SE438843B - PROCEDURE FOR PREPARING A WATER SOLUTION OF CALCIUM NITRIT WITH HIGH PURITY AND HIGH CONCENTRATION - Google Patents

Description

,., ._. The inventors have sought to achieve a process for producing an aqueous solution of high purity and high concentration calcium nitrite from nitric oxide gas and an aqueous slurry of calcium hydroxide. the inventors have found that the formation of calcium nitrate as a by-product can be prevented by bringing a gas containing more than about 3% by volume of nitrogen oxides into contact with a water slurry containing 3-10% by weight of calcium hydroxide and containing calcium nitrite to absorb the gas, thereby forming calcium nitrite with high efficiency.

The inventors have investigated and found a process for preparing an aqueous solution of calcium nitrite by (1 ') a step comprising bringing nitric oxide-containing gas containing 5-10% by volume of nitrogen oxides and a molar ratio of NO / NO 2 of 1.2-1.5 in contact with an aqueous slurry containing 20-40% by weight of calcium hydroxide at 40-70 ° C, until the calcium hydroxide content is reduced to within the range of 3-10% by weight in a first step, and (2 ') a step comprising separating unabsorbed and unreacted gas and oxidation of the separated gas to form a gas containing 1-3% by volume of nitrogen oxides with a NO / NO 2 molar ratio of 1.2-1.5 and (3 ') a step comprising bringing the resulting gas with low nitric oxide content in contact with the separated aqueous slurry containing 3 to 10% by weight of calcium hydroxide and containing calcium nitrite at 40-70 ° C to reduce the content of calcium hydroxide to less than 3% by weight as a second step in t (4 ') a step comprising filtering the obtained solution with high calcium nitrite concentration.

For the production of a large amount of water slurry of Ca (NO 2) 2 concentration exceeding, for example, 30% in an amount of a high degree of purity exceeding 95% with a high number of thousand tonnes per year, a large apparatus is required. To keep the temperature of the reaction slurry at 40-70 ° C upon absorption of a gas containing nitrogen oxides (a NO / NO 2 molar ratio of 1.2-1.5) at a temperature exceeding 150 ° C obtained by oxidation of ammonia in a water slurry of slaked lime with a concentration of 20-40%, it is necessary to use a large cooler to cool and a large reactor to oxidize the exhaust gas from the first reactor.

It is important to overcome these disadvantages.

The cooling effect can be improved by raising the reaction temperature, but when the reaction temperature exceeds 70 ° C below atmospheric pressure, the partial pressure of water vapor for the slurry is higher than the partial pressure of water vapor in the gas containing nitrogen oxides, so a concentration of the slurry is obtained. of water vapor, and the complex Ca (NO2) 2.Ca (OH) 2.2H2O precipitates and increases the viscosity of the slurry. The reaction will therefore not proceed without difficulty.

The inventors have further carried out studies to study the possibility of preventing the concentration of the slurry and it has been found that the amount of calcium nitrate as a by-product is increased in the reaction, so that an aqueous solution of high purity calcium nitrite (exceeding 95%) could not be obtained. - held. On the other hand, the inventors have found that although the solubility of slaked lime decreases due to temperature rise, the reaction rate remains essentially the same. Furthermore, the inventors have found that the increase in the amount of calcium nitrate as a by-product and the concentration of the slurry can be prevented by absorbing a gas containing nitrogen oxides (the NO / NO 2 molar ratio is higher) under a higher pressure at a temperature of the slurry exceeding 70 ° C.

It is an object of the present invention to provide a process for preparing an aqueous solution of high purity and high concentration calcium nitrite in excess of 30% by weight with high conversion. of more than 95%.

It is another object of the invention to provide a process for producing an aqueous solution of calcium nitrite with high productivity by minimizing the capacity of a cooler and a reactor at a higher temperature and under a higher pressure. It is another object of the invention to provide a process for preparing an aqueous solution of calcium nitrite of high purity and high concentration with high efficiency by combining simple steps of an aqueous slurry of calcium hydroxide and nitric oxide-containing gas. object of the invention is to provide a process for the preparation of an aqueous solution of calcium nitrite of high purity and high concentration with high efficiency under conditions which result in reduced loss of calcium hydroxide and nitric oxide gas.

