Dolomite is a carbonate-grade mineral that contains calcium and magnesium. The color of this mineral can vary from white to brown depending on the number of impurities. Powdered dolomite is easily decomposed by mineral acids while carbon dioxide is released. Although dolomite-bearing rocks are found all over the world, the most famous quarries are in the Midwestern United States, Canada, Switzerland, Spain, and Mexico [1]. Large dolomite quarries are also exploited in northern Lithuania. Dolomite is used as a thermal insulation material, also in agriculture as a liming material, electrical engineering, ceramics, paint, glass and building materials industry. This mineral can also be used as a source of calcium magnesium in the production of fertilizers by decomposing it with various concentrations of nitric acid.
Based on previous research in our department, different concentrations (35, 40, 45%) of nitric acid were used in this work for the decomposition of dolomite. Decomposition was performed at room temperature at 20–25 °C. Excess of 20% and 40% nitric acid was compared to the stoichiometric amount and used. It was found that the reaction is faster and decomposition takes 50 minutes, when 140 % acid is used. During the decomposition, the temperature increased to 40-50 °C and not only CO2 gas but also brown NO2 gas was released too. In this way, in the production of liquid fertilizers, it is necessary to provide for the purification of the different gases in the technological line. During the decomposition for 50 minutes, the highest concentration of CaO and MgO was obtained when 45% nitric acid was used in the dolomite decomposition process.
Solutions after the decomposition are very acidic, so neutralization needs to be performed. 25 % ammonia water was chosen for this process. Liquid nitrogen calcium magnesium fertilizer solutions of different pH values were prepared for neutralization using different amounts of ammonia water. The produced fertilizers contained ~10 % N, ~6 % CaO and ~4% MgO. The physical properties of the prepared fertilizer solutions, which are important for the development of the liquid fertilizer production technology, are given in the table.
| Sample No. | Ammonia water quantity, % | Crystallization temperature, °C | pH | Density, g/cm3 | Viscosity, mm/s2 | Electrical conductivity, mS/cm3 |
|---|---|---|---|---|---|---|
| 1 | 11.6 | -30 | 4.15 | 1.364 | 2.395 | 138.4 |
| 2 | 11.8 | -32 | 4.50 | 1.364 | 2.390 | 137.8 |
| 3 | 12.9 | -30 | 5.10 | 1.360 | 2.282 | 135.8 |
In order to increase the nitrogen concentration in the fertilizer, and after evaluating the solubility data of multicomponent systems available in the literature, additional nitrogen-increasing substances such as urea was added. These substances increased the nitrogen concentration in liquid fertilizers, but do not increase their crystallization temperature, which is one of the most important positive properties of liquid fertilizers. After decomposition of dolomite with nitric acid, neutralization of solutions and addition of urea, the nitrogen concentration increased to 22 %.
To produce the most efficient liquid fertilizers, a peat extract additive was added to the solutions, which acts as a bioactive substance, activates the processes taking place in the soil and improves the uptake of nutrients in plants [2]. UAB “Klasmann-Deilmann” Šilutė peat and potassium hydroxide were used to obtain the extract. Up to 20 % of the obtained extract was added to the liquid fertilizer. In the laboratory test, it was found that the addition of peat extract to liquid nitrogen calcium magnesium fertilizers reduced the nitrogen concentration from ~22 % to ~16%, and the addition of the extract did not affect the calcium and magnesium concentration. The pH value of liquid fertilizers with peat extract increases and varies between 5.5 and 7.0, the density increases from 1.265–1.270 g/cm3 to 1.332–1.335 g/cm3 and the viscosity increases from 0.55–0.75 mm/s2 to 0.65–0.84 mm/s2. The crystallization temperature of liquid nitrogen calcium magnesium fertilizers with peat extract varies in the range of -12.5 – -15 °C. All the specified physical properties comply with the requirements for liquid fertilizers.
In conclusion, a solution of the decomposition of dolomite with 45 % nitric acid, neutralized with 25 % ammonia water and with the addition of 38 % urea can produce liquid nitrogen calcium magnesium fertilizers containing 21.5% N, 3.8% K2O, 3.6% CaO and 2.5 % MgO with a crystallization temperature of 3 °C. Addition of 20% peat extract can be used to produce bioactive liquid nitrogen calcium magnesium fertilizers containing 15.1% N, 3.8% K2O, 3.6 % CaO and 2.5% MgO with a crystallization temperature of -15 °C.