Calcination

Calcination is a process of thermal dissociation or crystal transformation of mineral or man-made compounds. At this time, the compound is thermally dissociated at a certain temperature into a relatively simple compound or undergoes a crystal transformation to facilitate subsequent processing or to convert the chemical product into a form suitable for the user's needs. The response can be expressed as:

The main factors affecting calcination are temperature, gas phase composition, thermal stability of minerals, and the like. The general principle of thermal dissociation of compounds is now discussed by taking calcination (baking) of carbonate as an example. The thermal dissociation of a compound is generally a reversible reaction:
MCO ₃ ====MO+CO ₂
Under the condition that no solid phase exists between the solid phases, the equilibrium constant of the reaction is:
K _P =P _co2 (MCO ₃ )
The equilibrium partial pressure of the thermal dissociation of a compound at a certain temperature is called the decompression pressure of the compound, and its value can be used as a measure of the thermal stability of the compound. Some relations carbonate decompressed from the temperature shown in Figure 1, seen from the graph, when the same gas phase P _co2, calcite most stable, easily mineral siderite baking solution. Free energy change of the baking system

△G=△G°+RtinQ
=-RTinK ₂ +RtinQ
=RTinP _co2 -RtinP _co2(MCO3)
=4.576[lgp _co2 -lgp _co2(MOC3) ]
Where P _co2(MCO3 )—the equilibrium dissociation pressure of carbonates;
P _co2 — The actual partial pressure of carbon dioxide in the gas phase. [next]
When baking carbonate solution, the system is available free energy change values to measure the size of the metal oxide affinity for carbon dioxide. When P _co2 = 101,325 Pa, _ΔG = ΔG °, the affinity at this time is called the standard chemical affinity, and therefore, the carbonate can be measured by ΔG°. Thermal stability or affinity of metal oxides for carbon dioxide. When P _co2 >P _co2(MCO3) , the reaction proceeds in the direction in which carbonate is formed, otherwise the carbonate dissociates into metal oxides and carbon dioxide. Figure 2 is the decompression curve of some carbonates. If the operating conditions are selected at point a, the siderite and magnesite are baked, and the calcite is not baked. Calcination of calcite can be achieved by increasing the temperature or reducing the partial pressure of carbon dioxide in the gas phase. However, industrial heating is used to make calcite roasting. Since dolomite decomposition temperature of 750 ~ 800 ℃, resulting magnesite ore begins at 600 deg.] C and lysed instantaneous precipitation of carbon dioxide, calcite decomposition gold finish at 950 deg.] C conditions, when the temperature is higher than 1000 ℃ starts to generate a dense sintered Block, so the carbonate boiling temperature should be lower than 1000 ° C. Reduce the partial pressure of carbon dioxide in the gas phase, can reduce the initial temperature of carbonate baking, fine P _co2 = 101,325 Pa, the initial baking of calcite The temperature is 910 ° C, in the air (P _co2 = 303.98 Pa) is 800 ° C.

The decompression curve of some oxides is shown in Figure 3. From the curve, the magnetite is the most stable in the air, and the oxides of silver and mercury are more easily dissociated by heat, so silver and mercury can exist in the metal in the crust. in. Seen from FIG. 3, silicon, vanadium, titanium, zirconium, aluminum, barium, calcium affinity for oxygen and a minor silver, mercury, copper, lead and other affinity for oxygen. At high temperatures, copper, zinc , calcium, etc. have a greater affinity for sulfur, and the affinity of the metal for oxygen is greater than that for sulfur. When a high-priced compound is thermally dissociated, it begins to decompose into lower-priced compounds.

Due to the different thermal stability of various compounds, controlling the calcination temperature and the gas phase composition can cause some compounds to be thermally dissociated or crystallized, and then appropriately treated to achieve the purpose of removing impurities or enriching useful components. For example, siderite can be thermally dissociated into magnetite in a neutral atmosphere at 300-400 ° C and can be selected by a weak field magnetic separator. Limestone and magnesite can be calcined to calcium oxide at about 900 ° C. separating calcium oxide available digestion, the magnesium oxide available reselection recovered, carbonate calcined phosphorus ore can be used to digest a sorting process to give high phosphorus ore, manganese mineral conditions may be 600 ~ 1000 ℃ Calcination converts all manganese minerals into black manganese ore. This process can be used to treat refractory manganese ore and obtain manganese concentrate. Paramagnetic pyrite can be calcined at 700-1000 ° C to monoclinic pyrrhotite. The process can be used to remove pyrite from the ore in the molybdenum , and the a - spodumene (which does not react with sulfuric acid) can be converted into β-spodumene which can be effectively decomposed by sulfuric acid at a temperature of about 1000 ° C. At the same time of β-transformation, the volume of surrounding rock of spodumene changes. The fine-grade β-spodumene can be sorted from the surrounding rock by air classification. The beryl is heat-treated in an electric arc furnace at 1700 °C, and then it is made. Quenching, the beryl can be transformed into an amorphous (glassy) green column that is easily soluble in sulfuric acid.

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