The foregoing and other objects of the invention have been achieved by utilizing a process for preparing an aqueous solution of calcium nitrite by contacting a nitric oxide-containing gas with an aqueous slurry of calcium hydroxide, which is characterized in that a gas having a temperature of 190 -300 ° C and a nitrogen oxide concentration of 5-12% by volume and a molar ratio of NO / NO 2 of 1,6-2,5 are brought into contact with a water slurry with 20-40% by weight of calcium hydroxide content at 75-11 ° C without a pressure high enough to prevent concentration of the slurry until the calcium hydroxide content is lowered to a range of 2-10% by weight as a first step, and the unabsorbed and unreacted gas is separated and the gas is oxidized at 85-150 ° C to formation of a gas with a nitrogen oxide content of 1-5% by volume and a molar ratio of NO / NO 2 of 1.6-2.5 and that the resulting gas is brought into contact with an aqueous slurry, which is separated from the the first step and contains 2 to 10% by weight of calcium hydroxide and calcium nitrite, at 7 DEG-110 DEG C. under a pressure high enough to prevent the slurry from concentrating so that the calcium hydroxide content decreases to less than 3% by weight as a second step, and filtration of the resulting solution.

A first feature of the invention is that the aqueous slurry of calcium hydroxide is reacted with nitric oxide gases in two steps such as step (1) and step (3) under different concentrations.

The second feature of the invention is to reduce the loss of calcium hydroxide and nitric oxide in step (2) and step (3).

The third feature of the invention is to combine steps (1), (2), (3) and (4), whereby the formation of a by-product of calcium nitrate is reduced and the conversion to calcium nitrite increases and becomes more than 95%, so that one obtains an aqueous solution of calcium nitrite with high purity and high concentration with high efficiency.

In the process of the invention, the conversion to calcium nitrite of a ratio of calcium nitrite to the total amount of calcium nitrite to calcium nitrate is indicated.

The water slurry of calcium hydroxide with a high calcium hydroxide content can be easily obtained by dispersing a commercially available calcium hydroxide, for example slaked lime, in water.

When the calcium hydroxide content is less than 20% by weight, an aqueous solution of high concentration calcium nitrite can not be obtained, which is the object of the invention, even if steps (1), (2), (3) and (4) are combined.

When the calcium hydroxide content exceeds 40% by weight, the viscosity of the slurry of the reaction mixture in step (1) becomes too high due to the formation of a complex, whereby the absorption of nitric oxide gas can not be carried out undisturbed. Furthermore, the precipitation of calcium nitrite becomes disadvantageous.

The gas with high nitrogen oxide content used according to the invention can be easily obtained by oxidizing ammonia with air and usually has a pressure of less than 10 kp / cm 2 and a temperature exceeding 150 ° C, preferably a temperature of 190-300 ° C. It is important that the molar ratio NO / NO 2 in the gas amounts to 1.6-2.5.

When the molar ratio of NO / NO 2 is less than 1.6, the formation of the by-product calcium nitrate increases in step (1) including the contact with the aqueous slurry of calcium hydroxide for absorption, whereby an aqueous solution of high purity calcium nitrite can not be obtained.

When the molar ratio of NO / NO 2 exceeds 2.5, the conversion of nitric oxide in step (1) decreases, whereby the efficiency of the reaction remains low. However, it is not sufficient to define only the molar ratio of NO / NO 2 in the nitric oxide-containing gas. used in step (1), but it is appropriate to define the nitric oxide content of the gas to a specific range.

When the nitric oxide concentration is less than about 5% by volume, the conversion decreases so that it becomes uneconomical with regard to the apparatus efficiency.

When the nitric oxide content is higher, no problems are obtained.

However, it is convenient to use a nitric oxide-containing gas with less than 12% by volume of nitric oxide by oxidation of ammonia in industrial operation. "It is suitable to use nitric oxide-containing gas with a high concentration but less than 12% by volume in step (1) of the process according to the invention. 8006790-3 7 The nitric oxide-containing gas is supplied at a temperature of 190-300 ° C. If the temperature is below 150 ° C, moisture is condensed in the gas, leading to the formation of nitric acid and calcium nitrate as a by-product Condensation of the moisture in the gas can be prevented almost completely at temperatures exceeding 190 ° C. However, if the temperature exceeds 300 ° C, it increases the amount of heat which must be removed from the reaction zone according to step (1).

In the process of the invention, it is necessary to keep the temperature of the water slurry at 75-110 ° C in step (1), when the nitric oxide-containing gas is brought into contact with the water slurry of calcium hydroxide under higher pressure of, for example, 2.0-10 atm.

