Preparation Of Ammonium Sulfate Combined With Synthesis Gas Decarbonating In Slurry Of Ammonia- Phosphogypsum

Phosphogypsum, which is a by-product from wet phosphoric acid process, has restricted the sustainable development of phosphorus fertilizer industry due to its occupation of land and pollution in the environment. At present, many countries are paying more and more attention to dealing with phosphogypsum. The process which converts phosphogypsum to ammonium sulfate fertilizer with precipitated calcium carbonate as a by-product during the conversion is one of the effective approaches to the disposal of phosphogypsum. The process has already been industrialized and there is no difficulty in technology; but its cost is more expensive than high concentrated nitrogen fertilizer developed in 1970s, which seriously restrained its development. Therefore, improving the process and reducing cost are the most important way. In this paper, a new ameliorated process has been presented that the disposal of phosphogypsum is integrated into the carbonation process of synthesized ammonia, in which the utilization of phosphogypsum and the purification of raw material gas of synthesized ammonia is achieved at the same time. Preliminary cost estimates suggest that the process is economically feasible. Although some basic data can be obtained from literatures, there remains many fundamental research works to do. A process experiment has been designed to simulate the conditions in vapor-liquid-solid system. In order to understand the optimum process conditions, the influence of raw material proportion, material concentration, reaction time and temperature on the reaction of Phosphogypsum with aqueous ammonia and carbon dioxide to prepare ammonium sulfate and calcium carbonate under the conditions of vapor-liquid-solid system has been studied. According to the experimental investigation, the mechanism and kinetics of the carbonation process of the vapor-liquid-solid system hasve been analyzed adequately. On the basis of penetration theory, a simplified kinetics model has been proposed to calculate the dynamic absorption rate of carbon dioxide into suspension of phosphogypsum in aqueous ammonia.The experimental results indicated that the conversion of Phosphogypsum was more than surpassed 96%, which was not less than that of ammonium bicarbonate process. Such proved the new process was feasible. Meanwhile, the results revealed the conctact style and contact time played important roles in the process, suggesting that gas-liquid mass transfer be essential, even the determining step. So it is necessary to pay more attention to the mass transfer. The temperature of the vapor-liquid-solid system is also an important parameter. Lower temperature can reduce the volatilization of ammonia and promote the crystallization and growth of calcium carbonate. Under experimental conditions, 40℃ is compatible.Chemical reactions involved in the vapor-liquid-solid system were considered as three consecutive and reversible steps: the absorption reaction of carbon dioxide, the hydrolysis of carbamate and the dissolution of phosphogypsum, and other reactions included were instantaneous. The reaction rate equations and equilibrium constants were also provided. A model based on penetration theory has been developed to quantify the mass transfer within the liquid phase. The developed concentration profiles of the diffusing species and the depth of penetration have been determined for different penetration time and different carbonation degree. Moreover, the chemical enhancement factor and carbon dioxide absorption rate were calculated. Model simulations revealed that both the enhancement factor and the absorption rate decreased as carbonation degree increasesd from low level to high level.