GB2074993A

GB2074993A - Process for Producing Aqueous Solution of Calcium Nitrite

Info

Publication number: GB2074993A
Application number: GB8031397A
Authority: GB
Original assignee: Nissan Chemical Corp
Current assignee: Nissan Chemical Corp
Priority date: 1980-04-08
Filing date: 1980-09-29
Publication date: 1981-11-11
Also published as: SE8006790L; JPS56145106A; DK156168C; JPS5850926B2; DE3036605A1; SE438843B; NO154832C; DK156168B; NO154832B; GB2074993B; DK398680A; CA1114133A; NO802873L; DE3036605C2

Abstract

An aqueous solution of calcium nitrite having high purity and concentration is obtained by contacting a nitrogen oxides containing gas (5 to 12 vol %) with a high concentration aqueous slurry of calcium hydroxide (20 to 40 wt %) by feeding the gas under an elevated pressure to prevent a concentration of said slurry, the unabsorbed gas being oxidized. The resulting gas is fed to the slurry obtained in the first instance so as to convert the residual calcium hydroxide to calcium nitrite without forming a calcium nitrate by-product. The process is preferably carried out as continuous process having a first reaction zone and a second reaction zone.

Description

SPECIFICATION Process for Producing Aqueous Solution of Calcium Nitrite The present invention relates to an improved process for producing an aqueous solution of calcium nitrite by contacting a gas which contains NO and NO2 with an aqueous slurry of calcium hydroxide to absorb the gas.

Calcium nitrite is useful as an anti-corrosive agent and as an additive for cements. For industrial purposes calcium nitrite is preferably used in the form of an aqueous solution rather than in solid form, and especially in an aqueous solution containing about 30 to 40 wt.% of calcium nitrite.

The aqueous solution of calcium nitrite can be easily obtained by dissolving solid calcium nitrite in water. The commercially available solid calcium nitrite is obtained by concentrating and drying an aqueous solution of calcium nitrite and accordingly, the method of dissolving the solid calcium nitrite is of remarkably low efficiency.

it has been proposed in Japanese Patent Publication No. 35596/1976 to produce an aqueous solution of calcium nitrite. However, the known process requires many complicated steps such as filtration, aging, concentration, a second filtration, and a second concentration step, reducing its efficiency and resulting in a large loss of starting materials (nitrogen oxides and aqueous slurry of calcium hydroxide).

The inventors have studied ways of producing an aqueous solution of calcium nitrite having high purity and high concentration from a nitrogen oxides-containing gas and an aqueous slurry of calcium hydroxide, and have found the fact that the formation of calcium nitrate as a by produce can be prevented by contacting a gas having a nitrogen oxides concentration of not more than 3 vol.% with an aqueous slurry having a calcium hydroxide content of 3 to 10 wt% and containing calcium nitrite to absorb the gas whereby calcium nitrite is produced with high efficiency.

The inventors have developed a process for producing an aqueous solution of calcium nitrite by (1') contacting a gas having a 5 to 10 vol.% nitrogen oxides concentration and a molar ratio of NO/NO2 of 1.2:1 to 1.5:1 with an aqueous slurry having a 20 to 40 wt.% calcium hydroxide content of 40 to 700C until the calcium hydroxide content is reduced to within a range of 3 to 10 wt.% as a first stage; (2') separating the unabsorbed and unreacted gas and oxidzing the separated gas to form a gas having a 1 to 3 vol.% nitrogen oxides concentration and a molar ratio of NO/NO2 of 12:1 to 1 .5%1; (3') contacting the resulting gas having a low nitrogen oxides concentration with the separated aqueous slurry having a 3 to 10 wt.% calcium hydroxide content and containing calcium nitrite at 40 to 700C, to reduce the calcium hydroxide content to less than 3 wt.% as a second stage and (4') filtering the resulting solution having a high calcium nitrite concentration.

In order to produce a large amount of an aqueous slurry of Ca(NO2)2 having a purity higher than 95% at a concentration of higher than 30% for example, at a rate of several thousands tonnes per year, a large apparatus is required. In order to maintain the temperature of the reaction slurry in the range 40 to 700C in the absorption of a gas containing nitrogen oxides (a molar ratio of NO/NO2 of 1.2:1 to 1.5:1 ) at higher than 1500C obtained by the oxidation of ammonia into an aqueous slurry of slaked lime having a concentration of 20 to 40%, it is necessary to use a large cooler for cooling and a large reactor for oxidation of the discharged gas from the first reactor.

The efficiency of cooling can be improved by raising the reaction temperature, but when the reaction temperature is higher than 700C under atmospheric pressure, the partial pressure of steam in the slurry is higher than the partial pressure of steam in the gas containing nitrogen oxides whereby a concentration of the slurry is caused by evaporation of steam and the complex Ca(NO2)2.Ca(OH)2.2H20 is precipitated which increases the viscosity of the slurry. Therefore, the reaction does not proceed smoothly.

