Processing of Yandev quicklime for potential amelioration of acidic soil

This study focused on processing of Yandev quicklime for potential amelioration of acidic soil. It involved production of quicklime from the Yandev limestone, characterization and slaking of the quicklime. In a batch process, 10g of the limestone (90  m particle size) was calcined for 3hrs to produce the quicklime. Mineralogical composition of the quicklime was determined by X-ray Diffractometer (XRD), while scanning electron microscope (SEM) was used to examine its surface morphology. The CaO was hydrated for the production of slaked lime (Ca (OH) 2). The slaking process was carried out by digesting CaO in distilled water. During the slaking/hydration process, values of reactivity (rise in temperature) were recorded. Central composite design (CCD) tool of Design Expert Software 11 was used to design the experiment of the slaking process. Quicklime/water ratio, particle size and time were the considered slaking variables, while reactivity was considered as the response. Analysis of the results quicklime is made up of pure calcite with visible pores. Quadratic model adequately described the relationship between reactivity and the considered slaking factors of quicklime/water ratio, particle size and time. Optimum reactivity was obtained as 58.4 0C with the corresponding optimal factors of quicklime/water ratio (0.26 g/ml), particle size (93.0 µm) and time (16.4 minutes). Properties of the slaked lime showed that it is suitable for acidic soil amelioration.


Introduction
Most soils in sub-Sahara Africa are acidic [1]. The acidic soil causes poor plant growth. This problem can be solved by soil amelioration. Acidic soil amelioration is a process of reducing the acidity of the soil, thereby improving its quality. Reduction of soil acidity can be achieved through liming. Liming is defined as the application of lime on acidic soil. It is an agricultural practice that reduces soil acidity by counteracting the effects of excess H + and Al 3+ ions [2]. Liming is a necessary practice that enhances soil productivity. It improves plant root environment and is a basic condition for increasing crop yield. Acidic soil treated with lime can produce high crop yields; reducing acidity, toxic effect of Al 3+ and Mn 2+ and providing Ca and Mg to plants. Liming is a viable technique for treating acidic soil [3,4,5,6].
Soil pH is a measure of the number of hydrogen ions in the soil solution. It is an outstanding chemical indicator of soil quality. Farmers can improve the soil quality of acid soils by liming to adjust pH to the levels needed by the crop to be grown. Benefits of liming include increased nutrient availability, improved soil structure, and increased rates of infiltration. Understanding soil pH is essential for the proper management and optimum soil and crop productivity. Nigeria is blessed with numerous limestone deposits, which include that of Yandev. Yandev is a Town in Gboko Local Government Area (LGA) of Benue State, Nigeria, Africa. Limestone mining and cement production at Yandev, Nigeria commenced in 1980 [7]. The limestone and its burnt derivative lime are very important raw materials for the industry, enhancing the developmental economy of Nigeria [8]. The limestone exposed at Yandev quarry is a component of the sedimentary fill of the Benue trough. Though, Yandev limestone has largely been used for cement production, its application on lime production has not been fully developed. Thus, there is need to process Yandev quicklime in order to obtain slaked lime for potential acidic soil ameliorant

Production of Quicklime from Yandev Limestone
Quicklime (CaO) was produced from Yandev limestone by calcination process. In a batch process, 10g of the limestone of 90m particle size was weighed into pre-weighed empty crucible plate. The pre-weighed crucible plate with the limestone was set to laboratory furnace and heated at temperature 1000 0 C. The sample was removed at time of 3 hours. Then, it was allowed to cool for 15 minutes. The calcined sample was transferred to desiccator. The weight of the quicklime produced was measured.

Determination of Mineralogical Composition
Mineralogical composition of Yandev quicklime was determined by X-ray Diffractometer (XRD). Method used by previous researchers [9,10] the mineralogical composition of the sample. The X-ray diffraction pattern was taken using Empyrean Pan Analytical. The sample was analyzed using reflection transmission spinner stage using the theta-theta (X-ray beams at certain angles of incidence) settings. Two-theta (2Ɵ) starting position was 4 degrees and ends at 75 degrees with a two-theta step of 0.026261 at 8.67 seconds per step. Tube current was 40 mA and the tension was 45VA. A programmable divergent ship was used (with width mask) to determine the mineral content of the quicklime.

Determination of Surface Morphology of the Quicklime
Scanning electron microscope, (Phenom Pro X-ray, phenom world Emdhoven Netherlands) was used to examine the surface morphology of the quicklime, in accordance with the technique used by previous author [10]. The scanning electron microscope (SEM) produced the image of the quicklime sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample.

Slaking of the Yandev Quicklime
Method used by previous author [11] was employed in the slaying of Yandev quicklime. The quicklime (CaO) was obtained by calcination of Yandev limestone. Then, the CaO was hydrated for the production of slaked lime (Ca (OH) 2). The slaking process was carried out by digesting CaO in distilled water. After being dissolved, spontaneous chemical reaction (exothermic reaction) occurred between CaO and distilled water. During the slaking/hydration process, values of reactivity (rise in temperature) were recorded. The reactivity measurement continued till the reaction was completed. Central composite design (CCD) tool of Design Expert Software 11 was used to design the experiment of the slaking process. Quicklime/water ratio, particle size and time were the considered slaking variables, while reactivity was considered as the response.

