GEOCHEMISTRY, MINERALOGY AND BENEFICIATION OF IRONSTONE FROM BIDA BASIN, NIGERIA

ABSTRACT

The Bida Basin is a NW-SE trending inland sedimentary basin in North Central Nigeria that extends from Kontagora in the north to slightly beyond Lokoja in the south and its mesas consist of ironstone layers from which economic iron ore can be won. The stratigraphic succession in the basin comprises the Bida sandstone at the base in the northern part of the basin followed successively upward by the Sakpe, Enagi and Batati formations.  The  geochemistry,  mineralogy and  beneficiation  study of  the  ironstone samples from the study area in the northern Bida Basin were studied. Twelve representative samples from the study area were analysed for their elemental content and three samples for mineralogical characteristics using X-ray Fluoresce(XRF) and X- ray diffraction (XRD) techniques respectively while, beneficiation study was carried out on six (6) selected samples using magnetic separation technique. The result of the chemical analysis reveals that the following oxide with their indicated average values are present in the iron ore samples; SiO2 (24.64%), Al2O3 (2.24%), MnO (0.66%), CaO (0.086%), V2O5  (0.03%), P2O5  (0.03%) and Fe2O3(64.05%). From the XRD analysis, the dominant iron bearing mineral is the hematite which account for 71%, followed by quartz which is 21% while magnetite is 8% as the lowest mineral. The recovered iron content of the beneficiated samples ranges from 59.11% to 66.88% with an average of 62.99%. On the basis of the total iron content contained in the iron samples, a grade of 44.79% was determined which put the analyzed iron ore in a low grade class. From the discrimination plot, the analyzed ironstone plotted within the submarine –hydrothermal deposit and magnetite- silicate facies.

CHAPTER ONE

1.0      INTRODUCTION

1.1     Background to the Study

The most commonly used iron-bearing minerals contain iron compounds as follows: hematite, Fe2O3  (70% Fe), magnetite, Fe3O4  (72.4% Fe) and of much less importance are: limonite, 2Fe2O3.3H2O (60% Fe), siderite, FeCO3  (48.3% Fe); pyrite FeS2  (46.6% Fe). These iron percentages are in their pure state.

In  ores,  the  Fe  content  is  lowered  according  to  the  amount  of  impurities  present. Overall, the quality of iron ore is mainly judged based on the Fe content. More specifically, ores with Fe content above 65% are regarded as high-grade ores; 62-64% medium (or average) grade ores and those below 58% Fe are considered as low-grade ores.

Iron ore consumption for steelmaking was standing at 850million tonnes at the end of the twentieth century and was estimated to reach more than 1.3billion tonnes over the first quarter of the century (Katrak, 2008).The known world resources of crude iron ores are approximately 800billion tonnes containing about 230billion tonnes of Fe (Pollard, 2000). It is apparent that most of the known deposits contain low-grade ores with iron contents less than 30%. By contemporary growth the world consumption of iron ore (about 10% per year), the known resources of iron ores could run out within the next 64years. It is thus imperative to find new sources of iron ores to supplement the existing sources, in order to meet the growing demand. Therefore, revealing and exploiting new deposits of iron ores, particularly of high-grade is very important.

Iron constitutes 5% of the Earth’s crust, making it the fourth most abundant element. Iron oxides and hydroxides form the principal iron ore minerals, due to their high iron content and occurrence as large tonnage surface deposits. Almost 300 minerals contain some iron, but only a few are considered to be important ore minerals (Table 1). Nearly all of the mined iron ore (98%) is used in iron and steel production.

Iron is a tough, malleable, magnetic metal and forms important alloys with silicon, manganese, chromium, molybdenum, titanium, aluminium, carbon, nickel and tungsten to enable manufacture of a great variety of useful metal products (Sully, 1987). Steel, an alloy of iron and carbon, is the dominant metal commodity due to its unsurpassed versatility, relatively low production cost and availability of raw materials. Iron is also used in gas and water purification systems and in the manufacture of magnets, pigments (example; ochre), abrasive (for example; magnetite as a dense medium in coal washing) and high density concrete.

