The Gboko Limestone, Yandev, Benue State, Nigeria: Geology, Geochemistry and Industrial Potentials

The Cretaceous shallow marine Gboko limestone, Yandev, Nigeria is a component of the sedimentary fill of the 800 km NE-SW trending Benue Trough, Nigeria. The limestone is made up of thin bedded to massive limestone beds interspersed with laminated grey shale having foraminifera as the dominant fossil. The limestone has both mud supported and grain supported texture, and micrites constitute about 75% of the limestone. Bulk chemical composition analysis of the limestone reveals average CaCO3 of 92.41% and a range of 77.50% 99.00%. Mineralogical impurities include quartz, dolomite, pyrrhotite, fluorapatite etc. Trace elements concentration analysis was carried out using Energy Dispersive X-ray (EDXRF) spectrophotometry and showed the following trace elements: Mn (841.3 ppm), Sr (444.6 ppm), Fe (470 ppm), Zn (114.6 ppm) and Pb (116.4 ppm). Calcining the limestone in a laboratory muffle furnace at 1050 ̊C for 90 minutes produced a compact, soft burnt porous and reactive lime that does not crumble into fines. The lime so produced neither meets the requirements of the Steel Making Shop (SMS) of the Ajaokuta Steel Plant nor could it be used in the growing sugar refining industry in Nigeria. It can however be used in the food and the food by-products industry, environmental, agricultural and petroleum industries etc. The raw stone remains a major source of raw materials for cement manufacture for the ever expanding building industry.


Introduction
Nigeria in the comity of nations is rightly tagged developing country, a third world economy. The nation is definitely desirous of shedding this "developing" toga and putting on the enamored appellation of developed economy. In fact she wants to be among the top 20 developed economies by the year 2020. Attainment of this goal can only be via industrialization. Is it realizable? Yes, we can. There are abundant human and mineral resources. Many nations that do not have raw materials but have depended solely on importation have attained industrialized status. In this regard Japan and South Korea are noteworthy. But a situation where the raw materials are abundant as is the situation in Nigeria, only what is needed is the political will and focus on the part of government so as to harness the resources, exploit and utilize the raw materials to produce goods and services to uplift the standard of living and boost the Gross Domestic Product (GDP). This calls for very sound economic and political leadership to put right the priorities of the nation and acquire the needed technological expertise to utilise the abundant resources to produce goods both for export and local consumption. Nigeria is a vast market for diverse goods and services and given adequate time and focus will be satisfied and room for export created.
Limestone and its burnt derivative lime are very important raw materials for the industry. Lime on its own has more than two hundred applications and is reputed to be the largest bulk industrial chemical. Limestone is of course a sinequanon in the making of cement-a very vital ingredient in the construction industry especially housing, bridges and factories. The task ahead is to examine the role this monomineralic rock specifically the Gboko limestone can play in enhancing the developmental economy of Nigeria.

Geology of Gboko Limestone, Yandev, Benue State, Nigeria
The Benue Basin is an elongate NE-SW oriented sedimentary rift that extends inland for approximately 800 km from the Nigerian coastline. The Gboko limestone (Figure 1) actually exposed at Yandev quarry is a component of the sedimentary fill of the Benue trough. The limestone is Cretacious in age and is regarded as being part of Asu River Group of [1]. The Bima Sandstone forms the basal fill of the basin and consists of the braided river, lacustrine and deltaic clastics. [2] has given a detailed description of the stratigraphic sequence of deposition at the Yandev quarry. The rock unit consists of sandstone, limestone, and shales that were laid down during the initial phase of the first tectonic cycle of [3]. The section is made up of thin bedded to massive limestone bed interspersed with laminated grey shale. Foraminifera dominate the fossils. The limestone can be divided into lower and upper units. While the lower unit is composed of limestone with minor shale parting the upper unit is made of shale/limestone alternations. [2] reported that the limestones have both mud supported and grain supported textures in a ratio 3:1 and that micrites constitute 75% of the limestone. While the lower unit is composed of limestone with mudstone-wackestone texture and oncolite and mollusk packstone the upper unit is  an alternation of limestone with mudstone-wackestone-packstone texure and shales. The limestone is inferred to have been deposited in a shallow coastal hypersaline lagoon. The lagoon is interpreted to have been formed from marine incursion into a localized lacustrine body during the first transgression into the Benue Trough.

