Optimization of microwave-assisted manganese leaching from electrolyte manganese residue

2.1 Materials

The EMR used in this work was supplied from Guizhou Wuling Manganese Industry Co. Ltd., and before use its composition was analysed by X-ray fluorescence (XRF, XRF-1800, Shimadzu, Japan), with the results presented in Table 1. The results show the main constituents to be SiO₂, SO₃, Al₂O₃, CaO and Fe₂O₃amounting to approximately 98% of the total composition.

Figure 1 shows the XRD pattern of the EMR (D/Max 2200, Rigaku, Japan), indicates presence of quartz (SiO₂), gypsum (CaSO₄·2H₂O), groutite (Mn³⁺O(OH)), ammonium iron sulfate (NH₄)₃Fe(SO₄)₃, jacobsite (MnFe₂O₄) as the major minerals. The particle size distribution of the EMR is presented in Figure 2 (Master Sizer 2000, Malvern, UK.). It can be seen the average particle size of EMR is 21 μm and that 90% of the particles are smaller than 47 μm while 10% are smaller than 10 μm.

Figure 1

X-ray diffraction pattern of EMR.

Figure 2

Grain size distribution curves of EMR.

2.2 Experimental procedure and apparatus

The leaching tests were carried out in a self-made microwave reactor which had a temperature and stirring speed control system (Figure 3).

Figure 3

Schematic diagram of microwave reactor (1 – reflux condenser, 2 – three-necked flask, 3 – mechanically controlled stirrer, 4 – stirring controller, 5 – digital thermometer, 6 – LCD screen, 7 – temperature setting knob, 8 – power switch, 9 – emergency switch, 10 – microwave power setting knob).

First, the sulfuric acid solution was poured into a three-necked 500 mL flask reactor. Then, a certain amount of EMR sample was added to it. Following this, the flask was placed into the microwave reactor and the temperature raised to the desired value. The agitation speed and microwave power were kept constant at 300 rpm and 200 W respectively for all experiments. After the required duration was met and the solid and liquid separated, the concentration of manganese in filter liquor was determined by atomic absorption spectrometry (Z2000HITACHI). The percentage of manganese leached (E_m) was calculated according to the following equation:

where c and M₀ are the mass of Mn in the leaching solution and the original sample, respectively; V is the volume of the leaching solution after filtration.

2.3 Experimental design

At present work, four important parameters such as leaching temperature (X₁, °C), leaching duration (X₂, min), H₂SO₄ concentration (X₃, mol·L⁻¹), and dosage of citric acid (X₄, mL/g) were chosen as the independent variables. The low, middle, and high levels of each variable were identified as −1, 0, and +1 (Table 2).

The above stated upper and lower limits of the variables were selected based on preliminary experiments in addition to information gathered from literature sources. In order to evaluate the effects of independent variables on percentage of Mn extracted, batch tests were carried out. The coded values of the four significant parameters were generated based on Eq. 2:

where x_i is defined to be a dimensionless value of an independent variable, X_i is the real value of variable, X₀ is the real value of an independent factor at the center point, and ΔX_i is the step change.

The statistical models and data analysis were given using Design-Expert 8.0.6 software (trial version). The design is depended on the combination of 2^k factorial design with an incomplete block design. Total numbers of experiments required in a Box-Behnken design can be calculated according to the Eq. 3 [21]:

where N is the number of experiments, k is the factor number (k = 4), and (cp) is the replicate number of the central point. It resulted in 24 experiments with three repetitions at the center point to estimate the pure error. The generalized form of model equation relating the X’s and Y is given by Eq. 4:

where Y is the response, β₀ is a constant coefficient, X_i and X_j are the input factors, β_i, β_ii and β_ij are linear, quadratic and interaction coefficients, respectively, e is the random error. The quality of the fit of the model would be evaluated by ANOVA.

3.1 Data analysis

The number of experiments generated utilizing Box-Behnken design are listed in Table 3, which comprehensively provides the process conditions at which experiments were conducted along with the outcome of the experiment ‘Percentage Mn recovery’.

