Microwave-assisted and regular leaching of germanium from the germanium-rich lignite ash

2.1 Materials

The GA sample used in this work was obtained from Yunnan Province in PR China. The element composition of GA detected using X-ray fluorescence (XRF) technique is shown in Table 1; it can be seen that the main elements in the GA include Si, Al, S, Ca, Fe, K and Ge, and the accurate content of Ge in GA analyzed with chemical methods is 9263.2 g/t. The phase compositions that were analyzed with X-ray diffraction (XRD) are displayed in Figure 1, and it can be observed that GA consists of Al₂O₃, SiO₂, CaSO₄, Ca(PO₃)₂, KPO₃, and GeO₂. The microstructure and element distribution of GA were observed using scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) technique, as shown in Figure 2. It can be found that GA consisted of relatively smooth white particles with a size between 3 μm and 150 μm and amorphous powders with a size <20 μm. Specially, the EDS analysis results of point A and point B indicate that the Ge exists in two different types of phases. The oxidizing agent used in this research was MnO₂ (Tianjin Fengchuan Chemical Reagent technologies Co., Ltd.) in view of its industrial application in the future.

Figure 1:

XRD pattern of Ge-rich lignite ash (GA).

Figure 2:

SEM-EDS analysis of GA. (A) EDS spectrum of point A, and (B) EDS spectrum of point B.

2.2 Apparatus and procedure

Microwave-assisted leaching experiments are carried out using the micro-assisted leaching equipment. The equipment assembled in the Key Laboratory of Unconventional Metallurgy in Kunming University of Science and Technology consisted of high-pressure toughened glass reactor, NJL2-8 microwave oven (supplied by Nanjing Jiequan Microwave Development Co., Ltd), DSX-120 digital stirring (supplied by Hangzhou instrument electric co., Ltd.), thermoelement (supplied by Dongtai Xinrui Instruments Co., Ltd.), Erlenmeyer flask, and SHB-III circulating water vacuum pump (supplied by Zhengzhou Great Wall instrument co., Ltd.). The output power of the microwave was automatically controlled by the temperature. The schematic diagram of the micro-assisted leaching experimental setup is shown in Figure 3.

Figure 3:

Microwave-assisted leaching equipment diagram: (1) high pressure toughened glass reactor, (2) NJL2-8 microwave oven, (3) DSX-120 digital stirrer, (4) thermoelement, (5) conical flask, and (6) SHB-III circulating water vacuum pump.

2.00 kW was chosen as the microwave power output in this work. The solution temperature was measured by a thermoelement, which is plugged inside the reaction vessel in the 0°C–120°C range. A lid covered the vessel to ensure a tight seal and was connected with the stirrer, thermoelement and breather pipe, which was used to guide Ge tetrachloride vapor to the condenser pipe. The output power of the microwave oven can be continuously adjusted from 0 to 2.5 kW. When the solution temperature reaches the target temperature, the microwave-assisted leaching experiment begins. The SHB-III circulating water vacuum pump was opened to guide Ge tetrachloride vapor into condenser pipe and collect in conical flask after the leaching experiment was completed.

2.3 Exploratory experiment

Firstly, the glass reactor was loaded with 100 g of GA mixed with HCl and MnO₂; then placed in the microwave oven and heated to a certain temperature. Secondly, the solution was heated to 95°C after the leaching reaction. Thirdly, the circulating water vacuum pump was started and Ge tetrachloride vapor was collected in an Erlenmeyer flask through a condenser pipe. Finally, on completion of the experiment, the lixivium and the leaching residue were separated and the concentration of Ge was estimated by chemical analysis.

2.4 Calculation method of leaching rate of Ge

The leaching rate is calculated by following equation:

where X is the leaching rate of Ge, %; m₀ is the initial mass of GA in leaching experiment, g; C₀ is the content of Ge in raw material, g/t; m₁ is the mass of leached residue, g; and C₁ is the content of Ge in the leached residue, g/t.

3.1 Effect of leaching temperature on leaching rate of Ge

GeO₂ is reduced to metallic Ge in the process of microwave leaching of GA, and then Ge tetrachloride is evaporated into a gaseous phase and condensed on the low-temperature zone. The reaction between GeO₂ and HCl, and the standard Gibbs free energy (ΔG^o) are listed in Eq. (2) [23]:

The experiments were conducted in the temperature range from 40°C to 70°C; the concentration of HCl was 10 mol/l, the leaching time was 90 min while the stirring speed was 250 rpm. The relationship between the recovery of Ge and the reaction temperature is shown in Figure 4. The leaching rate increases with a temperature between 40°C and 65°C by the microwave-assisted method and it is similar from 40°C to 55°C by the regular method. The microwave treatment presented a higher leaching rate of Ge than that of the regular method (89.01% and 84.35%, respectively) when the temperature was 65°C.