When the temperature is high, the formation of the complex can be prevented. However, the vapor partial pressure of the water slurry is higher than the vapor partial pressure of the nitric oxide-containing gas, when the temperature exceeds 70 ° C at atmospheric pressure, so concentration of the water slurry is obtained and the reaction with contact and absorption of nitric oxide gas can not be carried out smoothly. that the reaction in step (1) is carried out at a temperature of 75-110 ° C under a pressure of 2-10 kp / cm 2 to sufficiently prevent concentration of the slurry.

For reaction at a temperature exceeding 75 ° C, the NO / NO 2 molar ratio should be as high as 1.6-2.5. When the reaction temperature is as high as 70-110 ° C, the formation of calcium nitrate as a by-product increases at a NO / NO 2 molar ratio of less than 1.6. However, the conversion of nitrogen oxides is lower at a molar ratio of NO / NO2 exceeding 2.5.

In the process of the invention, it is necessary to continuously separate the gas containing unabsorbed and unreacted nitric oxide from the reaction mixture in step (1) and to interrupt the supply of the nitric oxide-containing gas, when the residual calcium hydroxide content is reduced to the range. 2% by weight in 2-10% by weight to stop the reaction.

When the residual calcium hydroxide content exceeds 10% by weight, a long time for the reaction in step (3) is required to achieve a low conversion, even if a large volume of low concentration nitric oxide-containing gas is brought into contact with the reaction mixture. residual calcium hydroxide content; increases the formation of the by-product calcium nitrate in step (1), so that it becomes difficult to obtain an aqueous solution of calcium nitrite with a high degree of purity.

Thus, an aqueous solution of calcium nitrite with high efficiency and high processability can be obtained by preventing the formation of the by-product calcium nitrate and precipitating the complex in step (1). However, the resulting slurry of the reaction mixture in step (1) contains 2-10% by weight of calcium hydroxide. Thus, if residual calcium hydroxide is separated from the water slurry, a loss of calcium hydroxide is obtained and further a concentration step is required to obtain an aqueous solution of high concentration calcium nitrite, so a simple process can not be obtained.

In the process of the invention, the aqueous slurry of the reaction mixture containing calcium nitrite and low calcium hydroxide content obtained in step (1) is further contacted with nitric oxide-containing gas in step (3) to prevent the above-mentioned disadvantages.

However, it is necessary to define a nitric oxide concentration in the nitric oxide-containing gas used in step (3).

As stated above, according to the invention, the fact has been investigated and established that when a nitric oxide-containing gas is absorbed in a water slurry with 2-10% by weight calcium hydroxide content and containing formed 8006790-39, calcium nitrite, the formation of the by-product calcium nitrate increases due to increase of the nitrogen oxide concentration in the nitric oxide-containing gas, while the formation of the by-product calcium nitrate decreases and calcium nitrite is formed with high efficiency due to a reduction in the nitrogen oxide concentration- The nitrogen oxide concentration in the gas used in It is the ion. step (3) is preferably less than 5% by volume. however, it is not advisable to use too low concentrations due to the formation of calcium nitrite at a low rate. It is preferred that the nitric oxide concentration be 1-5% by volume.

The molar ratio of NO / NO 2 in the gas with low nitrogen oxide concentration used in step (3) is preferably in the range 1.6-2.5 for the same reasons as given for step (1).

The temperature of the slurry in step (3) is preferably in the range of 75-110 ° C for the same reasons as given for step (1). The reaction according to step (3) is carried out under a pressure of 2-10 kp / cm 2 in order to sufficiently prevent concentration of the slurry at a high reaction temperature of 75-11 ° C. In step (3), calcium hydroxide is converted to calcium nitrite. For complete conversion of the entire amount of calcium hydroxide, a long period of time is required, which makes an industrial process inefficient. > When nitric oxide gas is supplied to a slurry with a low concentration of calcium hydroxide for a long period of time, a tendency to form calcium nitrate as a by-product is obtained by reacting nitric oxide gases with formed calcium nitrite.

It is convenient to stop the reaction under conditions which result in less than 3% by weight and preferably about 1% by weight of calcium hydroxide remaining in the reaction mixture obtained in step (3). Unabsorbed and unreacted gas is continuously discharged from the reaction zone 1 step (3).