Different empirical models were attempted to relate the process variables and independent variable subjecting it to statistical analysis. Among the models tested the quadratic model was found to be the best and the regression model is expressed as Eq. 5:

where Y is the percentage Mn extraction, A is the reaction temperature, B is the reaction time, C is the H₂SO₄ concentration, D is the dosage of citric acid.

The adequacy of model to utilize the statistical methods was established by using diagnostic plot (normal % probability versus studentized residuals, studentized residuals versus run number). The probablity plot authenticates data to be considered as of normal distributed while the studentize plot helps to identify the outliers.

Figure 4a shows all the data falling on line except one authenticating that the sample data is from a normally distributed population. Figure 4b shows possibility of the one of the data could be an outlier. Figure 4c authenticates the validity of the model equation (Eq. 5), as the model prediction is very close to the experimental data [22]. The adequacy of the model was investigated using the sequential model sum of squares and the model summary statistics are illustrated in Table 4.

Figure 4

Diagnostic plot the model for manganese leaching: (a) normal % probability versus studentized residuals, (b) studentized residuals versus run number, (c) predicted versus actual data.

As seen from Table 4, the value of regression coefficient for quadratic model is 0.9893. The R2adjand predicted R² are 0.9767 and 0.9404 for the quadratic model, respectively. The high R² shows the proximity of the model prediction to the experimental data. In general, the difference between adjusted R² and predicted R² is considered to be good fitness within the range of 0~0.200 for the model [23]. At present work, the difference between the predicted R² and the adjusted R² for the given quadratic mode is only 0.0363. Adequate precision is a crucial criterion to judge the signal to noise ratio. A ratio greater than 4 is preferable, and the ratio of 29.122 (adequate precision) indicates an adequate signal [24]. Based on all the favorable facts detailed above it can be concluded that the model adequately fits the experimental data.

Table 5 shows the analysis results obtained from the ANOVA test for fitting quadratic model. The Model F-value 78.98 implies that the model is significant. There is only a 0.01% chance that such a large “Model F-value” will appear due to noise.

The significance of each coefficient was investigated based on F-test and P-test also as shown in Table 5. According to the F values of A, B, C and D, the influence degree of the four variables on manganese extraction by microwave assisted leaching are illustrated in Table 6.

A values of “Prob > F” less than 0.0500 imply proposed model is significant, while a values greater than 0.1000 indicate the proposed model is non-significant [25]. In this case A, B, C, D, AC, AD, BC, A², B², C², D² are significant model terms. It indicates the linear effects and square effects of temperature, H₂SO₄ concentration, dosage of citric acid and time are significant. In addition, the interactive effects of temperature and H₂SO₄ concentration (P = 0.0002), temperature and dosage of citric acid (P = 0.0184) and time and H₂SO₄ concentration (P = 0.0138) have an adequate influence on the extraction of manganese by microwave assisted leaching. Compared to the value of pure error, the “Lack of Fit F-value” of 3.17 indicates that the Lack of Fit is not significant. At the same time, there is a 26.35% chance that a “Lack of Fit F-value” this large could appear owing to noise, non-significant lack of fit is desired to ensure the high quality of the fit of the model [26].

3.2 Contour plots and response surface

The effects of variables on the manganese leaching efficiency is assessed by plotting 3D surface curves against any two independent variables, while keeping other variables at their center value. The 3D response surface plots and the contour plots from the interactions between the variable are shown in Figures 5 and 6.

Figure 5

Response surface plots show the effect of temperature (X₁), time (X₂), H₂SO₄ concentration (X₃) and dosage of citric acid (X₄) on % manganese extraction with microwave irradiation.

Figure 6

Contour plots show the effect between temperature (X₁), time (X₂), H₂SO₄ concentration (X₃) and dosage of citric acid (X₄) on manganese extraction.