Iron  ore  may  be  defined  as  a  natural  material  of  suitable  grade,  composition  and physical quality that can be mined and processed for profit or economic benefit (Gross, 1993). Iron ore deposits are widespread and have formed in a range of geological environments throughout geological time. Pratt (1993) has divided iron ore deposits into four major categories based on their mode of origin, using aspects of pervious classification by Gross (1970) and Klemic et al (1973).

1.   Sedimentary (Banded iron formation, oolitic, placer, swamp)

2.   Igneous (magmatic segregations and skarn)

3.   Hydrothermal (proximal and distal)

4.   Surficial enrichment (laterite and surpergene)

1.1.1   Sedimentary Deposit

Sedimentary deposits, particularly those in Banded Iron Formation (BIF), contain the bulk of the world’s iron recourses. Sedimentary iron formations generally form in a variety  of  marine  environments  and  rarely  in  continental  environment  (Kimberely 1989). (BIF)-hosted deposits are almost exclusively of Precambrian age and distributed worldwide. An extensive body of data indicates that BIFs form by volcanogenic or hydrothermal effusive process (Gross 1993), but the origin of BIF-derived iron ore deposits is still debated widely (Morris 1988) and (Powell et al., 1999). The most recent publications on the Hamersely deposits suggested that post-depositional hydrothermal enrichment processes played a significant role in the formation of high-grade hematite ore bodies (Barlay et al.,1999) and (Taylor et al., 2001).   BIF are classified into two types ; The superior-type which formed in a near-shore continental shelf environment in type, which are associated with dolomite, quartzite and shale, and Algoma type, which are associated with volcanic (Edwards et al.,1986).

Oolitic deposits are Proterozoic to cretaceous in age and were an important source of iron ore before 1970. They are lower in grade (30-50% Fe) relative to BIF-hosted deposits (55-65% Fe). Two types have been identified; the Clinton type consists of deep red  to  purple  ores  composed  of  hematite,  chamosite  and  siderite  the  Minette-type consists of brownish to dark greenish-brown ores composed mainly of siderite and iron silicate (berthieriene and chamosite). These deposit formed in shallow marine environments and accumulated along passive continental margins during times of quiescence, extension and global sea level change (Van Houten et al., 1990).

1.1.2    Igneous Deposit

Igneous deposits are formed either by magmatic segregation of an immiscible magnetic rich melt in association with layered mafic-ultramafic inclusions or by injection of magnetite-rich fluids into surrounding rocks (example; Fe skarns). The former occur as massive cumulate-textured seams and are often mined for their economic concentrations of titanium and vanadium (e.g. Bushveld complex, South Africa). Fe skarns (or pyrimetasometic) are mainly derived from granitic to mafic intrusive and can be hosted in a variety of rock types. These deposits are massive irregularly shaped tabular bodies that continue to be a source of iron ore in some countries (e.g. Peru and Russia).

1.1.3    Hydrothermal Deposits

Hydrothermal  iron  ore  deposits  are  formed  by  the  circulation  of  heated,  iron-rich aqueous solutions of magmatic, metamorphic or sedimentary parentage. These deposits form the basis of the most iron oxide copper gold (IOCG) style deposit (Hitzman et al., 1992).  Proximal  hydrothermal  deposits  (also  known  as  volcanic  hosted  magnetic deposit) are essentially magnetite-hematite bodies that have replaced non-ferruginous host rocks (example; Kiruna iron ores). These deposit usually have obvious magmatic signature  and  adjacent  wall  rocks  are  generally  intensely  altered  (Pollard  2000). Hematite bearing quartz veins within fault zones are also part of this group. Distal hydrothermal deposits are tabular to podiform stratabounds specular hematite-magnetite bodies that formed by the enrichment of an iron-rich protolith.

