With the gradual depletion of zinc concentrates, people are paying more and more attention to the development and utilization of low-grade zinc oxide ore. Yunnan LANPING been proven alkaline metal zinc zinc reserves 12898000 t, average grade of 5.74% zinc, low grade oxide ore wherein the ore and mixing extremely difficult to process 40% of the total, and minerals High gangue content, complex mineral composition and multi-metal symbiosis.

The traditional acid leaching treatment of zinc oxide ore requires a large amount of acid, and has the disadvantages of difficult solid-liquid separation, high impurity content in the leachate, and difficulty in purification. The ammonia leaching method has high selectivity and is widely used in the wet smelting fields such as copper , nickel and cobalt . However, the concentration of the leaching agent used for treating the zinc oxide ore by the ammonia leaching method is high, and the ammonium salt is easy to precipitate. At the same time, the concentration of ammonia is large, and the volatile loss causes environmental pollution, so it is still in the laboratory research stage.

Ultrasonic waves can produce mechanical and chemical effects on the reaction medium by acoustic cavitation, thereby accelerating or initiating chemical reactions. Therefore, the introduction of ultrasonic waves in the mineral leaching process has attracted the attention of many scientific and technological workers. Sun Jiashou and other studies have shown that the leaching rate of copper after leaching of copper sulfide concentrate by FeCl ₃ for 30 min can reach 80.2%, which is 5%~10% higher than that without ultrasonic treatment, and the time is shortened by 4 h. BESEtl8] studied the enhanced leaching of slag by copper electric converter. Under the optimal leaching conditions, the leaching rates of Cu, Zn, Co and Fe in the slag after ultrasonic induction were increased by 8.87%, 3.04%, 5.35% and 1.57%, respectively. AVVARU et al. studied the ultrasonic enhanced leaching of uranium under two different leaching systems of nitric acid and sulfuric acid. The results show that the strengthening effect of ultrasonic in the two leaching systems is different.

The authors focused on the leaching process of NH ₃ -NH ₄ C1-H ₂ O system for refractory low-grade zinc oxide ore, and applied the ultrasonic enhanced leaching process to study the basic law of the effect of ultrasonic radiation on zinc ammonia leaching process in zinc oxide ore. It provides the basis for exploring the application of ultrasonic radiation to enhance the new process of leaching zinc oxide ore.

First, the experiment

(1) Experimental materials

The experimental raw materials were from Lanping zinc oxide ore in Yunnan Province. The ore was crushed and sieved by a pulverizer. The size of the ore used in the experiment was less than 147 μm. The main chemical components and phase compositions are listed in Tables 1 and 2, respectively.

Table 1 Main components of ore

Table 2 Phase analysis of ore

(2) Experimental instruments

The main instruments used in the experiment are DKB-501A super constant temperature water tank, JJ-1 compact force electric stirrer (3 pieces of impeller with a radius of 50mm), 07HWS-2 constant temperature magnetic stirrer, JY-92DII ultrasonic cell pulverizer (Working frequency 20 kHz, output power 150 ~ 750w, continuously adjustable, probe diameter 25 mm), SHB--III water circulation multi-purpose vacuum pump.

(3) Experimental methods

The conventional leaching experiment was carried out in a 1 000 mL stainless steel vessel, and the temperature was maintained in a super constant temperature water tank. Each time 30 g (minus size less than 147 μm) was weighed into a reaction vessel, and the prepared NH ₃ was added at a certain liquid to solid ratio. NH ₄ C1-H ₂ O solution, mechanically stirred (200 r/min) for leaching. The ultrasonic enhanced leaching experimental apparatus is shown in Fig. 1, and ultrasonic waves are started simultaneously with magnetic stirring (600 r/rain). During the leaching process, an appropriate amount of ice water is added to control the temperature rise of the solution caused by the ultrasonic radiation. After leaching for a certain period of time under different conditions, vacuum filtration is carried out to carry out solid-liquid separation. The filter residue is washed a plurality of times, and the composition of the washing liquid is the same as that of the leaching agent, and the volume is about 20% of the volume of the leaching agent. The Zn content in the leachate was determined by the EDTA volumetric method, and the leaching rate (R) of zn was calculated by the following formula:

Where w(Zn) is the content of zinc in the leachate; w ₀ (Zn) is the zinc in the ore

The content.

