The subject of the invention is a method of calcium nitrate production. Calcium nitrate is used in agriculture, greenhouse cultivation, water and sewage processing and in concrete additives industry. The high quality of calcium nitrate, with very low content of insoluble substances, no anti-caking agents combined with low hygroscopicity is especially needed in greenhouse and hydroponic cultivations. Additionally, the finished product should be characterized by a low content of ferrous compounds, phosphates and heavy metals.

A patent description RP no. 172651 presents a method of liquid physiologically alkaline multi-component fertilizers production, in which ground calcium and magnesium raw materials are blown as a suspension with air into a reactor with circulating solutions of calcium nitrate and magnesium nitrate, containing 380 - 450 g of nitrates / dcm3, while reagents are simultaneously sprayed with calcium and magnesium nitrates, to which nitric acid has been added beforehand. In this way a decomposition of calcium and magnesium raw materials is achieved at the temperature of 30- 6O0C. Then gaseous ammonia and aqueous solution of acrylamide polymer are simultaneously fed into the reactor. In order to obtain a liquid physiologically alkaline fertilizer, at the final stage of the known process, such quantity of sodium nitrate solution is added that the following weight ratio of components is achieved: 2. 3.0 of calcium, 0.8-1.1 magnesium and 3.6 - 4.0 of sodium.

In the known processes of calcium nitrate fertilizer grade production, the form of the product obtained from calcium nitrate solution in a process of e.g. phosphorite reaction with nitric acid, is of great importance. It is important at the final production stages that calcium nitrate crystals have a stable form and don't contain impurities such as ferrous and fluorous compounds, which are insoluble in water and difficult to separate by the traditional methods of neutralization and purification.

A method of crystalline calcium nitrate production has been described, among others, in the patent description US 4569677. The known method is based on adding crystalline seeds to a concentrated calcium nitrate solution, which is later cooled below the crystallization temperature, so that in the final stage crystals Ca(NO3)2 are obtained, which are separated from the mother solution in separative processes. However, according to patent description no. WO 0183374 crystalline calcium nitrate is obtained through crystallization of the melt containing 5 moles of Ca(NU3)2 and 1 mol of NH4NO3, while the process is carried out in a fluidized granulator, cooled with air at the temperature of 30-400C, and relative air humidity below 30%.

Crystalline calcium nitrate has a low melting point (45-5O0C), as it is a strongly hygroscopic compound, and tends to cake, which makes it difficult to store the product. To raise the melting point 6-8% of ammonium nitrate is fed into the melt, whereas the tendency of the granulate and crystals to cake is minimized by adding anti-caking agents, in either solid or liquid form. According to the patent description no. EP 0255665 in order to reduce the hygroscopy of nitrates, and of calcium nitrate in particular, 0.1-0.5% by weight of anti-caking agent is added, containing 2-10% by weight of polyethylene wax, 20-35% by weight of microcrystalline wax, and 60-80% by weight of mineral oil. The aim of the invention was to develop a method of calcium nitrate production, which would lead to a high quality product, with a stable form, no anti-caking agents, and with a low content of phosphates, fluorides and heavy metals. It has been proved that the method of calcium nitrate production according to the invention fulfils this aim.

The method of the invention comprises firstly on the reaction in a continuous flow columns where the packing is limestone and secondly on a continuous belt crystallization of the final product. In the columns exothermic reaction takes place according to the following equation: CaCO3 + 2HNO3 D Ca(NO3)2 + H2O + CO2 D Assuming that the height of the packing is stable, and only the dosing rate of nitric acid into the reactor changes in the continuous process, the increase in the efficiency of the process is closely connected with the content of unreacted nitric acid in the reaction liquid. The correlation between the quantity of free nitric acid in the product, and the flow rate of nitric acid through the packing is an exponential function, closely related to the duration of the reaction. The above mentioned correlation is described by the following equation: 1.76 cw = 0.0578 - e τ [weight %] where: Cw - quantity of free nitric acid [% by weight] V V τ - — ^- « -**■ - duration of the reaction [h] r. +K K Vs - flow rate of nitric acid [dm3/h] V2 - flow rate of the filling material [dm3/h] V^ -space in the packing [m3] Calcium nitrate production in a continuous process is carried out in a flow columns operating in the range of Reynolds numbers characteristic for laminar flow:

Re, = l_g_. ", < P e (0,5 -10) 3 \ -ε η where: φ - shape factor Ws - nitric acid flow rate [m/s] <_/e - equivalent diameter of the packing [m] p - density in the reactor [kg/m3] ε - specific surface of the packing [m3/m3] η - viscosity [kg/ms]. Such a procedure guarantees the maximum degree of nitric acid reaction and obtaining a product, in which the quantity of free nitric acid amounts to 0.1- 0.5 % by weight, preferably 0.2% by weight.