Determination of Activation Energy and pH
The activation energy of the reactivity was determined using Arrhenius model [12,13], Equation 1, while pH meter was used to record the pH of the slaked lime.
Considering different temperatures of the slaking process, the linear form of the Arrhenius equation was used to obtain the activation energy, Equation 2: Where K is rate constant, Ko is pre-exponential (frequency) factor, T is absolute temperature, R is universal gas constant (8.314kJ/kmol.K), and Ea is the activation energy, K1 and K2 are the rate constants at T1 and T2 respectively.

Mineralogical Compositions of the Quicklime
The mineralogical composition of Yandev quicklime, as determined by XRD, is shown in Figure 1. It revealed that Yandev quicklime is made up of pure calcite. This is an indication that the Yandev quicklime is suitable for versatile applications [14,15,16].

SEM Analysis of the Quicklime
The scanning electron microscopic analysis of Yandev quicklime is shown in Figure 2. The surface morphology of the quicklime was revealed. The micrograph showed that the particles are packed together in powdered form with visible pores. Revealed visible pores indicate that Yandev quicklime has good hydration characteristics.

RSM Results of the Slaking Process
The RSM results of the reactivity of Yandev quicklime are shown in Table 1. The results showed the effects of the interactions among the factors of quicklime/water ratio, particle size and time on the reactivity of the quicklime. In the results of 20-run experiment, the maximum reactivity values were observed around the mid-points of the factors of quicklime/water ratio, particle size and time of the slaking process. It depicts parabolic relationship between response and the considered factors [13].
It was also revealed that minimum reactivity values were recorded at the extreme points of the considered factors. In The lowest reactivity of the quicklime occurred at the highest particle size. It showed that reactivity increases with increase in surface area (decrease in particle size). Minimum reactivity of the quicklime was obtained at lowest quicklime/water ratio, highest particle size and lowest time of slaking. This observation is in agreement with the previous reports [11,17,18].

Mathematical Model of the Reactivity of the Slaked Lime
The mathematical models of the reactivity of Yandev slaked lime is as expressed in Equation (3). The models (in terms of significant terms) can make adequate predictions about the response for given levels of each factor, and they are useful for identifying the relative impact of the factors by comparing the factor coefficients. As revealed by the analysis of variance, each model adequately described the relationship between the reactivity and the factors of quicklime/water ratio, particle size and time. Thus, the reactivity is a function of quicklime/water ratio, particle size and time. The positive signs in the model signified synergistic effect, while the negative signs signified antagonistic effect [19]. As such, there is a synergistic effect on the interaction of quicklime/water ratio and particle size on the model. On the hand, the negative sign of the coefficients of AC indicates antagonistic effect of the interaction of quicklime/water ratio and time on the model. The highest power of at least one of the variables is two, which showed that the mathematical model is a quadratic equation.

Graphical Analysis of the Reactivity of the Slaked Lime
Graphical representations of the quicklime slaked lime are presented in Figures (3 -6). Plot of predicted versus actual yield was used to test the performance of the generated model. It gave linear graph, with the points clustered along the line of best fit. The 3-dimentional surface plots showed the relationship between the factors and response of the process.
The revealed the optimum reactivity as 58.4 0 C with the corresponding optimal factors of quicklime/water ratio (0.26 g/ml), particle size (93.0 µm) and time (16.4 minutes).

Validation of the Result of the Slaking Process
Data for the validation of the results are presented in Table 3. The experimental result was validated by the determination of percentage deviation of experimental reactivity from the predicted reactivity. The percentage deviation is less than 5%, an indication that RSM is adequate for the optimization of the slaking process. It is also an affirmation that the generated model sufficiently described the slaking process [13].

Rate of the Reactivity, Activation Energy and pH of the Slaked Lime
The rate of the reactivity of Yandev slaked limes is shown in Figure 7. The graph showed the relationship between reactivity and time at various temperatures. The reactivity versus time plot revealed a straight line graph. The straight line graph was confirmed by linear equation obtained by Trend line Function of Microsoft Excel. The correlation coefficient (R 2 ) is close to 1. The rate constant was useful for the determination of the activation energy using Arrhenius Equation [12,23,20].  Table 4 presents the activation energy and pH of Yandev slaked lime. The activation energy of slaked lime sample was obtained using Arrhenius law and it was observed that the reaction temperature has a direct effect on the rate of reactivity. This observation corroborates with the assertion of previous author [20]. Yandev has moderate activation energy of 50.42 kJ/mol, indicating that it requires moderate energy to activate its reactivity. High pH of 11.1 showed that Yandev slaked lime is suitable for acidic soil amelioration.

Conclusion
XRD and SEM analyses revealed the characteristics of the Yandev quicklime. The quicklime is made up of pure calcite with visible pores that make it suitable for hydration and versatile applications Quadratic model adequately described the relationship between reactivity and the considered slaking factors of quicklime/water ratio, particle size and time. Optimum reactivity was obtained as 58.4 0 C with the corresponding optimal factors of quicklime/water ratio (0.26 g/ml), particle size (93.0 µm) and time (16.4 minutes).
Yandev has moderate activation energy of 50.42 kJ/mol, indicating that it requires moderate energy to activate its reactivity. High pH of 11.1 showed that Yandev slaked lime is suitable for acidic soil amelioration.