Utilization of lime-softening sludge to obtain calcium nitrate

• Main Authors: Olena Korchuganova, Iryna Afonina, Pavlo Prygorodov, Victoriya Mokhonko, Krystyna Kanarova
• Format: Article
• Language: English
• Published: PC Technology Center 2018-08-01
• Series: Eastern-European Journal of Enterprise Technologies
• Subjects: water treatment; lime softening; sludge; calcium nitrate; coagulants; dissolution; nitric acid; purification; deposition
• Online Access: http://journals.uran.ua/eejet/article/view/141007

The given research is devoted to the development of the technology of utilization of lime-softening sludge to obtain calcium nitrate. Water treatment waste by chemical composition differs from natural raw materials such as limestone, which is traditionally used to obtain calcium nitrate. Sludge obtained at the stage of lime-softening contains about 70% calcium carbonate, a fairly large amount of iron, which enters the precipitate with solutions of coagulants, as well as organic impurities. Organic impurities come from river water and precipitate as a result of coagulation. The process of extracting calcium by the acid solution is stable. The results are well reproduced on two kinds of waste from different enterprises. This is explained both by the high dissolution rate of calcium carbonate in nitric acid, and the similar chemical composition of the waste. Not only calcium compounds, but also iron ones together with the organic component of liming sludge fall into the solution. A process scheme is proposed for cleaning the solution, which should include the stages of oxidation of the solution and subsequent precipitation of iron. The concentration of iron in the experimental solutions was up to 6 g/l. With the help of the calculations of the precipitate-solution equilibrium, it has been stated that iron in the area of low concentrations of nitric acid precipitates in the solution, and calcium stays in it. The decrease of the acid concentration was carried out by the addition of pure calcium carbonate. Kinetic studies of the process of iron precipitation on model solutions of iron (III) nitrate have been carried out. The second-order kinetic equation is obtained. The calculation of the process activation energy is available in the paper. The value of the energy is ~37 kJ/ mole, which is the evidence of the precipitation process in the transition area. The undissolved residue contains about 40% iron and can be used to obtain coagulants. Thus, the application of the proposed method will allow the best use of water treatment waste