At the next stage of the process, according to the invention aluminium hydroxide and iron hydroxide, both of which are insoluble in water, undergo a process of separation, by methods according to the state of the art, using substances with high oxidation and reduction potentials. The resulting suspension, insoluble in water solutions, undergoes the process of flocculation at the temperature of 20-250C, by adding cationic polyelectrolyte, allowing the formation of colloidal sediment, which is later separated. The purification process of calcium nitrate is carried out at pH 5-7.

The resulting calcium nitrate in liquid form may be the end product used in agriculture, greenhouse cultivation or as a concrete additive. In order to obtain the product in the solid form, according to the invention, calcium nitrate solution is concentrated in evaporators with the addition of ammonium nitrate while securing the following weight ratio: 0.6 - 0.8 % N- NH4 :26.8 - 27.2 % CaO, leading to the optimal concentration of 26.8-27.2% by weight. At the time of evaporation the crystalline seeds are coming into being in situ in the quantity of 20-40% to the total mother liquor, preferably, 30%, while the size of the seeds varies in the range of 0.01- 0.5mm. The concentration process is carried out to secure constant temperature, and CaO concentration, thus adding either crystalline seeds obtained through mechanical grinding of the end product or anti-caking agents, which would later constitute the impurity of the end product, is not necessary.

At the final stage of the process according to the invention, calcium nitrate melt is crystallized after being cooled. Preferably, during the cooling of the crystallizing suspension boron compounds, and microelements from the group: Cu, Fe, Mn, Zn in the form of biodegradable chelates, or their mixture are added up to 2,0% by weight, which increases the range of the application of the end product.

The melt prepared in this way is pumped onto a belt crystallizer, which is cooled with water and operated continuously. Rapid crystallization and reducing the degree of development of the contact surface of calcium nitrate with air leads to a significant decrease in water absorption from the environment by the end product, in comparison with the end product obtained according to the known solutions, in which calcium nitrate in the form of 1-2 mm diameter granules is produced. The product obtained according to the invention has the form of flakes. The graph presented below shows water absorption by calcium nitrate at the temperature of 250C, and relative air humidity of 65%, with the calcium nitrate sample containing no anti-caking agents. The calcium nitrate produced by the method according to the invention contains: 17.0-28% by weight of CaO, 0.1- 0.3% by weight of MgO depending on the form, either liquid or solid of the end product, 0 - 0.2 % by weight of microelements such as: Cu, Fe, Mn, Zn in the form of biodegradable chelates, and of boron. The product obtained according to the invention, with its bulk density of 950 kg/m3 and solubility of 20Og in 100 g of water at the temperature of 2O0C, contains no more than 0.01% of insoluble substances.

Graph no. 1 The graph presenting the dependence of water absorption by calcium nitrate on time - at the temperature of 250C and relative air humidity of 65%. In comparison with other known technologies of production the obtained product is free of such impurities as : fluoride ions, phosphor ions, and heavy metals, therefore it may have various applications also in the greenhouse and hydroponic cultivations.

