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The so-called phase diagram is a phase equilibrium diagram, which is a graph showing the relationship between various composition conditions (temperature, concentration, pressure) of a system at equilibrium. Ammonia and carbon dioxide can form a series of salts, hydrates, and double salts in aqueous solution under different conditions. In the production of ammonium bicarbonate, it is necessary to understand under what conditions crystals will form in the solution, how much crystal will be produced, and what the composition of these crystals is; these aspects can be analyzed using the phase diagram of the NH3-CO2-H2O three-phase system. On the crystalization process analysis graph for the carbonation of ammonia water, the horizontal axis represents the concentration of ammonia, while the vertical axis represents the concentration of carbon dioxide, both expressed as weight percentages. The dashed lines represent the solid-liquid equilibrium lines at various temperatures (the curves of saturated salt solutions). There are five crystalline regions in the phase diagram, indicating that ammonia, carbon dioxide, and water can give rise to five types of salts. ABGH is the crystallization region of the NH4HCO3 and (NH4)2CO3.H2O double salt, GBCF is the crystallization region of the (NH4)2CO3.H2O salt, ABCDJ is the crystallization region of the NH4HCO3 and (NH4)2CO3.H2O double salt, and dcfe is the crystallization region of the NH2COONH4 salt. These four crystallization regions are all located in areas with relatively high ammonia concentrations. The NH4HCO3 salt we need has its crystallization region in the LHAJK zone; most of it is within a range of lower concentrations, while a portion is in a range with higher ammonia concentrations. HAJ is the boundary line for ammonium bicarbonate crystals, KL is the saturation line for carbon dioxide, and DE is the saturation line for ammonia. The final stage of the carbonization process is the formation of ammonium bicarbonate crystals, which are required to be large and uniform in size, with a solid structure. Such crystals are conducive to centrifugal separation, have good stability, and can reduce decomposition losses. Assuming that the carbonization starting solution, which is concentrated ammonia water, contains no CO2, its composition point lies on the horizontal axis. If the ammonia concentration in this concentrated ammonia water is 20%, the corresponding composition point on the graph is Q. As the carbonization reaction proceeds, the concentrated ammonia water continuously absorbs CO2 from the gas, resulting in an increasing level of CO2 in the solution. Therefore, the composition point of the carbonized solution moves along the line QR toward point R. This QR line represents the carbonization process line; once the carbonization degree of the solution reaches point P, cooling takes place. If it is cooled to 20°C, it enters the ammonium bicarbonate crystallization zone, where bicarbonate crystals are formed. Connect composition point I of ammonium bicarbonate to composition point P of the carbonation system, and extend the line to intersect the saturated line of ammonium bicarbonate at 20°C at point M. Point M represents the composition of the clear solution; the ratio of the lengths of segments PM to PI is the ratio of the amount of NH4HCO3 crystals to the amount of clear solution. Similarly, if cooled to 35°C, the composition of the clear solution corresponds to point N on the saturation line at 35°C; the ratio between the condition when NH4HCO3 crystallizes and that when it is in the clear solution form is equal to the ratio of the line segment PN to PI. It can be seen that the lower the cooling temperature, the more crystals precipitate; if the carbonization degree of the solution is higher than point P, for example, reaching point S, then more crystals will also precipitate