1.1.4   Surficial Enrichment

Surficial enrichment of iron ore deposits result from subaerial weathering process of generally low-grade ferruginous protore, commonly BIF. Mature laterites develop under a wet tropical climate and can form extensive duricrust horizons (ferricrete), rich in iron oxyhydroxides. Supergene enrichment of low-grade iron deposits essentially leaches silica and other deleterious constituents and concentrates the iron oxide minerals to produce high-grade ore that can be directly shipped. Fine earthly hematite and iron oxyhydroxides such as goethite, limonite, and lepidocrocite are the principal iron minerals produced from surface and near surface enrichment processes.

This classification system is a useful guide but transitions exists between groups and most deposits have undergone more than one Fe enchainment phase. Many BIF-hosted deposits have undergone varying degrees of hydrothermal and supergene enrichment/alteration so as to produce high-grade ore (>60% Fe). The importance of surficial enrichment (Pratt 1993) has been overshadowed by recent studies on some iron ore province which suggested that enrichment of BIF is largely due to the activity of tectonically-induced hydrothermal brines ( Taylor et al., 2001).

1.4       Statement of the Research

This research will provide important background on the geology, geochemistry and beneficiation processes to the recovery of the iron content in the ironstone.

It will form a crucial part in assisting the government interest in the development of the metrological sector of the economy and provide a platform for further research.

1.5       Justification of the Research

A consultant geologist Oyewola .O. In 2019 reported that Nigeria can earn about 860 billion annually from its iron ore deposit during a sensitisation forum on the opportunity in solid mineral sector. To sustain the meteorological industry in Nigeria, the source of raw material must not be restricted to the iron ore in Itakpe deposit but also explore the ironstone in the various part of the country. Their percentage content and beneficiation method must be established.

The stratigraphy of the ironstone in Bida Basin is well known, some researchers also analyse the mineralogical content but the beneficiation of the iron content is yet to be proven. Taylor et al., 2001 described the ironstone at Jima in Northern Bida Basin to be high grade ore or with an average of 58.86 wt.

This research will therefore investigate the geochemical, mineralogical of the ironstone with emphasis on the beneficiation of the iron content to determine the recovery content of ironstone.

1.6       Aim and Objectives of  the Research

The aim of this research is to study the geochemical characteristics and processibility of the ironstone from the study area.

The objectives of the study include;

1.   Field work for location of ironstone beds and their sampling.

2.   Geochemical  analysis  of  the  ironstone  to  unravel  the  geochemistry  and processibility.

3.   Determine possible beneficiation processes to recover the iron content in the ironstone

1.7       Scope of the Research

The scope of this research is to analyse the percentage composition of the ironstone in central part of Bida Basin and to determine what percentage of the metal content that can be recovered from the ironstone in order to determine its industrial application.

1.8       Location and Accessibility of the Study Area

The field study was carried out in part of central Bida Basin from Doko to Kutigi and along Lemu Road on latitude N 080 56′ 43′ ′ to N 090 12′ 02′ ′ and longitude E 050 57′ 26′ ′ to E 050  36′04′ ′. The area is easily accessible through Minna-Bida Mokwa road. The area is well expose during dry season when grasses are usually dry.

Figure 1: Location map of study area

1.9       Climate and Vegetation

The climate of Niger state is like much of West Africa. The daylight temperatures vary from 24oc  the middle of the wet season to above 35oC at the peak of dry season. The seasonal rainfall regime gives rise to a longer wet season of about seven months with average rainfall of 250mm and a dry season of about five months with little or no rains at all. The climate is basically controlled by interaction of two widely different air masses as well as the movement of the zone of convergence of those air masses (known as the inter-tropical convergence zone ITCZ) relative to the ground. Dry season occurs between the month of November and March while the wet or rainy season runs from around April to October. The dry season is characterized by a high day time temperature of about 35oC, prominent north east wind and virtually no rainfall. As a result of the seasonal rainfall regime of the study area, there is a longer wet rainy season of about seven months with an average rainfall of 250mm and a short dry season of five months with little or no rainfall. The vegetation is characterized by grasses, shrubs and trees which makes it falls under the guinea savannah of Nigeria. The vegetation tends to be thicker along the river channels with concentration of mango and other tress.

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