Example 1 292kg/h of limestone and 500 l/h of 53% by weight nitric acid have been continuously fed into the flow columns. During the continuous process of the reaction, with the flow of nitric acid secured at Reynolds number 1, 537 l/h of calcium nitrate solution have been obtained, containing 916 kg of Ca(NO3)2 in Im3 of the solution. After storing 10m3 of calcium nitrate the remaining free nitric acid has been neutralized with the use of 50 I of calcium suspension. The oxidation and separation of ferrous and aluminium compounds, which are both insoluble in water, have been carried out at pH = 7, with 4 I of sodium hypochlorite per Im3 of calcium nitrate used as the oxidizing agent. After quantitative separation of insoluble substances a flocculation of the sediment has been carried out with the use of 200 I of 0,1% cationic polyelectrolyte. The flocculating sediment has been removed, at the first stage, with the use of pressure filtration on a filtering press, and at the second stage with the use of candle filters. 1661 of water per Im3 of calcium nitrate has been added to a clear solution. A commercial product has been obtained with the following parameters: CaO - 17% MgO - 0,1% N-NO3 - 8,5% N-NH4 - 0% pH - 6,5 d - 1,500 g/cm3 Insoluble substances - 0,002% Example 2 537 kg/h of limestone and 900 l/h of 55% by weight nitric acid have been continuously fed into the flow columns. During the continuous process of the reaction, with the flow of nitric acid secured at Reynolds number 3, 966 l/h of calcium nitrate solution have been obtained, containing 959 kg of Ca(NO3)2 in Im3 of the solution. After storing 10m3 of calcium nitrate the remaining free nitric acid has been neutralized with the use of 10 I of 25% by weight of ammonia water. The oxidation and separation of ferrous and aluminium compounds, which are both insoluble in water, have been carried out at pH = 7, with 4,5 I of sodium hypochlorite per 1 m3 of calcium nitrate used as the oxidizing agent. After quantitative separation of insoluble substances a flocculation of the sediment has been carried out with the use of 200 I of 0,1% cationic polyelectrolyte. The flocculating sediment has been removed, at the first stage, with the use of pressure filtration on a filtering press, and at the second stage with the use of candle filters. Solid ammonium nitrate has been added to the clear solution according to the following proportion N-NH4 : CaO = 0.6 : 27.0. After gaining complete homogenity the solution has been concentrated until a concentration of 27% by weight of CaO has been secured, while 20% by weight of crystalline seeds have been produced in situ. The melt prepared in this way has been crystallized in the form of flakes on a belt crystalizer leading to a product with the following contents: CaO - 27% MgO - 0,2% N-NO3 - 14,2% N-NH4 - 0,6% Insoluble substances - 0,003% The maximum water absorption by the obtained product amounts to 7% with the relative air humidity up to 60% at the temperature of 250C, with the graph presenting the water absorption by the product obtained according to the invention shown in figure no. 1.

Example 3 710 kg/h of limestone and 1200 l/h of 55% by weight nitric acid have been continuously fed into the flow columns. During the continuous process of the reaction with the flow of nitric acid secured at Reynolds number 6, 1290 l/h of calcium nitrate solution have been obtained, containing 1150 kg of, Ca(NO3)2 in Im3 of the reaction mixture. After storing 12m3 of calcium nitrate the remaining free nitric acid has been neutralized with the use of 18 I of 25% by weight of ammonia water. The oxidation and separation of ferrous and aluminium compounds, which are both insoluble in water, have been carried out at pH = 6, with 4,5 I of sodium hypochlorite per Im3 of calcium nitrate used as the oxidizing agent. After quantitative separation of insoluble substances a flocculation of the sediment has been carried out with the use of 260 I of 0,1% cationic polyelectrolyte. The flocculating sediment has been removed, at the first stage, with the use of pressure filtration on a filtering press, and at the second stage with the use of candle filters. Solid ammonium nitrate has been added to the clear solurtion according to the following proportion N-NH4 : CaO = 0,8 : 26,8. After gaining complete homogenity the solution has been concentrated until the concentration of 27% by weight of CaO has been secured, while 16% by weight of crystalline seeds have been produced in situ. 35 kg of Na2B4O7 x 10H2O and 215 kg of chelated biodegradable microelements containing the following elements: Cu, Fe, Mn, Zn have been added. The melt prepared in this way has been crystallized in the form of flakes on a belt crystalizer, leading to a product with the following contents: CaO - 26,8% MgO - 0,2% N-NO3 - 14,6% N-NH4 - 0,8% B - 0,03% Cu - 0,02% Fe - 0,06% Mn - 0,05% Zn - 0,03% Insoluble substances - 0,003%

The maximum water absorption by the obtained product amounts to 7% with the relative air humidity up to 60% at the temperature of 250C