The discharged gas can, if desired, be used for the production No / NO 2 to the range 1.6-2.5, it is necessary to use nitric acid, since the gas still has a high pressure sufficient for use for production of nitric acid.

The loss of nitrogen oxides can be prevented by using an exhaust gas containing unabsorbed nitric oxide discharged from step (1) as the low nitrogen oxide gas used in step (3).

Step (2) is used for this purpose.

The molar ratio of NO / NO 2 in the exhaust gas not absorbed in step (1) is usually higher than about 4. To control the molar ratio, oxidize the unabsorbed exhaust gas.

The oxidation can be easily carried out using an oxidation tower with 4-5% by volume of oxygen in the unabsorbed exhaust gas.

The oxidation can be easily carried out by keeping the unabsorbed exhaust gas in the oxidation tower for a sufficient period of time.

The concentration of nitrogen oxides can be easily achieved by supplying nitrogen-containing gas in the desired amount. If a gas with 5-12% by volume of nitrogen oxides is used as a gas with a high nitrogen oxide concentration in step (1), the nitric oxide concentration in the unabsorbed exhaust gas from step (1) is in the range about 145% by volume, so the exhaust gas can be used without special control treatment. Since the gas leaving as exhaust gas from the reaction zone according to step (1) usually has a high pressure and a high temperature, i.e. about 90-130 ° C, it is convenient to maintain the gas at high temperature and high pressure for use in subsequent oxidation step (2) for I (direct use in the reaction step (3) after step (2). it is also possible to use an oxidation tower of a small type or a pipe in. 8006T9Û ~ 3 ll instead of the large tower previously used.

In the control of the exhaust gas, the oxidation is carried out at 80-150 ° C and the gas is regulated so as to obtain a nitric oxide-containing gas with a content of 1-5% and a molar ratio NO / NO 2 of 1.6-2.5 for use in step (3).

In the process according to the invention, an aqueous solution with a high calcium nitrite content can be obtained by combining steps (1), (2) and (3). However, the solution obtained in step (3) contains a small amount of calcium hydroxide and insoluble impurities contained in the starting materials. These insoluble materials are separated to obtain an aqueous solution of calcium nitrite of high purity and high concentration.

The solution obtained in step (3) can be easily filtered, so it is convenient to provide a filtration step such as step (4) for separating insoluble impurities.

The method according to the invention can consist of a batch system and a semi-continuous system as well as a continuous system.

In a batch system, two large reactors were used.

In the first reactor, the first step of the conversion of calcium hydroxide to calcium nitrite is carried out, after which the unabsorbed exhaust gas is fed to the second reactor to carry out the second step. Since the calcium hydroxide content in the first reactor varies in the batch system, the balance of the reactions is not suitable.

Thus, it is convenient to use the continuous pro- CGSSEII.

After a long time is required for the conversion of calcium hydroxide with nitrogen oxides to calcium nitrite, it is convenient to use two or more reactors in the continuous process as set forth below, but a continuous process with reaction in a pipeline using two piping systems can be used if desired.

In the first reactor, the water slurry of calcium hydroxide is fed from the upper part and the nitric oxide-containing gas is fed from the lower part and the reaction mixture is discharged from the bottom.

The unabsorbed exhaust gas can be separated in the reactor.

In the second reactor, the reaction mixture is fed from the upper part and the low nitrogen oxide gas is fed from the lower part and the reaction mixture is discharged from the bottom.

This continuous process can be illustrated in the following way. Gas containing nitrogen oxides 1 Preheated reactor - Slurry of calcium hydroxide I Oxidation tower I vf Second reactor -----) Exhaust w 'Filter ___-_ í-) Sludge Aqueous solution of calcium nitrite In this continuous system the water slurry with high calcium hydroxide content is fed (20-40 % by weight) continuously from the top and the gas with high nitric oxide concentration continuously from the wells of the first reactor while maintaining the calcium hydroxide content in the range 2-10% by weight for absorption of the nitric oxide-containing gas in the water slurry of calcium hydroxide and for reaction between - components. The reaction mixture is continuously transferred to the second reactor. 8006790-3 13 The unabsorbed nitrogen oxide-containing gas continuously discharged from the first reactor is continuously fed to the oxidation tower to oxidize nitrogen oxides to control the NO / NO 2 molar ratio. The resulting gas is continuously discharged from the tower and fed to the bottom of the second reactor for contact with the reaction mixture transferred from the first reactor to the second reactor. The resulting solution is continuously discharged from the bottom and fed to the filter for filtration, whereby the aqueous solution of calcium nitrite of high purity and high concentration can be obtained continuously.