Figure 4:

Effect of leaching temperature on leaching rate of Ge.

However, if the temperature is higher than 65°C, the leaching rate decreases, which may be due to the increased hydrolysis. When the system temperature is higher than 65°C, the HCl will decrease rapidly; parts of the HCl will evaporate and parts of the HCl will involve in fast reaction with other element at high temperature. The hydrolysis reaction is listed in Eq. (3):

The generated GeO₂ belongs to the residue, which will decrease the leaching rate of Ge. Similarly, the heating time of the regular method is longer than that of the microwave method; the higher the target temperature, the longer it takes. As a result, the hydrochloric acid concentrations are reduced, and Ge tetrachloride hydrolysis is reduced seriously.

3.2 Effect of leaching time on leaching rate of Ge

The effect of time on Ge leaching with microwave treatment and regular method was investigated from 30 to 150 min. These experiments were conducted at 65°C–55°C microwave-assisted/regular, and the stirring speed was 250 rpm and the concentration of HCl was 10 mol/l. The relationship between recovery ratio of Ge and time is shown in Figure 5. The leaching quality increases significantly from 30 to 90 min due to the crystal structure destroyed by microwave selectivity heating and emerges with more pores that increased the contact rate between Ge and HCl, and the similar phenomenon appears from 30 to 90 min when operated with regular method. However, in both methods, the leaching quality decreases greatly after 90 min.

Figure 5:

Effect of leaching time on leaching rate of Ge.

During the leaching process, some component including GeO₂ will react with HCl; moreover, HCl will volatilize at a relatively high temperature. As a result, the concentration of HCl will decrease. With the increase of chlorides and the decrease of the concentration of HCl, GeCl₄ is hydrolyzed to GeO₂, which will precipitate in the final slag.

Figure 5 shows that the optimal leaching rate of Ge with 90 min of microwave treatment is 88.59%, whereas the optimal leaching rate with 90 min of regular leaching is 83.62%. Water bath heating from 25°C to 55°C is needed for 20 min, and 2 kW microwave heating for only 2–3 min. It indicates that the heating time is reduced by as much as 90% and the leaching rate of Ge is enhanced by microwave leaching. The experimental result shows that leaching rate of Ge is enhanced and the experimental period distinctly shortened with the microwave method. It may be mainly attributed to the microwave selective heating and polarization effect, which destroy the crystal structure and emerge porously or react with additives to update the response interface, thus inducing increase in the contact rate between Ge and HCl [24], [25], [26], and the microwave absorption of water is extremely strong; the polar vibration of water molecules enhances the mass transfer, improves the kinetics of leaching, reduces the mass transfer resistance of HCl and products to enhance the effect [27], [28].

3.3 Effect of initial acid concentration on leaching rate of Ge

As the HCl solution is adopted as the leaching system, so the initial acidity is a vital factor in the process of Ge leaching. Figure 6 shows that the optimal concentration of HCl is 10 mol/l, and the leaching rate of Ge with microwave treatment is 4.33% higher than that with regular method (86.75% and 82.42%, respectively).

Figure 6:

Effect of initial acid concentration on leaching rate of Ge.

The leaching rate of Ge (84.24%) in 9 mol/l HCl with microwave treatment is even higher than that (82.42%) in the optimum 10 mol/l HCl with the conventional method. Some analyses conducted are as follows: firstly, the higher the acidity of the reactants the more favorable the leaching response balance moves to the resultant. In addition, an increase in the concentration of chloride ion as a coordination agent is beneficial to produce the coordination compound with chloride ion, which increases the leaching rate of Ge. In fact, the GeCl₄ is soluble in dilute HCl; with the increase in HCl concentration, its solubility increases. When the concentration of hydrochloric acid is up to 6–7 mol/l, GeCl₄ has the greatest solubility. If the initial concentration of HCl is too low, it will cause an incomplete reaction of Ge in the raw material; the resulting GeCl₄ is easily hydrolyzed to produce the GeO₂ precipitation [23], thereby reducing the leaching rate of Ge.

