Study on Leaching African Copper Oxide Ore by Ammonium Carbonate Solution

African copper oxide rich mineral resources, high grade copper, copper mine development value is much higher than domestic. Due to frequent wars and backward technology, the development and production level of copper mines in Africa is backward and the resource utilization rate is low. In recent years, some foreign investors have mainly used copper oxide ore containing more than 30% copper, and blast furnace reduction smelting in Africa to produce about 85% of copper-containing “black copper” exports; a large number of low-grade copper oxide mines have reached grades. Less than the requirements of fire smelting, it is regarded as waste mine and has not been exploited. Some smelters in China use copper oxide with only 1.0% copper for production. With the increasingly tight copper resources, Africa's abundant low-grade copper oxide resources have attracted great attention and have very optimistic development prospects.

Low-grade copper oxide ore oxidation rate Africa, mineral enriched difficult, so a hydrometallurgical process technology is the most reasonable choice. Due to the high content of alkaline gangue in the ore, the acid leaching treatment consumes too much acid, is economically unreasonable, and the acid leaching liquid has high impurity content, and the subsequent purification process is complicated. The traditional ammonia leaching method, that is, the treatment of copper oxide ore with ammonia or ammonia plus ammonium salt as leaching agent, not only the ammonia water is volatile and volatile, the operating environment is poor, and the long-distance transportation of ammonia water is inconvenient, especially not suitable for industrial and transportation. Used on the African continent.

Based on the problems of acid leaching method and traditional ammonia leaching method, combined with the characteristics of low-grade copper oxide ore in Africa, this study only uses ammonium carbonate as leaching agent to carry out ammonia leaching of copper oxide ore. The ore particle size and ammonium carbonate are mainly investigated. The effects of concentration, liquid-solid ratio, reaction temperature, reaction time, stirring speed and other factors on the leaching effect.

First, the principle of reaction

The ammonia leaching method utilizes a stable complex of copper and ammonia to dissolve the copper metal and copper compounds in the ore, thereby achieving separation from gangue and impurity metals. The traditional ammonia leaching method uses ammonia or ammonia plus ammonium salt as the leaching agent; in this study, only ammonium carbonate is used as the leaching agent, and the nature of NH2 is thermally decomposed in aqueous solution to realize the copper-ammonia complexing effect, and copper is in the form of complex ions. After entering the liquid phase, metal copper is obtained through extraction and electrowinning. During the heating leaching process, the main reactions occur are:

(NH ₄ ) ₂ CO ₃ =2NH ₃ +CO ₂ +H ₂ O (1)

CuO+2NH ₃ +(NH ₄ ) ₂ CO ₃ =Cu(NH ₃ )4CO ₃ +H ₂ O (2)

CuCO ₃ ·Cu(OH) ₂ +6NH ₃ +(NH ₄ )2CO ₃ =2Cu(NH ₃ ) ₄ CO ₃ +2H ₂ O (3)

Second, the experiment

(1) Experimental materials

The elemental analysis and phase analysis results of low-grade copper oxide ore in Africa are shown in Tables 1 and 2, respectively.

Table 1 Elemental analysis (mass fraction) of low grade copper oxide ore in Africa

Table 2 Phase analysis (mass fraction) of low-grade copper oxide minerals in Africa

The reagents such as ammonia water, ammonium hydrogencarbonate and ammonium carbonate used in the experiment were all analytically pure.

(Ii) Test Method

The ammonia immersion reaction was carried out in a 2 L three-necked flask. Weigh a certain amount of ammonium carbonate into a certain volume of water, stir to dissolve, then add to the African copper oxide ore of a certain particle size, heat the reaction system, warm up to a predetermined temperature, and then continue to stir for a period of time, the leaching is completed. The filtration adopts vacuum filtration, the leaching solution is sampled and analyzed for ammonia concentration, and the ammonia recovery rate is calculated; the leaching residue is washed and dried, and the copper content is sampled to calculate the copper leaching rate.

(3) Analytical methods

Analysis using iodine slag Method copper content, ammonia concentration of the leaching solution by distillation - Determination of acid capacity.

Third, the experimental results and discussion

(1) Selection of leaching agent

The selected leaching agent in this study requires both the desired leaching effect and the current situation of lack of raw materials and inconvenient transportation in Africa. Fixed size ore, leaching agents [the NH3] concentration _{equivalent to,} liquid-solid ratio, reaction temperature, reaction time, stirring speed, effect on copper leaching effect for ammonia, ammonia plus ammonium carbonate, ammonium carbonate, ammonium bicarbonate leachant Exploratory experimental studies were carried out and the results are shown in Table 3.

