ways. Different components of an ore body (oxides, sulphides, arsenides, etc) call for different process route and reagents. One of the problems often encountered in minerals processing is non–availability of specific technologies and reaction reagents. In spite of spectacular advances in the development of extraction reagents (acids, alkali, collector, depression, activator, etc), the minerals processing industry is still in need of specific and selective processes to cope with the nature of complex ores (Murray, 1979). The donor atoms on the extraction and floating chelating agents as well as those associated with the minerals in the ores play significant roles in the separation of the constituent components of an ore.
1.1.6 Multi-Stage Processes
Table 1.1 reveals the complexities of some of these ore minerals. This often necessitates the use of several process steps to reach the desired mineral. These processes may first involve beneficiation steps, in which the complex ore is crushed and ground, and the siliceous gangue materials reduce as much as possible (Wills, 1981).
The beneficiation stage may be preceded by concentration processes of flotation and magnetization to provide concentrates suitable for roasting and smelting operations. The more complex an ore is the more complex, and perhaps the greater is the number of process steps involved in the separation of the constituent minerals and the winning of the valuable metal(s). The procedures adopted to separate gangue from the valuable minerals are generally termed “ore dressing”. Ore beneficiation is also sometimes used to describe these processes, but the term when correctly applied also includes such pyro-metallurgical pre–extraction processes as calcining, sintering, pelletizing and roasting. Beneficiation, with regard to copper ores encompasses all of the methods used to process the ore to improve its physical and chemical characteristics that will make it a more desirable feed for the converter furnace or autoclave for pressure leaching. Because of the differences in structural complexities and mineral contents of ores from different deposits, beneficiation methods also vary considerably (Mason and More, 1982; Read, 1972).
The beneficiation stage may be followed by roasting and agglomeration
MS + O2 MO + SO2 (1.1) M = Zn, Cu, Ni, etc.;
leaching and precipitation by a precipitant (HL) or by hydrolysis
Mn+ + nHL → MLn + nH+ (1.2)
Or
Mn+ + 3OH– → M(OH)3(s) (1.3)
The procedures adapted to extract metals in useable forms from concentrates may also depend on the mineralogical composition as well as on the degree of metallization of the component of interest. The choice of any extraction route will largely depend on the cost per ton of metal produced, the type of ore, availability and cost of fuel, and the quantity and quality of products of such a process (Ogwuegbu, 1999). Sometimes the separaton of a particular ore may involve the three processes outlined in the extraction of Al from bauxite. Others may involve only two process steps.
1.1.7 Cassiterite:
Cassiterite is the chief ore of tin. It is otherwise known as tinstone. Nigeria is one of the richest countries of the world as far as mineral resources are concerned, including cassiterite and iron ores. The estimates of workable ore deposits stand in excess of 2.5 billion tones most of which belongs to cassiterite and haematite, magnetite, cassiterite and goethite grades found in Itakpe (Alafara et al., 2005), Other ores include calcite, sphalerite, chalcopyrite, galena, ilmenite, etc. Extraction of metals from their ores and recovery from their various industrial wastes are a major step towards judicious utilization and conservation of our mineral resources depleting very fast (Jena, 2006).
Cassiterite also known as tin ore is the only tin mineral from which tin can be extracted in a commercial quantity. Cassiterite ore occurs both as alluvial and lode deposits. The ore deposit especially the lode ores are usually found associated with other minerals which generally hinders tin refining to some extent (Ogwuegbu et al., 2000; Onyedika et al., 2012). The presence of iron is deleterious during smelting while other elements such as As, Sb, Ti, Bi, Pb, Cu and Ag necessitate further process steps during refining. Iron is particularly troublesome owing to the fact that it has a goes into solution with tin. In the usual temperature range at which tin refining is carried out, the solid in equilibrium with liquid tin is FeSn2, the so called “hard head” containing 19% iron by weight. Present method of removing iron from tin deposit is crystallization of FeSn2 from liquid tin at low temperatures.
Various proposals have been made to extract tin from its ores by the reduction of cassiterite to tin metal and then extracting the tin with aqueous inorganic acids. Martell (1983), describes a process wherein low temperature (ca. 750o C) reduction is followed by countercurrent leaching and electrolytic deposition of tin, giving a tin-depleted leach liquor for reuse. He suggested that the combination of hydrogen reduction, leaching and electrolyte deposition of tin offered an economical method, with greater ease of operation than conventional smelting, and was capable of producing the highest grade of metal from complex ores. The most effective of all methods of isolation is yet to be discovered.