3.4 Effect of oxidizing agent amount on leaching rate of Ge

Due to some elements of the raw material existing in an elementary substance, there is a strong reducibility, making the oxidation-reduction potential of Ge higher, so the leaching process must have enough oxidants to oxidize these elementary substances [29].

Cl₂ gas is usually adopted as the oxidizing agent in the industries, but there are some potential safety hazards while using Cl₂ during the operation. In the experiments, the solid-state MnO₂ is chosen to replace Cl₂ gas. The amount of oxidizing agent is very important to oxide metals, which are in a state of incomplete oxidation during the leaching stage.

The effects of the amount of oxidizing agent on the leaching behavior of Ge are shown in Figure 7. It can be found that Ge leaching can be enhanced when the addition of oxidizing agent reaches 4 g. At the optimal leaching condition of this section, the leaching rate of Ge by microwave treatment is 6.12% higher than the regular method (88.36% and 82.24%, respectively). The leaching rate of Ge reduces with excessive oxidant due to the concentration of HCl changing with the adding of MnO₂. The chemical reaction given as follows will occur:

Figure 7:

Effect of oxidizing agent amount on leaching rate of Ge.

It can be observed that Cl₂ produced in this reaction, works as an oxidant, which influences the GeCl₄ solubility.

3.5 Effect of stirring speed on leaching rate of Ge

To observe the effect of stirring speed on the leaching rate, experiments were carried out using five different stirring speeds in the range of 200–400 rpm. These experiments were conducted at 65°C/55°C (microwave-assist/regular), leaching time was 90 min and the concentration of HCl was 10 mol/l. The experimental results that are given in Figure 8, show that both methods of the leaching quality increase inconspicuously from 200 rpm to 250 rpm, and leaching rate has slight fluctuation from 250 to 400 rpm; the leaching rate is nearly independent of the stirring speed. The 250 rpm chosen in this experiment is to prevent solid and liquid-state admixture to the container wall due to high stirring speed.

Figure 8:

Effect of stirring speed on leaching rate of Ge.

The experimental parameters are gradually optimized, and the optimal conditions of microwave-assisted leaching were as follows: leaching temperature of 65°C, leaching time of 90 min, oxidizing agent amount of 10 g/l, initial acid concentration of 10 mol/l, and stirring speed of 250 rpm. The maximum leaching ratio is 89.49%, which is significantly higher than that of conventional leaching method.

3.6 Characteristics of the residual slag after leaching process

The phase composition of leaching residue (leaching conditions: temperature 65°C, leaching time 90 min, HCl concentration 10 mol/l, stirring speed 250 rpm) is shown in Figure 9, and it can be found that the residual slag consists of SiO₂, Al₆Si₂O₁₃, and Al₂O₃. The other phases have reacted with HCl during leaching process. The residual GeO₂ in the reaching residue cannot be detected by XRD due to its very low content.

Figure 9:

XRD pattern of the residual slag after microwave-assist leaching process.

The microstructure and element distribution of leaching residue were observed with SEM-EDS, as shown in Figure 10, and it can be found that the microstructure of leaching residue is different from that of GA. The leaching residue mainly consists of amorphous powders with the size <30 μm. The peak strength of Ge is weak, which indicates that the content of Ge in the leaching residue is low, as shown in the EDS spectra of point A and point B. The microwave absorbing ability and thermal conductivity coefficient of various crystalline phases in the GA are different, and the temperature differences between different phases generated in the microwave-assisted leaching process will result in the local crushing stress and tension stress [30]. The formation of internal tiny cracks of slag particles increases the effective reaction interface area and therefore promotes the rate of leaching reaction [31].

Figure 10:

SEM-EDS analysis of the residual slag after microwave-assisted leaching process. (A) EDS spectrum of point A, and (B) EDS spectrum of point B.

3.7 Leaching behavior of the impurity ions from the GA

The leaching behaviors of the impurity ions such as Al³⁺, Ca²⁺, Fe³⁺, K⁺, etc. in the microwave-assisted leaching process were analyzed with XRF, and the element composition of the leaching residue (leaching conditions: temperature 65°C, leaching time 90 min, HCl concentration 10 mol/l, stirring speed 250 rpm) is shown in Table 2. It can be deduced that the S, Ca, Fe, Zn and Pb-containing phases are extensively dissolved. In contrast, the Si, Al and K-containing phases are not soluble in HCl system.