Table 3 Effect of different leaching agents on leaching effect

It can be seen from Table 3 that when ammonia water is used as the leaching agent alone, the effect of copper ore is not ideal, and NH ₃ easily escapes from the ammonia water, causing loss of ammonia, affecting copper leaching rate and causing environmental pollution. When ammonia water and ammonium carbonate are used as the leaching agent, the NH ₃ -(NH ₄ ) ₂ CO ₃ -H ₂ O system is heated and the copper leaching rate can reach 90%. However, due to the volatile loss of ammonia water, the ammonia recovery rate is low. In addition, due to the poor traffic conditions in Africa, ammonia is not easy to transport and is not suitable for use as a leaching agent. When only ammonium carbonate is used as the leaching agent, it fully decomposes and releases NH ₃ at 60 ° C in the solution, and cooperates with copper in time. The copper leaching rate is 92.2%, and the ammonia recovery rate is 96.7%. In the NH ₄ HCO ₃ -H ₂ O system, the copper leaching rate and the ammonia recovery rate are both ideal, but the ammonium hydrogencarbonate heating decomposes to generate a large amount of CO ₂ and NH ₃ , which is highly likely to cause a trough.

In summary, it is suitable to use only ammonium carbonate as the leaching treatment of low-grade copper oxide ore in Africa, avoiding the traditional technical route of controlling the concentration of NH ₃ by adding ammonia water, the transportation of raw materials is convenient, the operating environment is good, and the leaching effect is ideal.

(2) Single factor experiment

In the ammonia leaching test, the effects of ore particle size, ammonium carbonate concentration, liquid-solid ratio, reaction temperature, reaction time and stirring speed on the leaching rate were investigated. On this basis, the best leaching conditions are summarized.

1. Effect of particle size on leaching effect

The fixed ammonium carbonate concentration was 1.55 mol/L, the liquid-solid ratio was 4:1, the temperature was 60 ° C, the time was 2 h, and the stirring speed was 350 r/min. The effect of the average particle size of the ore on the leaching effect is shown in Figure 1.

Figure 1 Effect of average ore particle size on leaching effect

Figure 1 shows that when the average ore size of the ore varies between 0.060 and 0.300 mm, the copper leaching rate has an optimum value; while the ammonia recovery rate increases with the ore particle size, 89.4% has risen to 95.7%.

In general, the finer the ore particle size, the more complete the liquid-solid two-phase contact, the more complete the leaching reaction, the higher the leaching rate; after the particle size is reduced to a certain extent, the reaction is transferred to the thermodynamic control step, and the leaching rate is affected by the change. small. Moreover, this study found that copper ore with too small particle size also caused difficulty in filtration and washing, and caused excessive copper and ammonia complex ions to be mixed in the filter residue, resulting in different degrees of reduction in copper leaching rate and ammonia recovery rate. Therefore, the ore particle size should be selected appropriately to ensure the best effect of copper leaching; and the ammonia recovery rate is increasing with the increase of particle size. Taking into account the above two factors, the average particle size of copper ore is 0.150mm.

2. Effect of ammonium carbonate concentration on leaching effect

The fixed ore has an average particle size of 0.150 mm, a liquid-solid ratio of 4:1, a temperature of 60 ° C, a time of 2 h, and a stirring speed of 350 r/min. The effect of ammonium carbonate concentration on the leaching effect is shown in Figure 2.

Figure 2 Effect of ammonium carbonate concentration on leaching effect

Figure 2 shows that with the increase of ammonium carbonate concentration, the copper leaching rate has been increasing. When the concentration is higher than 1.55 mol/L, the copper leaching rate increases obviously. This is because when the concentration of ammonium carbonate is increased, the ammonia concentration is increased, thereby promoting the ammonia leaching reaction to proceed sufficiently, the copper-ammonium complex ion-stabilized region is increased, and the copper leaching rate is gradually increased, but after the ammonium carbonate concentration reaches a certain level, It is no longer the main controlling factor affecting the leaching reaction, and the influence on the copper leaching rate is reduced. The ammonia recovery rate gradually decreases as the concentration of ammonium carbonate increases, because the higher the ammonia concentration, the more volatile the loss. In addition, from the perspective of raw material cost, the ammonium carbonate concentration should not be too high, reaching 1.55 mol/L.

3. Effect of liquid-solid ratio on leaching effect

The fixed ore has an average particle size of 0.150 mm, an ammonium carbonate concentration of 1.55 mol/L, a temperature of 60 ° C, a time of 2 h, and a stirring speed of 350 r/min. The effect of liquid-solid ratio on the leaching effect is shown in Figure 3.

Figure 3 Effect of liquid-solid ratio on leaching effect

Figure 3 shows that as the liquid-solid ratio increases, the copper leaching rate gradually increases. When the liquid-solid ratio increased from 2:1 to 4:1, the copper leaching rate increased particularly sharply, from 83.3% to 88.3%. Under the premise of keeping the concentration of ammonium carbonate unchanged, increasing the liquid-solid ratio can promote the full contact of the liquid-solid phase, and the ammonia leaching reaction proceeds in depth, and the copper leaching rate is gradually increased. When the liquid-solid ratio is more than 4:1, the cost of raw materials such as ammonium carbonate increases, but the copper leaching rate rises slowly, and the copper concentration in the leachate is too low, and the recovery cost increases. At the same time, the ammonia recovery rate is less affected by the liquid-solid ratio, and is basically maintained at about 95%. Taking into account the above factors, a liquid to solid ratio of 4:1 is suitable.

