Cobalt is a very important non-ferrous metals, often used in the battery, petrochemical, machinery, ceramics, paint and other industries in the form of cobalt powder, cobalt oxide and cobalt compounds. China is a country with scarce cobalt resources. Cobalt is mainly recovered as a by-product from the comprehensive utilization of mineral resources such as copper , nickel and iron . At present, China mainly relies on the import of cobalt concentrate, cobalt slag and other raw materials to produce cobalt products. Since 2000, the annual average annual consumption growth rate of domestic cobalt has reached 19.7%. It is estimated that by 2010, China's cobalt consumption will reach 18,500 tons, ranking first in the world. The surge in cobalt consumption has caused a shortage of cobalt resources. In recent years, the recovery of cobalt from secondary resources such as spent catalyst, used batteries and smelting slag has caused widespread concern in countries around the world. Cobalt-containing catalysts are widely used in petroleum refining processes in the petrochemical industry. Catalysts are permanently ineffective due to poisoning during use and become spent catalysts. Due to the large amount of valuable metals, spent catalysts become important secondary resources for metals such as cobalt. The recovery of cobalt in the spent catalyst is mainly carried out by acid-soluble precipitation method, and the cobalt product obtained is mainly cobalt oxide, followed by cobalt powder and cobalt naphthenate. These methods are used to recover metals such as cobalt and nickel to some extent, but the recovery of other valuable metals such as vanadium , molybdenum and aluminum is not ideal; some processes are complicated and the cost is too high, which is not conducive to industrialization. At present, there are few studies on the kinetics of cobalt leaching during the recovery process. Here, based on the comprehensive recovery of valuable metals such as aluminum, vanadium, molybdenum, nickel and cobalt in a spent catalyst, the authors studied the nickel-cobalt slag obtained during the treatment of spent catalysts. The influence of relevant process conditions on cobalt leaching and the kinetics of leaching cobalt were discussed.
First, the experiment
(1) Raw materials and reagents
The raw materials used in the experiment were derived from the solid catalyst discarded in the production process of the petrochemical industry, and the carrier was corundum alumina. The spent catalyst is treated as follows: after the spent catalyst is mixed with a certain amount of caustic soda, the reaction is calcined at a high temperature for a period of time, and then the calcined product is leached with hot water. After filtration, aluminum, vanadium, molybdenum, etc. enter the leachate, while nickel and cobalt are enriched in the water leaching residue (nickel-cobalt slag). The contents of the main components in the material are shown in Table 1. In the roasting-water immersion process, the conversion of nickel and cobalt to oxides and the dissolution of most of the alumina create extremely favorable conditions for the subsequent sulfuric acid leaching of nickel-cobalt slag. The sulfuric acid used in the experiment was of analytical grade and was produced by Zhuzhou Chemical Industry Research Institute with a concentration of 18.4 mol/L and a density of 1.84 g/mL. Before the experiment, several different concentrations of sulfuric acid were prepared.
Table 1 Main chemical composition of materials
(2) Experimental methods
The leaching experiment was carried out in a 500 mL three-necked flask. The three-necked flask was placed in a constant temperature water bath and connected to an electric mixer. A certain concentration of sulfuric acid solution was added at a solid-liquid ratio of 1 ׃ 10, stirring was started and heated, and after reaching a predetermined temperature, 30 g of nickel-cobalt slag was quickly added. During the leaching process, the temperature fluctuation range is ±1 ° C, and the stirring speed is controlled within the range of 0 to 1500 r/min. The supernatant was aspirated from the sampling port at intervals of 2 mL. After constant volume, the concentration of Co 2+ in the liquid was analyzed by PS−6 vacuum type ICP−AES. In order to keep the volume of the liquid in the three-necked flask unchanged, the same volume of water was added after each sampling. The leaching rate of cobalt is calculated as follows:
Where: x(Co) is the leaching rate of cobalt, %; Ï i is the mass concentration of cobalt in the i-th sampling liquid, g/L; i is the number of sampling; m is the mass of nickel-cobalt slag, g.
Second, the results and discussion
(1) Sulfuric acid leaching of nickel-cobalt slag
The sulfuric acid leaching of nickel-cobalt slag is a liquid-solid phase non-catalytic reaction. During the leaching process, the main chemical reactions occur:
There are many factors affecting the leaching rate of cobalt. This study mainly examines the influence of particle size, stirring speed, sulfuric acid concentration, reaction temperature and time on cobalt leaching rate.