According to the second embodiment, the unabsorbed nitric oxide-containing gas discharged as exhaust gas in step (3) is recovered and fed to step (2 ') to control the NO / NO 2 molar ratio and, if necessary, the concentration of nitrogen oxides as step (2). The reaction mixture obtained in step (3) is contacted with the low nitrogen oxide gas obtained in step i2 ') to absorb the nitrogen oxides in step (3') and the solution obtained in step (3 ') is filtered.

Similarly, multiple steps or multiple steps (2 "), (2" ') ... and (3 "), (3"') ... may be added to carry out the reaction of nitrogen oxides in the absorption, until the calcium the hydroxide content has been reduced to less than 3% by weight.

In the method according to the invention, these multi-steps can be combined, but it is optimal to combine steps (1), (2), (3) and (4) in the stated simple way for carrying out the process with high efficiency in the apparatus. The method according to the invention is particularly suitable for large-scale industrial production with a compact apparatus.

Thus, in the process of the invention, steps (1), (2), (3) and (4) are combined to obtain an aqueous solution of calcium nitrite with more than 95% by weight of calcium nitrite content and with a high degree of purity with a yield exceeding 95%. This aqueous solution can be used as an anti-corrosion agent and added to cement without any treatment.

A better understanding of the invention can be obtained with the following specific embodiments, which are intended to illustrate the invention without limiting its scope.

Example 1.

In a first reactor with a diameter of 2.0 m and a height of 3.8 m, a slurry containing 1750 kg of slaked lime, 85 kg of calcium nitrite and 4500 kg of water was charged. From a porous nozzle arranged at the bottom of the tank, a gas containing nitrogen oxides was supplied in an amount of 9.4% by volume obtained by oxidation of ammonia with air (molar ratio NO / NO 2 1.7) and with a temperature of about 230 ° C with a flow of 1300 Nm3 / h for reaction under a pressure of 2.8 kp / cmz for 8 hours. During the reaction, the reaction mixture was cooled so as to maintain the temperature at 78-83 ° C and to maintain a pH in excess of 11, obtaining 8.5 tons of a reaction mixture containing 30.4% by weight of calcium nitrite and 1, 1% by weight of calcium nitrate and 3.4% by weight of calcium hydroxide. Unabsorbed gas was discharged continuously. The content of nitrogen oxides in the exhaust gas was 2.0% by volume.

The reaction mixture was transferred from the first reactor to the second reactor.

In the next batch reaction, the entire amount of unabsorbed gas discharged from the first reactor was passed through an empty tower (space tower) to control the NO / NO 2 molar ratio to 1.7 and passed through the porous nozzle arranged at the bottom. The reaction was carried out at 75-80 ° C and pH exceeding II under a pressure of 2.5 kp / cm 2 for 8 hours. Unabsorbed gas was continuously discharged from the reaction system to give 8.6 tons of reaction mixture containing 34.0% by weight of calcium nitrite and 1.4% by weight of calcium nitrate, 1.2% by weight of calcium hydroxide and 1.4% by weight of other solid components. The content of nitrogen oxides in the exhaust gas was 0.4% by volume.

The reaction mixture was cooled and filtered to give an aqueous solution containing 33.1% by weight of calcium nitrite and 1.5% by weight of calcium nitrate.

Example 2.

In the first reactor used according to Example 1, 8.5 tons of a slurry containing 29.5% by weight of calcium nitrite, 1.1% by weight of calcium nitrate and 4.0% by weight of calcium hydroxide and a slurry of slaked lime with a concentration of 29 .8% by weight was added continuously in an amount of about 750 kg / h. In addition, a gas containing nitrogen oxides was added in the amount of 9.4% by volume obtained by oxidation of ammonia with air (molar ratio NO / NO 2 1.7) and with a temperature of about 230 ° C in an amount of 1300 Nm 3 / h for reaction during the pressure 2.8 kp / cm 2. During the reaction, the reaction mixture was cooled so that the temperature was maintained at 78-83 ° C and the concentration of calcium hydroxide in the reaction slurry was about 4% by weight. The reaction mixture was continuously discharged from the first reactor in an amount of about 100 kg / h and fed to the second reactor used according to Example 1. The concentration of unabsorbed nitric oxide in the exhaust gas from the first reactor was about 1.9% by volume.