4, the effect of temperature on the leaching effect

The fixed ore has an average particle size of 0.150 mm, an ammonium carbonate concentration of 1.55 mol/L, a liquid-solid ratio of 4:1, a time of 2 h, and a stirring speed of 350 r/min. The effect of temperature on the leaching effect is shown in Figure 4.

Figure 4 Effect of temperature on leaching effect

Figure 4 shows that as the reaction temperature increases, the copper leaching rate gradually increases. At 50 to 60 ° C, the copper leaching rate increases rapidly, but when the temperature is higher than 60 ° C, the growth rate becomes slower. Because ammonium carbonate decomposes in the solution with increasing temperature, the complex molecule NH _{3 is} released to promote the ammonia leaching reaction. When the temperature is higher than the decomposition temperature of ammonium carbonate at 60 ° C, the ammonia concentration tends to be stable, and the copper leaching rate increases. Slow down. The ammonia recovery rate decreases as the reaction temperature increases, because the higher the solution temperature, the larger the partial pressure of ammonia, and the more easily the volatilization loss is caused. Considering the factors of copper leaching rate and ammonia recovery rate, the temperature selection is suitable at around 65 °C.

5, the impact of time on the leaching effect

The fixed ore has an average particle size of 0.150 mm, an ammonium carbonate concentration of 1.55 mol/L, a liquid-solid ratio of 4:1, a temperature of 60 ° C, and a stirring speed of 350 r/min. The effect of leaching on the leaching effect is shown in Figure 5.

Figure 5 Effect of ammonium carbonate concentration on leaching effect

Figure 5 shows that in the early stage of the reaction, the copper leaching rate increased significantly with time. When the leaching time was 2, the copper leaching rate reached 88.6%; the increase of reaction time had little effect on the copper leaching rate. This is because the control factors in the ammonia leaching reaction change, the main control factor in the early stage of the reaction is kinetics, and the latter is mainly controlled by thermodynamic factors. In addition, too long a leaching time will increase the loss of ammonia, resulting in a decrease in ammonia recovery rate and an increase in investment costs and operating costs. Therefore, the leaching time selection 2 is suitable.

6. Effect of stirring speed on leaching effect

The fixed ore has an average particle size of 0.150 mm, an ammonium carbonate concentration of 1.55 mol/L, a liquid-solid ratio of 4:1, a temperature of 60 ° C, and a time of 2 h. The effect of stirring speed on the leaching effect is shown in Fig. 6.

Figure 6 Effect of stirring speed on leaching effect

Figure 6 shows that the stirring speed has a great influence on the copper leaching rate and ammonia recovery rate. In the liquid-solid reaction process, when the stirring speed is slow, diffusion is an important factor affecting the progress of the reaction. The reaction is mainly controlled by kinetics. Therefore, as the stirring speed increases, the copper leaching rate increases significantly. When the stirring speed is higher than 350r/min, the thermodynamics becomes the main factor affecting the ammonia leaching reaction, and the influence of the stirring speed on the copper leaching rate becomes small. At the same time, the increase of the stirring speed will accelerate the evaporation of ammonia in the solution, and the ammonia recovery rate will continue to decrease. In combination with the operability of industrial production, a stirring speed of 350 r/min is suitable.

(3) Leaching effect under optimal leaching conditions

Through the above experiments, the optimal leaching conditions for low-grade copper oxide ore in Africa are as follows: fixed ore average particle size 0.150mm, ammonium carbonate concentration 1.55 mol/L, liquid-solid ratio 4:1, temperature 65 °C time 2h, stirring speed 350r/ Min. Under the optimal conditions, the copper oxide ore obtained an ideal leaching effect, the copper leaching rate reached 92.4%, and the ammonia recovery rate reached 95.5%.

Fourth, the conclusion

In this study, only ammonium carbonate was used as the leaching agent, and it was used in the solution to be susceptible to thermal decomposition to produce NH ₃ . The ammonia leaching treatment of low-grade copper oxide ore in Africa was carried out. The optimum leaching conditions were obtained through experimental research: the average particle size of fixed ore was 0.150. Mm, ammonium carbonate concentration 1.55 mol / L, liquid-solid ratio 4: 1, temperature 65 ° C time 2 h, stirring speed 350 r / min. Under the optimal conditions, the copper oxide ore obtained an ideal leaching effect, the copper leaching rate reached 92.4%, and the ammonia recovery rate reached 95.5%.

The ammonia leaching process is simple, the raw material cost is low, the transportation is convenient, the copper leaching effect is ideal, the ammonia recovery rate is high, and the ammonia can be recycled in combination with the extraction process, and is particularly suitable for use in an African country with underdeveloped infrastructure. Local low-grade copper oxide resources have very positive significance.

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