Results and discussion
3.1. Characterization of Abu Dabbab sample
The XRD pattern of the raw sample shows the presences of several mineral phases dominated by plagioclase of sodic type (albite), quartz, mica and muscovite. The silicate minerals represent the main composition of the rock with little amount from metallic minerals. The present metallic minerals are cassiterite and rare earth mineral tantalite, Figure 3. Such mineral composition was reflected in the chemical composition of the sample.
The complete chemical analysis of the major oxides and trace element content of the sample appear in Table 2. The sample composed mainly of 60% SiO2, 20% Al2O3, 10% Na2O, and 5% K2O. Such oxides related to the presences of quartz, mica and feldspar minerals as major constituents. Minor amounts from cassiterite, iron, tantalum and niobium oxides are presented.
The successful beneficiation of albite and mica from other heavy minerals such as iron, tin, tantalum and niobium oxides depends on the big difference in specific gravity and their response to the magnetic field. The separation process starts with the gravity separation and then followed by the magnetic separation.
3.2.1. Shaking table separation
Shaking table separation is applied in order to separate light minerals such as albite and mica from heavy minerals such as iron oxides, cassiterite and rare earths due to the large difference in specific gravity between heavy minerals and the silicate minerals.
Shaking table separation is applied on the feed sizes -0.125 +0.080 mm. Three parameters are studied during the separation process. The parameters are namely; inclination angle, stroke length and water flow rate.
Applying Box-Behnken Experimental Design. Box-Behnken Design, as an experimental design technique, was utilized in order to optimize the recovery of albite and mica using shaking table separator and also to evaluate different parameters interaction. This allows the study of the effect of each factor stroke length, inclination angle and water flow rate, and the interactions effect between factors on the separation process.
According to the experimental design, the optimum conditions are estimated using a second order polynomial function by which a correlation between response and studied factors was achieved. The general form of this equation is: (Abd El-rahman et al., 2009; Youssef et al., 2009)
Y = bo + b1 X1 + b2 X2 + b3 X3 + b12X1X2 + b13X1X3 + b23X2X3 + b11 X12 + b22 X22 + b33 X32 (1) where Y is the predicted response; albite recovery %, X1, X2 and X3 are studied variables stroke length, inclination angle and water flow rate; ?ij are equation constants and coefficients.
The analysis of variance , ANOVA, was utilized to estimate the statistical parameters. The fitting extent of the experimental results to the polynomial model equation was expressed via the determination coefficient (R2 ). The F-test was utilized to estimate the significance of the terms in the polynomial equation within 95% confidence interval. The adequate Precision is used to measure the signal to noise ratio, a ratio greater than 4 is desirable and enhances an adequate signal. The data of ANOVA for the system illustrates the well fitting of the experimental results to the polynomial model equation and therefore the accuracy of this model, Table 3. The model F-value of 122 implies the model is significant. The “Adeq Precision” ratios of 35.5 indicates an adequate signal.
Figure 4a shows the shaking table separation with the optimum parameters of the Box-Behnken design for the size fraction (-0.125 + 0.080 mm) which are inclination angle (4 degree), stroke length (2.5 cm), feed rate (280 gm / min), and water flow rate (20 l / min), table 4, figure 5a. two fractions are obtained, a heavy fraction with valuable contents of Nb2O5, Ta2O5, SnO2 and Fe2O3. The other fraction is the light fraction which contains mainly albite, silica and mica.
3.2.2. Magnetic Separation
Magnetic separation was achieved for both shaking table heavy and light fractions in order to separate mica and albite from heavy minerals and from each other.
Applying Box-Behnken Design; In order to optimize the dry magnetic separation, factorial design is used. In this design, three factors are taken into consideration, namely; splitter inclination, belt speed, and feed rate.
The best optimum conditions are estimated using a second order polynomial function by which a correlation between response and studied factors was achieved using equation 1.
The data of analysis of variance , ANOVA, for the system illustrates the well fitting of the experimental results to the polynomial model equation and therefore accuracy of this model.
Figure 4b shows the response surface for the separation process at different values of the studied factors at maximum field intensity. Successful separation of albite from mica was achieved with high recoveries at low values of roll speed and feed rate. Slow feed rate and belt speed enables the mica particles to be separated efficiently from the albite particles at splitter inclination of 76 degree. On the other hand increasing both splitter inclination and belt speed more than 76 degree and 72 rpm respectively leads to a decrease in the separation efficiency of albite from mica.
The best optimum parameters of the Box-Behnken design of beneficiation using magnetic separator are: belt speed (72 rpm), feed rate (120 g/min) and splitter inclination (76 degree), table 4, figure 5b.
Figure 6 shows the suggested flowsheet for the beneficiation process for albite and mica from Abu Dabbab area. It is shown that albite and mica could be successfully separated from iron oxide and other heavy minerals using shaking table separation technique. These valuable contents of minerals would be subjected for further separation processes in order to separate them from each other for industrial applications. Then albite and mica are successfully separated from each other using RER magnetic separator technique due to the paramagnetic properties of mica (biotite form), table 5.
Figures 7, 8 show the X-ray diffraction of the magnetic and non magnetic fractions. It is obvious that the principle mineral included in the non magnetic concentrate is albite, with little content of silica. While the magnetic fraction mainly consists of mica (biotite form) with some rubidium oxide. The SEM images, figure 9, evidences the effective separation of mica flakes from albite.