I. Theoretical basis of leaching (I) Overview
Leaching is a process in which a solvent selectively dissolves a component of a mineral raw material, the purpose of which is to separate the useful component from the impurity component or the gangue component. The reagent used for leaching is called a leaching agent, the leaching solution is called a leaching solution, and the residue after leaching is called leaching slag. The mineral raw materials entering the leaching operation are generally refractory mineral raw materials that cannot be treated or treated uneconomically by physical ore smelting method or traditional smelting method under current technical conditions, such as refractory ore, physical ore selection, difficult to select mixed ore, Difficult to choose coarse concentrate, tailings, lean ore and off-balance. According to the characteristics of the raw materials, calcination may be carried out in advance, followed by leaching or direct leaching. Therefore, leaching is a common operation in the chemical beneficiation process.
In practice, the leaching rate of the target component, the selectivity of the leaching process, and the reagent consumption are commonly used to measure the efficiency of the leaching process. The leaching rate of a component is the ratio of the amount of the component transferred to the solution at the time of leaching to the total amount in the raw material, namely:
Where ε immersion - the leaching rate of a component, %;
Q———the dry weight of the material to be immersed, t;
A—the content of a component in the immersed material, %;
V———the volume of the leachate, m 3 ;
c——— the concentration of this component in the leachate, t/m 3 ;
m———dry weight of leaching residue, t;
Θ—the content of this component in the leaching residue, %.
The selectivity of the leaching process is the ratio of the leaching rates of the two components during leaching:
At present, there are many leaching methods, which can be divided into water solvent leaching and non-aqueous solvent leaching depending on the reagents used. According to the material movement mode, it can be divided into agitation leaching and percolation leaching. Stirring leaching is a leaching process in which the ground material and the leaching agent are vigorously stirred in a stirred tank. The diafiltration leaching process is a leaching process in which the leaching agent passes through the fixed material layer from top to bottom or under pressure by gravity under the action of gravity. According to the leaching method, it can be divided into three types: in-situ percolation leaching (underground leaching), percolation heap leaching and percolation tank leaching. According to the leaching temperature and pressure, it can be divided into normal pressure leaching and hot pressure leaching.
The choice of leaching agent mainly depends on the characteristics of raw materials and economic factors. The principle is thermodynamically feasible, high leaching rate, fast leaching speed, good selectivity, and low cost. [next]
(2) Thermodynamics of the leaching process
The leaching process is a multiphase chemical reaction process in an aqueous solution. According to the essence of the chemical reaction during leaching, it can be divided into two major categories: oxidation-reduction reaction and non-oxidation-reduction reaction. Each major class can be divided into two sub-classes: hydrogen ion participation and hydrogen-free ion participation. For example, the ε-pH diagram of the Fe-H 2 O system is shown in Figure 1. Each line in the figure represents an equilibrium condition, and the intersection of the lines indicates that the potentials and pH values ​​of the respective equilibrium equations are equal, and the faces in the figure represent groups. The stable zone of the minute.
In the ε-pH diagram, a (O 2 line) and b line (H 2 line) indicating the stability of water are also drawn. The stable upper limit of water precipitates oxygen, and the reaction formula is:
O 2 +4H + +4e=2H 2 O
O2/H 2 O=1.229-0.0591Ph+0.01481gP o2
=1.229-0.0591pH (when P o2 =101325Pa)
The lower limit of the stability of water precipitates hydrogen, and its reaction formula is:
2H + 2e=H 2
ε H +/H2=-0.0591pH-0.0295lgp H2
=-0.0591pH (when P H2 =101325Pa)
When the reaction temperature is not equal to 298K and the activity of each component is not 1 and other values ​​are balanced, it shall be calculated according to the conditions given and plotted with the calculated values. Therefore, each specific ε-pH map is only applicable to a certain reaction temperature and specific component activity conditions.
If the aqueous solution contains complexing agent object components, complex formation will change the redox metal ions. Let the complexation reaction of Mn+ and complex L (which can be charged or not, where there is no charge):
Mn+ZL=MLn+z
Then the standard potential of the metal complex and the metal pair is:
Similarly, the reaction of the same metal ion in different valence states to form a complex is:
M m+ +(mn)e===M n+ (m>n)
Let the highest coordination number complex be ML m+ p and ML n+ p , respectively, and the electrochemical reaction between them is:
ML m+ p +(mn)e===ML n+ -p
Where K m , K n — the complexation constant of high-valent ions and low-valent ions.
It can be seen from the above that the more stable the complex formed by the metal ion and the complex is, the smaller the standard potential value of the electric pair is, that is, the corresponding metal is more easily oxidized and transferred into the solution as a complex ion form. Similarly, if the high-valent complex ions of the same metal are stable at lower valence ions, the lower-priced ions are more likely to be oxidized and exist in the form of high-complex ions. In production practice, this principle is often used to leach some of the components that are difficult to oxidize (such as cyanide gold, etc.). In most cases, high-valent metal complex ions are stable at lower valence metal ions, but there are exceptions, such as Fe 3+ and The complex formed by Fe 2+ with bpy (bipyridyl) or phea (phenanthroline) belongs to this.
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