Geochemistry
The major elements abundance in the representative samples of the limestone were determined using Atomic Absorption Spetrophotometry (AAS). After digestion of the samples the concentration of Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, and S in the limestone samples were measured with a single element light source.
Calibration for each element was achieved using four AAS standard solutions of known concentrations. Elemental values were recalculated to oxides using appropriate conversion factors. Summary of the chemical results is presented in Table 1. Determination of trace elements concentrations in the samples was carried out using Energy Dispersive X-ray Fluorescence (EDXRF). Pellets 19 mm diameter were prepared from ground samples of less than 125 microns. Then measurements were performed using an annular 25mCi 109 Cd as the excitation source that assists Ag-K X-rays in which case all elements with lower characteristic excitation energies were accessible for detection in the samples. Quantita-  The low Sr content of ancient limestones like the Gboko limestone cannot be due to the variations in the mSr/mCa ratio of sea waters from which the limestones were precipitated for the mSr/mCa ratio of sea waters has remained remarkably constant throughout the Phanerozoic era ([4] [5]). Although ancient limestones are considerably depleted in Sr they exhibited a wide range of Sr content from a low of 50 ppm to a high of 1000 ppm [5].

Mineralogy
X-ray diffraction analysis was done on pulverised (1 micron) bulk samples of the limestone using Philips P.W 1800 X-ray diffractometer. Cu-K radiation was em- because very many applications of the limestone is in the form of lime prepared from it. Lime manufacturers choose quality raw stone because quality of the lime is dictated by quality of the limestone and by its manufacturing processes. A compact highly reactive lime is desired by the industries. A general requirement for commercial lime production is a rock with minimum of 95% of CaCO 3 [7] but

Crystal Structure
The On the other hand, the fine grained limestones can easily be calcined to form lumps of lime because the crystals resist the temperature stress .The Gboko limestone is fine-grained ( Figure 3) and is therefore favourably disposed to calcination into lumps of lime.

Calcination
( ) 3 2 CaCO CaO lime CO ∆  → + As hinted earlier, calcination is an important factor in quality lime production.
The objective always is to produce a compact, softburned, highly reactive lime.
To achieve this, the limestone has to be calcined in laboratory muffle furnace through a range of temperatures at varied retention times and the resultant lime then tested technologically for strength and reactivity to determine the best burning parameters. Samples of the Gboko limestone trimmed to size (2 cm Generally larger stone sizes are more difficult to calcine uniformly and require more time. This is because dissociation always penetrates gradually from the surface to the interior of the stone [8]. To expel the CO 2 from such large stone high temperatures are necessary to generate sufficient CO 2 pressure in the interior of the crystal lattice for the escape of the gas. Frequently these high temperatures overburn the surface of the stone causing excessive shrinkage, which narrows and closes the pores. [9] prescribed that the ideal stone to calcine for optimum quality, uniformity and thermal efficiency would be of small size (1.25 cm) and be of uniform size and shape.

Temperature and Retention Time in the Furnance
Since the temperature and time can easily be controlled, they were varied so as to determine the best calcination parameters for the Gboko limestone by testing the quality of the lime product using technological tests such as weight loss, decrepitation, loss on ignition of lime, mechanical strength, and reactivity and by inference porosity and surface area of the lime. The calcination duration was 60, 90, and 120 minutes.

Weight Loss and Loss on Ignition of Lime
After calcining at the desired temperature and time, the lime products were allowed to cool to room temperature in a dessicator before weighing accurately to  (Table 3).

Decrepitation and Mechanical Strength
The various calcines 950˚C/60minutes, 1000˚C/90minutes etc were cooled to room temperature in a desicator then poured onto 1.18 sieves and shaken gently.
The quantity of minus 1.18 sieve material generated was measured and expressed as percentage of the original weight are then expressed as percent decrepitation (Table 3).
Screen analysis was carried out on the calcined stone. After shock calcining the samples at the appropriate temperature the lime products were subsequently  The less than 0.6 mm sieve material for Gboko limestone is less than 1.0 grammes. This is not significant compared to 5.6 gm of Jakura marble which generates fines. The result is an indication that the lime will not generate fines in the kiln and during handling. gives an indication of the tendency of the stone to decrepitate in the kiln [9]. The result is presented in Table 4. From the test one can see that Gboko limestone does not show any tendency to decrepitate in the kiln judging from the minus 0.6 mm material.