Figure 5a demonstrates the combined effect of temperature with reaction time. It is clear from Figure 5a that % leaching increases significantly with increasing temperature. An increase in temperature from 40°C to 80°C increased the percentage leaching remarkably from 65.02% to 93.61%. The corresponding contour plot is shown in Figure 6a The combination of the response surface along with the contour plot helps to identify the combination of time and temperature that could provide the maximum percentage recovery. The combined factors reach an asymptote at % Mn recovery higher than 90%. An increase in the percentage Mn extraction with increase in temperature could be attributed to the higher rates of mass transfer due to increased diffusion coefficient. While an increase with increase in time could be due to the time available to complete the extraction.

Figure 5b shows the combined effect of H₂SO₄ concentration and leaching temperature on the % leaching. As can be seen from Figure 5b manganese recovery increased with increasing both: temperature and H₂SO₄ concentration. However, at higher H₂SO₄ concentration effect of temperature on the percent recovery of Mn was significant, while at lower H₂SO₄ concentration only a minor effect was observed. An increase in the concentration of H₂SO₄ would increase the rate of reaction and due to reduced the mass transfer resistance aiding diffusion of H₂SO₄ to the site of Mn containing mineral phase, facilitating higher % leaching. Figures 5b and 6b help to identify the combination of factors that maximize the % Mn recovery.

Figure 5c illustrates the combined effect of citric acid dose with temperature. The effect of increase in citric acid concentration seems to be insignificant at low temperatures while at high temperatures it exhibits an increase with increase in concentration. Again, Figures 5c and 6c provide the combination factors that offer the maximum of % Mn recovery.

Figures 5d and 5e show the effect of H₂SO₄ and citric acid dose in combination with time on the percentage manganese recovery respectively. The effect of these parameters was discussed in detail earlier. The combination of H₂SO₄ concentration as well as time is found to have significant influence on the percentage Mn recovery while the combination of the citric acid dose and the time has only marginal effect on the percentage Mn recovery. Figures 6d and 6e provide the combination of factors that could maximize the % Mn recovery [27].

The interactive effect of H₂SO₄ concentration with citric acid dose is shown in Figure 5f It can be observed that an increase in both H₂SO₄ concentration as well as citric acid dose increases the percentage leaching. An increase in the amount of sulfuric acid affects the dissolution rate of manganese in leaching of EMR much more than the citric acid dose. Although the increase in the citric acid dose doesn’t seem to increase the extraction rate significantly, the increase in citric acid dose promotes conversion of high valence of Mn into low valence of Mn, inhibiting the leaching of other gangue minerals. In other words, citric acid offers not only high manganese recovery and selectivity of leaching but also contributes to reduced acid consumption rendering the process more environmentally benign. Above mentioned result is also revealed by the corresponding contour plots presented in Figure 6f

3.3 Optimization

Although the effect of process variables and combinations thereof that could maximize the percentage Mn recovery is evident from plots developed, the optimal combination of factors that would maximize the percentage recovery may be identified with the help of process optimization. Within the range of parameters assessed in the present work, the process optimizer identified the optimal conditions to maximize the percentage Mn recovery as a temperature of 77°C, time of 55 min, H₂SO₄ concentration of 0.76 mol·L^-1, dosage of citric acid of 3.51 mg/g, corresponding to a maximum Mn recovery of 94.25% with the desirability of 1. The pictorial representation of the optimized process parameters as shown the software is provided in Figure 7.

Figure 7

Ramps of the numerical optimization.

In order to verify the results of predicted optimum conditions and understand the role of microwave irradiation, the experiments were carried out at identified optimized process conditions under conventional heating and microwave fields, the results are shown in Table 7.

Table 7 revealed that about a half of the manganese retain in the leached residue under conventional leaching process. On the contrary, microwave-assisted leaching ensures the perfect recovery of manganese from EMR. This can be explained that the different microwave absorber character between manganese-bearing mineral and other components result in intergranular and transgranular thermal stress cracking, increasing the contact area between lixiviant and ores. Therefore, microwave-assisted leaching led to the higher recovery efficiency. Moreover, the experiment at identified optimized process condition was carried out in triplicates and the average value of 93.28% for Mn leaching efficiency is sufficiently agree with the predicted value. Hence, this validation confirms the adequacy of the developed quadratic model for Mn leaching.