1.1.8 Cassiterite occurrences in Nigeria
The bulk of Nigeria cassiterite occurs in the grantic rocks of the Jos Plateau. The main area of occurrence may be divided into two, corresponding to the Bukuru and Ropp granites, which are separated by the Kassa basalt flows. In the north, the important deposit belongs to the Delimi,
Ngeli and Korot – forum drainage. The Ray-filed plains at the head of the Delimi and Shen, and the junction tributaries of the Forum are of almost equal importance. The Dorowa area has been important for dredging. The Delimi, Ngeli, Ropp and Forum drainage. The Ray-field plains at the head of the Delimi and Shen, and the junction tributaries of the Forum are of almost equal importance. The Dorowa area been important for dredging, the Delimi, Ngeli, Ropp and Forum drainage account for about eighty percent of the entire plateau tin (Ofor 1994). Only minor deposits are found on the Jarawa, Sha, Kagoro and Rukuba granites, and the environs of the Ganawuri mass have few important deposits.
1.1.9 Associated minerals
Some minerals are natively associated with cassiterite. The ones associated with cassiterite in the mineralized zones are secondary mica, quartz, topaz, fluorite, tantalo- niobites and wolfram. Sulphides are widely distributed in small amounts. Chalcopyrite and galena are occasionally present while sphalerite and molydenite are very rare. Cassiterites from alluvial deposits are found to contain plentiful ilmenite of relatively coarse size. Ilmenite is practically absent in cassiterite from the sub-fluvio-volcanic series. It is considered that the modern alluvials received ilmenite from the Newer Basalts whereas no such rich source existed for the earliest tin deposit. This zone is the country’s only known source of radioactive minerals. Deposit from which cassiterite and the associated minerals, principally columbite and wolfram, have been won, may be classified in the following manner, in the order of economic importance (Ofor. 1994).
- i) Alluvial deposits: deposits of cassiterite with columbite, cassiterite and minor columbite cassiterite with rarely columbite, ii) Top soil cassiterite and lateritic deposits of cassiterite. iii) Eluvial deposit: cassiterite occurs rarely with wolfram.
- Primary deposits: Stockworks – cassiterite and more rarely wolfram.
- Lodes – cassiterite and wolfram.
1.2.0 Deposits in the present drainage system
About ninety percent of the cassiterite which has hitterto been mined has come from the modern streams and their old channels. The columbite mined on the plateau has all come from the deposits except minor amounts from the leads under the fluvio- volcanic series. The ratio of columbite to tin throughout the field is between 1:20 and 1:25. The tin has, in general varied in size from 40 mesh to pea size. Coarse tinstone is occasionally found in these deposits and is usually more properly attributable to the eluvial class.
1.2.1 Deep leads beneath the newer basalts
Morphologically, the older and smaller leads occur beneath outliers in relatively elevated position, while the younger leads are found as extensive areas occupying the position of former major valleys. The tin is essentially similar to that in the youngest alluvial, but no important columbite has been located. The depths of these leads vary from 6 to 16 meters. The more recent newer basalts are known to have deposit below them which are 50 to
60 meters beneath the surface (Ofor, 1994).
1.2.2 Deep leads beneath the flurio – volcanic series
Characteristically, these are in hill deposits since most of the occurrences of the series so far recognized form high outliers. These hill deposits are not easy to work if the overburden is deep. The tin is usually fairly fine and the concentrate is very much cleaner than river tin. These leads tend to be narrow and richer than those of the modern deposits.
1.2.3 Deposits in the top soil
Appreciable quantities of cassiterite have been won from the top few meters or centimeters of the soil, despite any deposit which may lie near the bedrock. Sheet erosion has been responsible for the formation of these deposits by repeated removal of the light matter of the top soil resulting in lowering of the surface with high proportion of cassiterite (Ofor,1997)
1.2.4 Lateritic deposit
Rarely, high laterite may contain cassiterite. This has not been of economic importance but rather of interest in showing that the laterite concerned once constituted a surface on which drainage flowed. Such a deposit was worked in the Werren hills, where the laterite is situated on top of a fluvio-volcanic hill (Ofor,1997).
1.2.5 Eluvial deposits
Cassiterite from these deposits usually contain finer water – worn tin in addition to the course angular mineral. Most of the interesting mineral specimens and crystals of the tin fields have come from this kind of deposit.