The entire amount of unabsorbed gas was passed through a space tower to control the molar ratio of NO / NO 2 to 1.7 and introduced into the other reactor for reaction at 75-80 ° C under a pressure of 2.5 kp / cm 2. The concentration of calcium hydroxide in the reaction mixture was maintained at 1.5% by weight. The reaction mixture was continuously discharged from the second reactor in an amount of about 1000 kg / h and cooled and filtered to obtain an aqueous solution containing 32.4% by weight of calcium nitrite and 1.6% by weight of calcium nitrate.

Claims (6)

1. 8006790-3., _: Ib in CLAIMS 1. Process for the preparation of an aqueous solution of calcium nitrite of high purity and high concentration by contacting a gas containing nitrogen oxides with an aqueous slurry of calcium hydroxide, characterized by , that a gas with a temperature of 190-300 ° C and a nitrogen oxide concentration of 5 to 12% by volume and a molar ratio of NO / NO 2 of 1.6 to 2.5 is brought into contact with a water slurry containing 20 to 40% by weight of calcium hydroxide at 75-110 ° C under a pressure high enough to prevent concentration of the slurry until the calcium hydroxide content is reduced to 2 to 10% by weight as a first step, after which the unabsorbed and unreacted gas is separated and the gas is oxidized at 85-150 ° C to a gas. with a content of nitrogen oxides of 1 to 5% by volume and a molar ratio of NO / NO 2 of 1.6 to 2.5 and the resulting gas is brought into contact with the one separated from the first stage the aqueous slurry having a shark of 2 to 10% by weight of calcium nyaroxia and containing calcium nitrite at 75-110 ° C under a pressure sufficiently high to prevent the slurry from concentrating the calcium hydroxide content to less than 3% by weight as a second step. , after which the resulting solution is filtered. a 2. A process for the preparation of an aqueous solution of calcium nitrite according to claim 1, characterized in that the gas containing nitrogen oxides used in the first step is obtained by oxidizing ammonia and has a pressure of 2-10 kp / cmz. 3. A process for preparing an aqueous solution of calcium nitrite according to claim 1 or 2, characterized in that the calcium hydroxide content of the aqueous solution obtained in the second step is kept lower than about 1% by weight. 'i aoosvøu-3 4. A process according to claim 1 for the continuous preparation of the aqueous solution of calcium nitrite, characterized in that (a) a water slurry containing 20 to 40% by weight of slaked lime and gas containing nitric oxide with a temperature of 190-300 ° C is continuously fed , a pressure of 2-10 kp / cm 2 and with a content of nitrogen oxides of 5 to 12% by volume and a molar ratio of NO / NO 2 of 1.6 to 2.5 obtained by oxidation of ammonia in a first reaction zone, so that the reactants contacting at a temperature of 75-110 ° C and a pressure high enough to prevent concentration of the slurry, until the calcium hydroxide content is reduced to 2 to 10% by weight, (b) continuously discharging the unabsorbed gas and the reaction mixture from the first reaction zone and separates and continuously supplies the reaction mixture to a second reaction zone and continuously supplies the unabsorbed gas to an oxidizer v id 85-150 ° C, (c) oxidizes the unabsorbed gas in the oxidizer at 85-150 ° C to control the nitrogen oxide concentration to 1 to 5% by volume and a molar ratio NO / NO 2 of 1.6 to 2.5 and adds the resulting gas to the second reaction zone, (d) continuously contacting the resulting gas with the water slurry separated from the first reaction zone and having a content of 2 to 10% by weight of calcium hydroxide at a temperature of 75-110 ° C and a pressure of high enough to prevent concentration of the slurry to lower the calcium hydroxide content in the second reaction zone to less than 3% by weight and (e) continuously discharges the unabsorbed gas and reaction mixture from the second reaction zone and continuously separates and feeds the reaction mixture to a calcium hydroxide filter. . 8006790-3; - 'ß A process for preparing an aqueous solution of calcium nitrite according to claim 4, characterized in that the calcium hydroxide content of the reaction mixture obtained in the second reaction zone is kept lower than about 1.5% by weight. A process for preparing an aqueous solution of calcium nitrite according to any one of the preceding claims, characterized in that the calcium hydroxide used as starting material in the first step is slaked lime.