Pretreatment of arsenic-bearing gold ore with microwave-assisted alkaline leaching

2.1 Raw materials

The mineral samples used in this work were gold concentrates obtained from Henan Province, China. The proportion of mineral grains with a diameter of <200 mesh is >80%. The main minerals in the refractory gold concentrate were arsenopyrite, pyrite, realgar, and quartz according to the X-ray diffraction (XRD) analysis, as shown in Figure 1. The grades of some of the important constituents of the concentrate were 18.5% S, 6.87% As, 20.2% Fe, and 60.1 g/t Au. The sodium hydroxide solution used for leaching was of analytical grade.

Figure 1:

XRD pattern of arsenic-bearing gold ore.

2.2 Methods

Microwave-assisted leaching was carried out in an experimental microwave oven. A certain concentration of sodium hydroxide solution and a 20-g ore sample were added into a one-necked round-bottomed flask, which was positioned on an insulating board at the center of the base of the microwave chamber. A reflux condenser was set up on the glass reactor to prevent the loss of solution due to evaporation. A glass tube with a diameter of 6.0 mm was inserted into the sample bottom to pump in air, in order to oxidize and resolve the gold concentrate ore in the leaching process. Air agitation was used in the leaching process, and the air flow rate was 200 ml/l. After the leaching process, the slurries were separated by vacuum filtration and the filter cake was dried at 100°C. The chemical compositions of the obtained residues were analyzed. The leaching ratio was calculated by using the following equation:

where w is the arsenic or sulfur leaching ratio (%), a is the arsenic or sulfur quantity of raw ore (g), and b is the arsenic or sulfur quantity of the leaching residue. The phase composition of the residues was analyzed by using the XRD method.

3.1 Effect of sodium hydroxide concentration

Experiments were carried out in the solution with a liquid-to-solid ratio of 5:1 under a microwave power of 539 W and an air flux of 200 ml/min for 90 min. The concentrations of sodium hydroxide were 5, 10, 12, 15, and 18 wt.%. The effect of the sodium hydroxide concentration on the arsenic removal rate is shown in Figure 2. It can be seen that the sodium hydroxide concentration has a greater influence on arsenic removal. According to the E-pH diagram of the FeAsS-H₂O system [19], FeAsS existed in the low electric potential and it can be decomposed by O₂ and other oxidants. With the increase of alkaline in the solution, the stable regions of HAsO₄^2- and AsO₄^3- were enlarged and, thus, FeAsS is easily transformed into arsenate. Ferric arsenate does not dissolve in the low-alkalinity solution; however, it could be converted to soluble arsenate under high alkalinity. That is to say, the increase in sodium hydroxide concentration is beneficial to the removal of arsenic, which is consistent with the experimental data, as shown in Figure 2.

Figure 2:

Effect of sodium hydroxide concentration on arsenic removal rate.

3.2 Effect of the liquid-to-solid ratio

The ore was leached in the solution containing 18 wt.% sodium hydroxide under a microwave power of 539 W and an air flux of 200 ml/min for 90 min. The ratio of liquid to solid was from 5 to 8. The effect of the liquid-to-solid ratio on the arsenic removal rate is shown in Figure 3. It can be seen that the arsenic removal rate increased with the increase in the liquid-to-solid ratio. The higher arsenic removal was about 78.22 wt.% at a liquid-to-solid ratio of 7:1.

Figure 3:

Effect of liquid-to-solid ratio on arsenic removal rate.

3.3 Effect of leaching time

Experiments were carried out in the solution with a liquid-to-solid ratio of 7:1 and containing 18 wt.% sodium hydroxide under an air flux of 200 ml/min. The sample was radiated at 539 W versus various times. The effect of leaching time on arsenic removal from the arsenic-bearing gold ore is presented in Figure 4. Within 90 min, the arsenic removal rate increased with the increase of leaching time, and it reached equilibrium after 90 min. Leaching time is related to the degree of reaction and the leaching efficiency. A long leaching time means that the interaction between the ore sample and the leaching reagent will proceed thoroughly. However, with the leaching time extension, the leaching rate of arsenic was maintained at a stable value when the reaction reached the greatest extent.

Figure 4:

Effect of leaching time on arsenic removal rate.

3.4 Effect of microwave power

Experiments concerning the effect of microwave power on arsenic removal were carried out in the solution with a liquid-to-solid ratio of 7:1 and containing 18 wt.% sodium hydroxide under an air flux of 200 ml/min for 90 min. The microwave power was from 120 to 700 W. Figure 5 shows the effect of microwave power on the arsenic removal rate. It was found that the arsenic removal rate increased with the microwave power. The temperature of ore pulp under the effect of microwave irradiation is primarily determined by its microwave-absorbing property. With the increasing energy of the ore pulp, the polar molecule rapidly changes direction into such a high-speed vibration that the ore pulp temperature would be higher especially during the initial leaching time, which would provide suitable thermodynamic conditions for the leaching process. In addition, sulfides are favorable to microwave absorption; however, gangue cannot be heated by microwaves [20]. Therefore, a temperature gradient existed between sulfide phases and gangue. At high microwave power, sulfide phases were heated up faster than the gangue, and thus a temperature gradient would arise between them. The two temperature gradients mentioned above would create thermal stresses that are helpful in improving the removal rate of arsenic.

Figure 5:

Effect of microwave power on arsenic removal rate.

3.5 Orthogonal experiment

The overall effects of reaction time, sodium hydroxide concentration, liquid-to-solid ratio, and microwave power on the removal of arsenic were investigated by using an L₉ (3⁴) orthogonal array under an air flux of 200 ml/min. The parameters and their levels in the leaching experiment are shown in Table 1. Table 2 shows the orthogonal test conditions and the results of alkaline leaching of arsenic under microwave irradiation.

Table 2 shows that reaction time and microwave power were the important influencing factors. The optimum conditions analyzed were reaction time, sodium hydroxide concentration, liquid-to-solid ratio, and microwave power of 60 min, 18 wt.%, 5:1, and 700 W, respectively. Under these conditions, the removal rate of arsenic was approximately 81.46 wt.% and, at the same condition, the sulfur removal rate was about 55.56 wt.%. The temperature of the leaching system under the conditions of experiment no. 8 has been measured. We found that the temperature of the ore pulp could reach 110°C within 6 min, and it would not increase with the increase of time. Conventional leaching experiments under the conditions of reaction time, sodium hydroxide concentration, and liquid-to-solid ratio of 60 min, 18 wt.%, and 5:1, respectively, were also carried out at this measured temperature, and the experimental results show that the removal rate of arsenic was 80.10%, which is less than the 81.46% rate under microwave-heated conditions. The XRD pattern of the leaching residue under the conditions of experiment no. 8 is shown in Figure 6. It can be seen that FeAsS and AsS have been completely decomposed by the NaOH solution during the leaching process, while FeS₂ has not.

Figure 6:

X-ray diffraction of leaching residue.