Recovery of molybdenum from alkali leaching solution of low-grade molybdenum concentrate by ion exchange

2.1 Experimental materials

The flow chart of Mo recovery from alkali leaching solution of low-grade Mo concentrate is presented in Figure 1. The refractory Mo ores was from Henan province and the Mo grade was 0.12% in raw ore. After the beneficiation process, the low-grade Mo concentrate was obtained and its main mineral compositions are shown in Table 1.

Figure 1:

The flow chart of molybdenum recovery from alkali leaching solution of low-grade molybdenum concentrate.

In the pretreatment process of Mo concentrate, roasting at 600°C for 1.5 h followed by the alkali leaching step were employed to dissolve Mo to be recovered. The alkali leaching process was conducted under the following conditions: sodium carbonate dosage was 40%, liquid solid ratio was 3, leaching temperature was 85°C and leaching time was 1 h. The main compositions of the obtained alkali leaching solution were Mo 4.96 g/l, S 3.48 g/l, P 0.08 g/l.

2.2 Ion exchange procedure

The resins were firstly soaked for 24 h in deionized water to make the resins swell sufficiently, then converted in 4% NaOH solution for 8 h, and rinsed with deionized water until the pH of the washing solution was close to neutral. Furthermore, the resins were soaked and stirred in 2 mol/l HCl solution to exchange the resins from OH type to Cl type.

The static tests were carried out in a beaker containing 5 g resins and 500 ml alkali leaching solution. The beaker was heated to a specific temperature while being magnetically stirred. After the required time, the Mo content in the adsorption solution was analyzed by inductively coupled plasma atomic emission spectroscopy to calculate the adsorption capacity.

The dynamic tests were conducted in an ion exchange column (Ф20×200 mm). Treated resins (20 g) were poured into the column, and the operation was performed by downstream flow at a constant flow rate which was controlled by a pump. Samples were collected periodically from the column effluent and analyzed to determine Mo concentration. Then, the loaded resins were rinsed with deionized water and desorbed using 10% ammonia solution.

The feed solution in both static and dynamic experiments was 4.96 g/l Mo. The Mo concentration loaded into resin was obtained by mass balance. Adsorption percentage of Mo was calculated by Eq. (1):

where θ is the adsorption capacity, α is the Mo concentration in initial solution (g/l) and β is the Mo concentration in adsorbed solution (g/l).

According to previous investigations, D201 and D314 resins were employed in adsorption and desorption experiments. D201 and D314 resins are weak basic anion exchange resins. The chemical and structural properties of the resins are shown in Table 2.

The ion exchange resins of D201 and D314 obtained from Zibo Dongda Chemical Company (Zibo, Shandong, China) were used in this study. All chemical reagents used in this study were of analytic grade.

3.1 Static tests for adsorption

The pH value of alkali leaching solution was about 11, and hydrochloric acid was adopted to adjust the pH value for the requirement of the experiments. The effect of pH value on adsorption capacity of Mo is shown in Figure 2.

Figure 2:

Effect of pH value on static tests for adsorption capacity of molybdenum (Mo).

From Figure 2, it can be observed that the pH value of the alkali leaching solution significantly affected the adsorption capacity of Mo for D201 and D314 resins. With increasing pH value from 2 to 3.5, the adsorption capacity linearly increased and reached the maximum value of 93.50% and 95.47% when the pH value was 3.5. This is because when the pH value was low, Mo existed as poly-molybdate salt ions state, which could be easily adsorbed on the resins [24, 25]. Nevertheless, with further increase of pH value, the adsorption capacity gradually decreased.

According to above results, it could be concluded that the adsorption capacity of D314 was higher than that of D201. Therefore, D314 resin was chosen for further experiments.

3.2. Dynamic tests for adsorption

For the alkali leaching solution, hydrochloric acid was adopted to adjust the pH value to 3.5, and the flow rates were 1 ml/min and 2 ml/min, controlled by a constant flow pump. The dynamic adsorption curve of D314 resin is shown in Figure 3.

Figure 3:

The dynamic adsorption curve of D314 resin.

It can be seen that Mo concentration in effluent at 2 ml/min was much higher than at 1 ml/min, which indicated that the flow rate of the alkali leaching solution should be slow. For the flow rate of 1 ml/min, the average Mo concentration in effluent was 0.16 g/l when Mo concentration in the leakage point was 0.35 g/l. Consequently, it could be calculated that the adsorption capacity of Mo was 96.77% in dynamic tests.

3.3 Desorption tests

Ammonium solution (10%) was adopted to deposit Mo ions, which were released to the desorption solution as ammonium molybdate. The elution curve of D314 resin is presented in Figure 4.

Figure 4:

The deposition curve of D314 resin.

It can be seen that 10% ammonium solution performed excellently in depositing Mo ions. The results indicated that the maximum value of Mo concentration in eluate was 122 g/l. When the volume of desorption solution was double the size of the resins, the desorption process basically completed, while Mo concentration in eluate was 0.07 g/l.

3.4 Precipitation tests

The main compositions of desorption solution were Mo 59.21 g/l, P 1.26 g/l and S 0.94 g/l, wherein P content was too high to disrupt the precipitation process. Therefore, P was eliminated by magnesium chloride saturated solution under the following conditions: stirring temperature 50°C, final pH value 8.3–8.5 and stirring time 1 h. As a result, P was barely observed in the desorption solution.

After P was removed, 50% nitric acid solution was added to the desorption solution until the pH value reached 2.5 under a stirring temperature 50°C. The desorption solution continued to be stirred for 20 min, and the residue was filtered. Furthermore, the obtained residue was put into the resistance furnace at 550°C for 1 h, and the Mo oxide products were finally obtained. The main compositions of Mo oxide products are shown in Table 3.

It could be observed that the content of Mo was 55.75% and the amounts of impurities such as S and P were rather low. The quality of Mo oxide products met the requirement of YMo 55 national standard in GB/T24482-2009 and Grade A standard of ASTM A146-04 (2014). In conclusion, the Mo recovery was 97.81% in the precipitation process.