Figure 1 Ultrasonic enhanced leaching experimental device

Second, the results and discussion

(1) Conventional leaching without ultrasonic radiation

Considering the volatility and leaching efficiency of ammonia, according to the literature, the mixed solution of c(NH ₄ Cl)=5.0 mol/L and c(NH ₄ OH)=2.5 mol/L is used as the leaching agent to leach the zinc oxide ore under mechanical stirring. The maximum leaching of zinc in the ore was carried out by single factor condition experiment. The effects of liquid-solid ratio, leaching time and reaction temperature on the leaching effect were investigated to determine the optimum leaching conditions. The effects of various factors on the leaching effect are shown in Figures 2 to 4. The experimental results show that the liquid-solid ratio, leaching time and reaction temperature have different effects on the leaching effect of zinc in the selected experimental conditions. It can be seen from Fig. 2 that the influence of the liquid-solid ratio on the leaching rate is remarkable. When the liquid-solid ratio is less than 5:1, the leaching rate increases remarkably with the increase of the liquid-solid ratio, but when the liquid-solid ratio is greater than 5:1, the leaching rate tends to be gentle. Therefore, the optimum liquid to solid ratio is 5:1. It can be seen from Fig. 3 that when the leaching time is less than 3 h, the leaching rate increases remarkably with the increase of time, but when the leaching time exceeds 3 h, the leaching rate tends to be gentle. Therefore, the optimal leaching time is 3 h. It can be seen from Fig. 4 that when the temperature is lower than 30 °C, the leaching rate increases with the increase of temperature, which is due to the increase of temperature, which is conducive to the diffusion of solid-liquid and the mass transfer rate, thereby improving the leaching. rate. But when the temperature is higher than 30 ℃, elevated temperatures can cause volatilization of NH _3, NH ₃ concentration in the leachate reduced, resulting in reduced leaching rate of zinc. Therefore, the leaching experiment temperature is generally 30 ° C.

Figure 2 Effect of liquid-solid ratio on leaching rate

Figure 3 Effect of leaching time on leaching rate

Figure 4 Effect of temperature on leaching rate

In summary, for the Lanping low-grade zinc oxide ore, the best ammonia leaching conditions are: c (NH ₄ CO = 5.0 mol / L, c (NH ₄ OH) = 2.5 mol / L, liquid solid The ratio of 5:1, the leaching time was 3 h, the reaction temperature was 30 ° C, and the leaching rate was 69.4%, that is, the leaching rate of zinc in the zinc oxide ore in the mixed ore was 97.4%.

(2) Leaching behavior of low-grade zinc oxide ore under ultrasonic irradiation

1. Effect of ultrasonic irradiation time on leaching rate Ultrasonic waves were introduced under the optimal conditions of conventional leaching, and the ultrasonic power was 300W. The leaching effect under different ultrasonic times was investigated. The results are shown in Fig. 5. It can be seen from Fig. 5 that under the same leaching conditions, the leaching rate of Zn after ultrasonic leaching for 20 min is 63.9%, that is, the leaching rate of Zn is 80 min after conventional leaching, and it can be seen that the ultrasonic wave can significantly increase the leaching rate of Zn. With the prolongation of ultrasonic time, the leaching rate of zn is significantly improved. When the ultrasonic leaching time reaches 80 mill, the leaching rate of Zn is 69.2%, which basically reaches the leaching rate of Zn after 180 minutes of conventional leaching. When the ultrasonic time is longer than 80 min, the ultrasonication time is prolonged, and the leaching rate of Zn is not greatly improved. This is because the agitation generated by the ultrasonic radiation wave enhances the mass transfer of the solid-liquid problem and accelerates the reaction rate; the washing and pulverization of the solid surface and the erosion of the solid surface by the micro-jet generated by the ultrasonic cavitation reduce the mineral particles. The particle size reduces the thickness of the diffusion layer, which is beneficial to increase the contact area between the leaching agent and the mineral particles, thereby accelerating the reaction. Therefore, ultrasonic radiation significantly changes the kinetic behavior of zinc oxide ore leaching, but has no effect on the equilibrium state of the reaction.