1.2.6 Primary Deposits
Primary deposits occur within the bedrock, and are often uncovered by alluvial workings. Wolfram occurs along with cassiterite and sometimes the occurrence of chalcopyrite, biolite and galena are observed.
Besides the Jos – Bukuru area, new deposits are found in Kuru, a few kilometers from the Saiye and Shokolo hills in neighboring Bauchi State. The most common rock type exposed here in the porphyritic rhyolite with pink and white feldspar. The rhyolites overlie the biolite granite.
The rhyohite – biolite granite contact is the dominant structural control for cassiterite mineralization. Cassiterite occurs in both rock types in the vicinity of the contact in the zones of altered granite and siliceous and green mica gneissen. Most cassiterite occurrence is on the granite side of the contact, nonetheless, some occurrences are observed within the rhyolite itself. The biotite-granite in this area is columbite-bearing although little of it may be exposed.
1.2.7 Cassiterite properties
The general properties of Cassiterite ( SnO2 ) are; It has a black reddish brown or yellow colour, Luster is adamantine or greasy; transparent crystals, crystal system in tetragonal 4/m, 2/m, cleavage is good in two direction forming prisms. Other common properties of cassiterite are; Molecular formula – SnO2 ; Molar mars,150.708g/mol; appearance is white powder , Melting point, 11270 C; Crystal points is tetragonal, Geometry is 0.3 trigonal planar; Hardness is 6-7 mohs; specific gravity 6.8-7.1; fracture is subconcoidal to even; cleavage is poor prismatic.
The bulk of the Cassiterite found in Nigeria occurs as black mineral usually rounded, and opaque, above 60 microns in thickness. This habit is considered a matter of attrition since specimens from eluvial and primary deposits shows that much if not most of the Cassiterite was well and sometimes perfectly crystallized.
Wood tin has been found at various localities notably Gindi, Akwuti and South Ropp. The bending varies in with from 0.1 millimeter with 0.2 to 0.3 millimeter space bands, down to 0.01millimeter wide bands with 0.04 millimeter spacing. The wood tin is microcrystalline. Wood tin is considered to be a high temperature colloidal product. The high melting point of tin oxide (SnO2) accounts for the high temperature required for it in decomposition in reverberatory furnace (1200 – 13000 C). (Greenwood, 1921; Mackay et al,
1949)
1.2.8 Uses of Cassiterite
Cassiterite in its pure form or combined state can be used in a number of chemical applications. When solidified, SnO2 / NaOH melt is dissolved in water, it gives Na2[Sn(OH)6 ]2. This complex is called preparing salt, which is used in the dying industry.
Cassiterite (SnO2) is used as a catalyst for oxidation of aromatic compounds in the synthesis of carboxylic acid anhydrides when combined with vanadium oxide. Thick layers of SnO2 doped with antimony ion and fluorine ion are electrically conducting and is used in electroluminiescent devices.
Cassiterite is used as a pigment in the manufacture of glasses, enamels and ceramic glasses. Pure SnO2 gives a milky white colour. Other colours are achieved when mixed with other metallic oxides such as Sb2O5 grey blue. It is used in sensors of combustible gases.Figure 1 shows the map of Kuru , a typical mining area, 20 km away from Jos, capital of Plateau State of Nigeria.
Figure 1.1: Map of Jos, Nigeria showing Kuru site (A) for Cassiterite ore deposit.
1.2.9 Microwave applications
Microwave is the name for electromagnetic waves arising as radiation from electrical disturbances at high frequencies. Microwave power is the technology of the future – with economical benefits including energy conservation and reduced processing time as well as being more environmentally friendly. It has been used successfully in a number of applications including the food and telecommunications industry (Vorster, 2001).
Valuable minerals do not, as a rule, occur freely in nature, but are frequently locked away within gangue particles. A typical example is gold which usually occurs in extremely low concentration in nature and is finely dispersed within the host rock (mostly gangue). The mineral processing industries are responsible for the production and supply of raw materials in sufficient quantities and grades required for further processing. In order to accomplish this, the mineral needs to be effectively and efficiently separated from the gangue before it can be further produced. Microwave techniques are becoming valuable tools in achieving these objectives.
