Oxidative roasting and sulfation roasting
Mineral sulfide is heated in an oxidizing atmosphere condition, all (or part) and the removal of sulfur into the corresponding metal oxide (or sulfate) in the process, referred to as roasting (or sulfuric acid roasting). Whether the sulfide mineral is heated in air is partially sulfated or fully oxidized, depending on the firing conditions. Under calcination conditions, the main reactions of metal sulfide minerals are:
2MS+3O 2 =2MO+2SO 2
2SO 2 +O 2 ===2SO 3
MO+SO 3 ===MSO 4
Where MS - metal sulfide;
MO - metal oxide;
MSO 4 - metal sulfate.
In the oxidative calcination, the reaction of the metal sulfide to the metal oxide and sulfur dioxide is irreversible, and the other reactions are reversible. The equilibrium constants of the above respective reaction formulas are:
Where P so2 - the partial pressure of sulfur dioxide in the furnace gas;
P o2 - the partial pressure of oxygen in the furnace gas;
P so3 - the partial pressure of sulfur trioxide in the furnace gas;
P so3 (MSO4) - decomposition pressure of metal sulfate.
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The relationship between P so3 and P so3 (MSO4) in the furnace gas during calcination is shown in Fig. 1. The curve in the figure shows that when the temperature is low and there is a high concentration of sulfur trioxide in the furnace gas, the metal sulfide will be converted into the corresponding metal sulfate. When the temperature rises to 700 to 900 ° C, the sulfide will oxidize to the corresponding metal oxide. Since the decomposition temperature and decomposition free energy of various metal sulfates are different, the composition of the calcined product can be controlled by controlling the calcination temperature and the composition of the furnace gas to achieve the purpose of selective sulfuration roasting. For example, the state diagram of the Cu-Co-SO system at 680 °C is shown in Fig. 2. The solid line in the figure is Co-SO system, and the broken line is Cu-SO system. If the furnace gas composition is 8% SO 2 , 4% O 2 , the copper- cobalt sulfide is converted into the corresponding sulfate, which can produce 97% soluble copper and 93.5% soluble cobalt. If the calcination conditions are controlled in zone A, only soluble cobalt sulfate and water-insoluble copper oxide are produced.
The temperature at which the sulfide mineral is oxidized and calcined should be higher than the ignition temperature of the corresponding sulfur mineral. The ignition temperature of sulfide minerals is related to their particle size. In practice, the calcination temperature often fluctuates between 580 and 850 ° C and should not exceed 900 ° C, otherwise the material will melt to form a poorly soluble co-solvent.
Oxidative roasting and sulphating roasting are widely used to treat iron , copper, copper- nickel , cobalt, molybdenum , zinc , antimony and other sulfide minerals, so that heavy metal sulfides can be converted into easily soluble metal oxides or sulfates, making iron into insoluble The iron oxide can change the mineral structure, make it porous and porous, and can cause arsenic , antimony, selenium and lead to volatilize in a gaseous state. This method can thus be used to extract or remove these components from mineral raw materials. Under the oxidative roasting conditions, the volatilization rates of some metals are: %10~20, ln5~10, Ta5O~70, As60~5~7.
80, Sb20～40, Bi10～1.5, Se25～50, Cd5～20, Pb5～10, Zn5～7 The quality of oxidative roasting is usually measured by the desulfurization rate or the degree of sulfation of the target component.
Calcination is a multiphase chemical reaction process. The main influencing factors are: temperature, physical and chemical properties of reactants and products (such as particle size, porosity, chemical composition, mineral composition, etc.), gas flow characteristics (flut), gas phase The concentration of oxygen and sulfur dioxide, etc. When the calcination temperature is greater than the ignition temperature of the sulfide, the heat evolved by the reaction is sufficient to cause the oxidation process to proceed spontaneously throughout the material layer. The temperature has different effects on different sulfides, and the calcination temperature must be lower than the melting temperature of the material. Therefore, the appropriate firing temperature must be selected according to the material characteristics and subsequent process requirements, and the cooling device is often used to control the temperature. The particle size is small, the porosity is large, and the phase boundary area is large, and the oxidation process is easier to carry out. Some sulfide minerals (such as pyrite, chalcopyrite) will disintegrate and disintegrate under roasting conditions, making the particle size smaller and depositing elemental bowls, so it is easier to oxidize and roast. The galena mine does not dissolve or dissolve, so it is difficult to oxidize and is easy to agglomerate.
The oxidizing roasting process can be used as a roasting equipment according to the production scale, using a roasting pot, a reverberatory furnace or a rotary kiln that continuously operates, a boiling furnace, a multi-layer roasting, and the like.
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