Figure 5 Effect of ultrasonic time on leaching rate

2. Effect of temperature on leaching rate

In the leaching agent, c(NH ₄ Cl)=5.0 mol/L, c(NH40H)--2.5mol/L, the liquid-solid ratio is 5:1, the ultrasonic power is 300 W, and the leaching rate of zinc at different temperatures The relationship with the ultrasonic time is shown in Figure 6. It can be seen from Fig. 6 that the effect of the ultrasonic wave on the leaching rate weakens with the leaching time at the same temperature leaching. When the ultrasonic time increases from 10 min to 80 min at 25 ° C, the leaching rate decreases from 18% to 5.9. %. When the ultrasonic time is the same, the effect of ultrasonic on the leaching rate is weakened with the increase of temperature. When the ultrasonic leaching time is 10min, the temperature increases from 25°C to 50°C, and the increase of ultrasonic leaching rate decreases from 18% to 18%. 5.1%. It can be seen from Fig. 6 that at a lower temperature, the effect of ultrasonic on the leaching process is rapidly decreased with the leaching time, and when the temperature is high, the change tendency becomes gentler. This is because as the temperature increases, the cavitation caused by the ultrasonic radiation is weakened, so that the erosion of the solid surface by the micro-jet generated by the cavitation is weakened, and the effect of the enhanced leaching is also reduced. When the temperature rises, the molecular motion in the solution increases, the diffusion coefficient between the solid and liquid increases, the reaction rate of zn in the mineral contact with the leaching agent increases, the reaction is affected by mass transfer, and the conventional leaching rate is high. At the same time, at higher temperatures, the cavitation caused by ultrasonic radiation is weaker than the cavitation in the solution at low temperatures, so the effect of ultrasonic enhanced leaching at higher temperatures is not as pronounced at lower temperatures.

Figure 6 Effect of ultrasonic waves on leaching rate at different temperatures

3. Effect of leaching agent concentration on leaching rate

In the leaching agent, c(NH ₄ Cl)/c(NH ₄ OH)=2, liquid-solid ratio 5:1, temperature 30 ° C, ultrasonic power 300 W, when the concentration of leaching agent is different, the leaching rate of zinc with ultrasonic time The change is shown in Figure 7. It can be seen from Fig. 7 that under the action of ultrasonic waves, zinc is leached with these two concentrations of leaching agent, and the leaching rate is improved. At the same concentration, the effect of ultrasonic on the leaching rate weakened with the leaching time. When the leaching agent c(NH ₄ Cl)=4mol/L, the leaching time increased from 10min to 60min, the leaching rate decreased from 11.7%. To 6.8%. The concentration of the leaching agent is different, and the effect of the ultrasonic wave on the leaching rate is different under the same leaching time. With the increase of leaching agent concentration, the effect of ultrasonic on the leaching rate is weakened. When the ultrasonic leaching time is 60 min and the c(NH ₄ Cl) is increased from 4 mol/L to 6 mol/L, the leaching rate decreases from 6.8%. To 3.1%. Because the low concentration of NH ₄ Cl is used as the leaching agent, the viscosity of the leaching agent is small, cavitation bubbles are easily generated, and the energy loss during the mass transfer process is small, which facilitates the cavitation of the ultrasonic waves and the cavity near the solid particles. Asymmetric collapse promotes intraparticle diffusion and increases mass transfer rate to shorten reaction time. When the concentration of NH ₄ CI is higher, the leaching agent diffuses faster on the surface of the solid particles, and the reaction rate of Zn in contact with the leaching agent in the mineral is also accelerated, and the conventional leaching rate is higher. At the same time, the surface tension and viscosity coefficient of the high concentration solution become larger, and the "cavitation" is much weaker than the "cavitation" in the dilute solution. Therefore, the effect of the ultrasonic enhanced leaching is not as effective as when the concentration is low.