1.3 Justification
Many processes have been reported for the extraction of tin from ores and low grade concentrates. A process in which cassiterite is made acid soluble by reduction to SnO2, which stabilizes by glass formation and the tin was extracted by leaching the glass with sulphuric acid, followed by electrowinning was reported (Holt and Pearson, 1977; Nixon and Prosser,1973). This research has been directed towards finding the best route of beneficiating Nigerian cassiterite ores to increase the economic values of export. Application of microwave treatment on the cassiterite ore may reduce the thermal energy and time required for both pyro-metallurgy and hydrometallurgical processes of tin. This research is supposed to provide some basic information through the applications of more modern analytical instruments on mineral characterization of the ore, improved method of reducing iron content. The result will be used to develop a predictive model for SnO2 extraction.
1.4 Notable Clarification
There have been recent controversies over the effects of microwave on ore minerals (Rybakov et al., 1999). The distinction between conventional processes and microwave processes is in the nature of the energy source.
In conventional processes, heating of materials is by conduction, convection and radiation. On the other hand, microwave(dielectric) heating depends on the molecular vibration to heat materials from the inside (as a result of internal friction). Therefore, this research is primarily concerned with the thermal effects of microwaves on the Nigerian cassiterite ore components prior and its subsequent effect on leaching.
1.5 The Research Question
The mineral processing and metal industries which are the major consumers of hydrometallurgical products are demanding materials of high grade and purity that meet engineering specifications.
Can the existing hydrometallurgical route be modified to achieve a considerable increase in the recovery of tin, iron and titanium?
How would microwave treatment of the ore increase the liberation and leachability of the valuable minerals?
What are the time and cost implications of the separation processes?
Is the development of an extraction model possible, and can it be validated?
1.6 The Problems Statement
Although hydrometallurgical processes for extraction of valuable minerals are available, the methods themselves are expensive and require high energy and time. The major problems with the current mineral
processing of cassiterite are:
- Poor information on ore mineralogy and composition
- High energy consumption during communition and leaching
- Low efficiency of separation
- Long time requirement during processing.
The practical implications, if not properly addressed would be loss of income; and if these issues are addressed, huge economic benefits are possible.
1.7 Hypothesis
When polar materials absorb microwaves, the different molecular components with varying bond types vibrate at different frequencies. This leads to internal friction. This friction generates heat. The energy change within the material may lead to breakdown of the chemical bonds and structure which may result to phase transformation.
It is therefore, hypothesized that since cassiterite ore is made up of different minerals, the various mineral components would absorb microwaves and vibrate with different frequencies. The resulting differential heating may lead to cracks within the ore body and breakdown of bond structures. This transformation by microwave is both energy efficient and should result in lower processing time.
1.8 Objectives of the Study
There is virtually, no available works on the leaching of microwave treated Nigerian cassiterite ore. Though, reports on leaching of Nigerian cassiterite, galena , sphalerite and ilmenite are available ( Baba et al., 2009; Baba et al., 2007, 2005; Adebayo, et al., 2006). Objectives of this research are the following;
- Characterization of the mineral phases in Nigerian cassiterite ore using scaning electron microscope (SEM-EDS).
- X – ray diffractometer (XRD) to further fully quantify the elemental compositions of this ore.
- Use of inductively coupled plasma – optical emission spectroscopy (ICP – OES) to elucidate the structures obtained before and after microwave treatment.
- Optimization of microwave treatment to obtain quickly beneficiate
cassiterite.
- The evaluation of the viability of microwave application in the efficiency of minerals extraction during leaching.
- Comparison of the behavior of mineral components in conventional leaching and leaching of microwave treated ore. vii The development of a model for the effect of microwave treatment for the predictive extraction of SnO2 from the ore.
1.9 Relevance of the study
- This research study is designed to investigate the thermal effects of microwave irradiation on Nigerian cassiterite ore and its beneficial effects on leaching will be a major contribution to the existing methods. The benefits of thermally assisted ore treatment include increase in mineral liberation (Walkiewicz, 1991; Vorster, 2001).
- The use of modern instruments and technologies in ore
characterization with the view to finding out and understanding the best possible practice or beneficiating route by which tin oxide could be possibly extracted.
- The development of a model for the leaching process for cassiterite ore will assist metallurgists to produce higher purity of tin in the tin or metal base metal industry to predict the optimum possible metal recovery from the ores.
MICROWAVE – ASSISTED HYDROMETALLURGICAL BENEFICIATION OF A CASSITERITE ORE IN KURU, NIGERIA