1. Effect of material particle size on cobalt leaching rate
Particle size is a very important factor in the leaching process. In this study, three types of nickel-cobalt slag were prepared: (0.100 to 0.200) mm, (0.074 to 0.100) mm, and (0.043 to 0.074) mm. Figure 1 shows the relationship between the particle size of the material and the cobalt leaching rate in the nickel-cobalt slag. The leaching conditions are: sulfuric acid concentration of 4 mol/L, temperature of 60 ° C, and stirring speed of 800 r/min.
Figure 1 Effect of material particle size on cobalt leaching rate
It can be seen from Fig. 1 that the particle size has a great influence on the leaching of cobalt. The leaching rate of cobalt has a good correspondence with the particle size of the raw material, and the raw material with a particle size of (0.043-0.074) mm has the best leaching effect. However, solid-liquid separation of the leaching system under such conditions is difficult, and therefore, (0.074 to 0.100) mm is used as a suitable particle size for leaching in subsequent experiments.
2. Effect of stirring speed on cobalt leaching rate
The leaching process of many fine-grained materials requires stirring to disperse the suspended particles and thoroughly mix the leaching solution. The experiment of the effect of stirring speed on the leaching effect of cobalt was carried out under the following conditions: the particle size of nickel-cobalt slag was (0.074-0.100) mm, the concentration of sulfuric acid was 4 mol/L, the temperature was 60 ° C, and the stirring speed was controlled at 400-1200 r/min. as shown in picture 2. It can be seen from Fig. 2 that the leaching rate of cobalt has little relationship with the stirring speed, which indicates that the outward diffusion rate of the reactant H 2 SO 4 and the product CoSO 4 on the surface of the solution and the particles is faster, and therefore, the leaching rate of cobalt and The speed does not matter much. In order to ensure the consistency of the stirring speed, the stirring speed is selected to be 800r/min.
Figure 2 Effect of stirring speed on cobalt leaching rate
3. Effect of sulfuric acid concentration on cobalt leaching rate
The effect of sulfuric acid concentration on cobalt leaching rate is shown in Fig. 3. The leaching conditions are: nickel-cobalt slag particle size (0.074-0.100) mm, temperature 60 °C, and stirring speed 800 r/min. It can be seen from Fig. 3 that the concentration of sulfuric acid has a great influence on the leaching rate of cobalt, and the leaching rate of cobalt increases remarkably with the increase of sulfuric acid concentration. When the reaction time is 180 min and the concentration of sulfuric acid is 2, 4, 6 mol/L, nickel cobalt The leaching rates of cobalt in the slag were 24.3%, 63.0% and 85.7%, respectively. Therefore, the sulfuric acid concentration was chosen to be 6 mol/L.
Figure 3 Effect of sulfuric acid concentration on cobalt leaching rate
4. Effect of reaction temperature on cobalt leaching rate
Examination of the effect of temperature on the leaching effect can estimate the apparent activation energy, activation enthalpy and activation entropy of the reaction. The effect of reaction temperature on the leaching rate of cobalt is shown in Fig. 4. The particle size of the raw material used in the leaching process is (0.074-0.100) mm, the concentration of sulfuric acid is 6 mol/L, the stirring speed is 800 r/min, and the leaching temperature varies from 40 to ～. At 80 ° C, the solid-liquid ratio of the leaching was 1׃10. It can be seen from Fig. 4 that the reaction temperature has a certain influence on the cobalt leaching rate, and the leaching rate of cobalt increases with the increase of the reaction temperature. The reaction time has a great influence on the cobalt leaching effect; at the same temperature, the cobalt leaching rate increases with the extension of the reaction time. When the leaching temperature is 80 ° C and the reaction time is 180 min, the leaching rate of cobalt can reach 94.2%.
Figure 4 Effect of reaction temperature on cobalt leaching rate
(2) Kinetics of leaching of nickel-cobalt slag
1. Reaction rate control model
In a liquid-solid reaction system, the reaction rate is usually controlled by the slowest step in the following steps: out-diffusion on the surface of the liquid film, internal diffusion of the product/reactant surface, or chemical reaction on the surface of the reaction particle. In the chemical reaction, there are mainly three control models: chemical reaction control model, diffusion control model and hybrid control model on the particle surface.