Figure 7 Effect of ultrasonic wave on leaching rate under different NH ₄ Cl concentrations

In the leaching agent, c(NH ₄ Cl)=5mol/L, temperature 30°C, liquid-solid ratio 5:1, ultrasonic power 300 W, ultrasonic time 60 min, adjust the molar concentration ratio of NH ₄ Cl and NH ₄ OH, study the ratio At different times, the effect of ultrasonic waves on the leaching process is shown in Figure 8. As can be seen from Fig. 8, the molar concentration ratio of NH ₄ C1 to NH ₄ OH has a significant effect on the leaching rate. The leaching rate of zinc decreases remarkably with the increase of the molar concentration ratio of NH ₄ Cl and NH ₄ OH, and the degree of strengthening of the leaching process increases with the increase of the molar ratio of NH ₄ Cl to NH ₄ OH. When the molar concentration ratio of NH ₄ Cl to NH ₄ OH was increased from 1:1 to 3:1, the leaching rate increased from 0.4% to 5.2%. As the molar concentration ratio of NH ₄ Cl to NH ₄ OH decreases, the concentration of NH ₄ OH in the solution increases, that is, the content of soluble ammonia in the solution increases, since the dissolved gas is the nucleation point of the ultrasonic cavitation bubble, The higher the content of ammonia gas, the easier the cavitation bubble is formed, and the stronger the ultrasonic cavitation effect, that is, the effect of ultrasonic enhanced leaching is more obvious.

Fig. 8 Effect of ultrasonic wave on leaching rate under different c(NH ₄ Cl)/c(NH ₄ OH) ratio

(III) Selectivity of ultrasonic enhanced ammonia leaching

When the leaching agent has c(NH ₄ Cl)=5 mol/L, c(NH ₄ OH)=2.5 mol/L, temperature 30° C., liquid-solid ratio 5:1, ultrasonic power 300 W, leaching time 60 min, The results of ICP (plasma spectroscopy) analysis of the leachate under the process conditions are listed in Table 3.

Table 3 Composition analysis of leachate

It can be seen from Table 3 that for conventional leaching and ultrasonic enhanced leaching under the process conditions, the contents of impurities in the leaching solution are relatively high except for Cd and Ca, and other metals are very low, so NH ₃ -NH ₄ Cl- The H ₂ O leaching system has a high selectivity to Zn. Ultrasound-enhanced leaching compared with conventional leaching leaching solution significantly enhances the Zn content in the leaching solution, while the content of other metal impurities increases little, so the ultrasonic is also selective for the enhanced leaching of metal zinc.

Third, the conclusion

(1) The optimum leaching conditions for the leaching of Lanping low-grade zinc oxide ore by NH ₃ -NH ₄ Cl-H ₂ O system without ultrasonic radiation are: c(NH ₄ Cl)=5.0mol/L, c(NH) ₄ OH) = 2.5 mol/L, liquid-solid ratio 5:1, leaching time 3 h, temperature 30 ° C, maximum leaching rate 69.4%. Under the same conditions, ultrasonic radiation significantly shortened the leaching time. It takes 80 minutes to use Zn leaching rate of 61.8% without ultrasonic radiation, and only 20 minutes when using ultrasonic radiation leaching.

(2) Ultrasonic enhanced ammonia leaching of low-grade zinc oxide ore is sensitive to leaching agent concentration, reaction temperature, molar ratio of NH ₄ Cl to NH ₄ OH. When the reaction temperature is low and the concentration of the leaching agent is low, and the molar concentration of NH ₄ Cl and NH ₄ OH is relatively large, the ultrasonic wave has a remarkable effect on the zinc leaching strengthening effect in a short time. Therefore, ultrasonic irradiation is expected to reduce the reaction temperature of ammonia-immersed low-grade zinc oxide ore and the concentration of NH ₄ OH required for leaching, and greatly shorten the leaching time.

(3) Ultrasonic enhanced ammonia leaching has a high selectivity for zinc leaching.

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