Reactivity of the Gboko Lime
The maximum temperature reached through the exothermic reaction of quicklime with water is a good indicator of the quality of the lime in terms of the available CaO [10]. The ASTM specified the use of 100 gm of lime in 400 ml of distilled water at 25˚C. The sample was prepared to pass 2.36 sieve as quickly as possible and then left in a desicator to cool to room temperature before slaking test using a thermos flask, thermometer, and a stop clock, and a rod as stirrer, a pestle and a mortar for crushing the quicklime. Stirring was done continuously and reading of temperature taken at 30 seconds interval. Readings were recorded until less than 0.5˚C temperature change was noted in each of three consecutive readings were taken. The initial temperature was subtracted from the final temperature to obtain the total temperature rise. Also the initial temperature was subtracted from the temperature at 2 minutes to obtain temperature rise in 2 minutes (Table 5). Suitable curves showing temperature rise as the ordinate and time as the abscissa were then plotted ( Figure 4). The results were reported as A. B. Ofulume et al. Optimum reactivity is defined as >40˚C rise after 2 minutes; Therefore 1050˚C/90minutes calcine yielded a highly reactive lime that is a softburned, porous lime with high surface area and that does not decrepitate.

Results and Discussion
All parameters did not meet the specification except CaO (Table 6). Gboko limestone cannot be used in the growing Nigerian Sugar industry.  95.5% and silica less than 1.0% (Table 8).
From Table 8, the sulphur content of Gboko limestone (0.20%) far exceeds the less than 0.05% required for steel making at Ajaokuta. Consequently this limestone will not be used in the Ajaokuta steel industry, Nigeria.

Environmental Use of Gboko Lime
Lime is useful in the sewage biosolids and sludges. Both quicklime and hydrated lime have been in use for biological organic wastes for more than 100 years.
Lime neutralizes acid wastes generated in industry thereby impeding corrosion and protecting the natural environment. Lime removes silica, manganese, fluorides, iron and other impurities from water. Most importantly lime is used in the treatment of drinking water, softening, pH adjustment, coagulation. The Gboko lime can be of use in the treatment of hazardous wastes currently generated or abandoned materials for example, the hazardous waste that was imported and dumped in Koko, Delta state, Southern Nigeria.

Gboko Lime in Food Industry
In the dairy industry, lime water is often added to the cream separated from whole milk so as to reduce acidity prior to pasteuralization when butter is produced. Lime is also beneficial in preparation of the common types of baking powder, and the preservation of fruits and vegetables. The need for the setting up of dairy industry in the northern states especially Benue state that prides itself as the foods basket of the nation is highlighted with Gboko lime as adequate and available compliment for this food industry ( Figure 5).

Gboko Lime for Agricultural Purposes
Gboko lime or the ground limestone can be useful in pH adjustment of agricultural soils, but the focus seems to be the use of lime in conjuction with nitrogen fertilizers because the lime allows reduced usage of fertilizers resulting in reduction of nitrogen leaching from the soils. Gboko lime can also serve useful purpose in compositing, poultry litter and pesticides and certain fertilizers ( Figure   6).

Gboko Lime in Petroleum Industries
Various

Gboko Limestone for Cement Making
The Gboko limestone has for many years been the source of limestone for quality cement manufacture at the Benue cement factory at Yandev. The output capacity of the factory has received a tremendous boost by the acquisition of the factory by the Dangote Group. The Group presently is the greatest cement maker in the whole of Africa and owns cement factories in many African countries.
Cement making by and large remains the major use of the Gboko limestone, and the diversification of its application is called for with the introduction or incoporation of lime manufacturing facilities in the factory at Yandev. Table 9 shows the average chemical composition of Gboko Limestone.

Conclusions
The Cretaceous Gboko limestone is made up of thin bedded to massive limestone beds interspersed with laminated grey shale. It has foraminifera as the do- However, it can be utilized in the food and the food by-products industry, environmental, agricultural and petroleum industries. Cement manufacturing by Dangote cement remains the main user of the raw stone.