The most common reaction model for liquid-solid phase non-catalytic reactions is the Shrinkingcore model. The reduced core model is further divided into a particle size constant shrinkage core model and a particle shrinkage reduction core model. The particle size constant shrinkage core model is characterized by the formation of a solid phase product layer, and the particle size does not change during the reaction. The feature of the particle reduction and shrinkage model is that during the reaction, the reactant particles are continuously reduced, there is no solid phase product layer, and the product is dissolved in the solution. During the reaction of nickel-cobalt slag with sulfuric acid, aluminum present in the form of corundum does not participate in the reaction, and solid product CaSO 4 is also formed. The reaction kinetics can be studied by using the particle size constant shrinkage model.
When the chemical reaction is a control step, the dynamics equation of the reduced core model is:
Where: x(Co) is the leaching rate of cobalt; t is the reaction time; t f is the complete reaction time; Ï is the nickel-cobalt slag density; r is the initial reaction radius of the nickel-cobalt slag particles; k is the reaction rate constant; b is Cobalt oxide metering coefficient; M is the relative molecular mass of cobalt oxide; k' is the reaction rate constant; c is the fluid reactant (sulfuric acid) concentration. For a fixed system, and the fluid reactant concentration is approximately constant, t f can be regarded as a constant, and 1/t f can be expressed as an apparent reaction rate constant k.
Substituting the data in Figure 4 into equation (4), plot the relationship between 1−(1−x(Co)) 1/3 and reaction time at different reaction temperatures, as shown in Figure 5. It can be seen from Fig. 5 that the linear relationship between the data in the figure and equation (4) is not obvious and the straight line does not pass through the origin. Therefore, it is inferred that the leaching reaction does not conform to the chemical reaction control model.
Figure 5 Relationship between 1−(1−x(Co)) 1/3 and reaction time at different reaction temperatures
When the diffusion in the solid product layer is a control step, the dynamics equation of the reduced core model is:
Where: D is the diffusion coefficient of cobalt ions in a porous medium. Substituting the data in Figure 4 into equation (6), plot the relationship between 1−2/3x(Co)−(1−x(Co)) 2/3 and reaction time at different reaction temperatures, as shown in Figure 6. . It can be seen from Fig. 6 that the data in the figure has a good linear relationship with equation (6), and the correlation coefficient R 2 is greater than 0.999, which indicates that the nickel cobalt slag sulfuric acid leaching reaction is controlled by the rate of diffusion in the product layer.
Figure 6. Relationship between 1−2/3x(Co)−(1−x(Co)) 2/3 and reaction time at different reaction temperatures
2. Calculation of reaction activation energy
In a chemical reaction, the reaction rate constant k is a function of temperature, and the effect of temperature on the reaction rate constant can be expressed by the Arrhenius formula:
The slopes of the straight lines obtained in Fig. 6 are the reaction rate constants k at different temperatures. The lnk is plotted against 1/T, and the result is shown in Fig. 7. The slope of the straight line is obtained according to Fig. 7. The apparent activation energy E obtained by the formula (8) is 16.34 kJ/mol, indicating that the leaching reaction is controlled by diffusion, which is consistent with the results of the previous control model. The relationship between the reaction rate constant k and temperature is: k=0.334exp(−1965.7/T)
Figure 7 lnk and 1/T relationship diagram
(III) Recovery of cobalt and nickel
The suitable conditions for the leaching were as follows: the concentration of H 2 SO 4 was 6 mol/L, the reaction temperature was 80 ° C, the reaction time was 180 min, and the particle size of the raw material was (0.074-0.100) mm. The stirring speed was 800 r/min, and the solid-liquid ratio was 1 ׃10. During the leaching process, nickel is also leached along with the cobalt. From the analysis results, it was found that under the leaching conditions, the leaching rates of cobalt and nickel were 94.2% and 93.5%, respectively. The obtained nickel-cobalt acid leaching solution can be separated from nickel and cobalt by a conventional solvent extraction technique, which is a relatively mature process in the industry.
Third, the conclusion
(1) During the leaching of nickel-cobalt slag, the particle size, sulfuric acid concentration and reaction temperature have a great influence on the cobalt leaching rate, while the stirring speed of 400-1200r/min has little effect on the cobalt leaching rate.
(2) Leaching nickel-cobalt slag with sulfuric acid, when the reaction temperature is 80 ° C, the reaction time is 180 min, the particle size of the raw material is (0.074-0.100) mm, the concentration of H 2 SO 4 is 6 mol/L, and the stirring speed is 800 r/min. When the solid-liquid ratio is 1׃10, the leaching rate of cobalt is 94.2%, and the leaching rate of nickel is 93.5%.
(3) The leaching of cobalt in the nickel-cobalt slag is controlled by diffusion in the product layer, and the apparent activation energy of the leaching reaction is 16.34 kJ/mol.
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