The inventors have further studied ways of preventing the concentration of the slurry and have found that the formation of calcium nitrate is a by-product is increased in the reaction so that an aqueous solution of calcium nitrite having high purity (higher than 95%) could not be obtained.

On the other hand, the inventors have found that even though the solubility of slaked lime decreases with increasing temperature, the reaction velocity is substantially the same.

Moreover, the inventors have found that the increase in the formation of calcium nitrate as a by-product and the concentration of the slurry can be prevented by absorbing a gas containing nitrogen oxides (the molar ratio of NO/NO2 is higher) under an elevated pressure and at a slurry temperature higher than 700C.

The present invention provides a process for producing an aqueous solution of calcium nitrite of high purity and high concentration wherein a gas at a temperature of 1 90-3000C and having a nitrogen oxides concentration of 5 to 12 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5:1 is contacted with an aqueous slurry containing 20 to 40 wt.% of calcium hydroxide at 75 to 1 00C under an elevated pressure sufficient to prevent condensation of said slurry until the calcium hydroxide content is reduced to within a range of 2 to 10 wt.% as a first stage; the unabsorbed and unreacted gas is separated and the gas is oxidized at 85-1 500C to form a gas having a nitrogen oxides concentration of 1 to 5 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5:1 and the resulting gas is contacted with the aqueous slurry separated from the first stage and having a 2 to 10 wt.% calcium hydroxide content and containing calcium nitrite, at 75 to 1 100C under an elevated pressure sufficient to prevent concentration of said slurry, to reduce the calcium hydroxide content to less than 3 wt.% as a second stage and the resulting solution is filtered.

Using the process of the present invention it is possible to produce an aqueous solution of calcium nitrite of high purity and with concentration higher than 30 wt.% at a rate of conversion ogreaterthan 95%.

In a preferred form, the invention provides a continuous process for producing an aqueous solution of calcium nitrite of high purity and high concentration by contacting a nitrogen oxidescontaining gas with an aqueous slurry of calcium hydroxide, which process comprises: (a) feeding continuously an aqueous slurry containing 20 to 40 wt.% of slaked lime and a nitrogen oxides-containing gas having a temperature of 1 90-4000C, a pressure of 210 kg/cm2, a nitrogen oxides concentration of 5 to 12 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5::1 obtained by oxodizing ammonia into a first reaction zone to contact them at a temperature of 75 to 1 00C and an elevated pressure sufficient to prevent concentration of said slurry until the calcium hydroxide content is reduced to within a range of 2 to 10 wt.%; (b) discharging continuously the unabsorbed gas and the reaction mixture from the first reaction zone, separating and feeding the reaction mixture continuously to a second reaction zone and feeding continuously the unabsorbed gas into an oxidizing device at 85-1 500C; (c) oxidizing the unabsorbed gas in the oxidizing device at 85 degrees to 1 50 degree C to adjust the nitrogen oxides concentration therein to 1 to 5 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5::1, and feeding the resulting gas into the second reaction zone; (d) contacting continuously the resulting gas with the aqueous slurry separated from the first reaction zone and containing 2.10 wt.% of calcium hydroxide at a temperature of 75 to 1 100C and a elevated pressure sufficient to prevent concentration of said slurry to reduce the calcium hydroxide content in the second reaction zone to less than 3 wt%; and (e) discharging continuously the unabsorbed gas and the reaction mixture from the second reaction zone while separating and feeding continuously the reaction mixture to a filter to separate the remaining calcium hydroxide.

The process of the invention can suitably be carried out in stages (1) to (4), corresponding essentially to stages (1') to (4') of the known process described above. An important feature of the present invention is the reaction of the aqueous slurry of calcium hydroxide with nitrogen oxides gas in two stages (1) and (3), different concentrations.

An advantage of the process of the present invention is the reduced loss of calcium hydroxide and nitrogen oxide in the step (2) and the step (3).

By combining the steps (1), (2), (3) and (4), the formation of calcium nitrate as a by-product can be reduced and the conversion to calcium nitrite increased to greater than 95%, giving an aqueous solution of calcium nitrite of high purity and high concentration and with high efficiency.

In the process of the present invention, the conversion to calcium nitrite is given by the ratio of calcium nitrite to total of calcium nitrite and calcium nitrate in the final mixture.

An aqueous slurry having a high calcium hydroxide content can be easily obtained by dispersing a commercially available form of calcium hydroxide such as slaked lime in water.

When the calcium hydroxide content is less than 20 wt.% the aqueous solution of calcium nitrite obtained does not have sufficiently high concentration even when the steps (1), (2), (3) and (4) are combined.

When the calcium hydroxide content is greater than 40 wt.%, the viscosity of the slurry of the reaction mixture in the step (1) is too high because of the formation of a complex whereby the absorption of nitrogen oxides gas cannot proceed smoothly. Moreover, calcium nitrite is precipitated.

The gas having high nitrogen oxides concentration used in the present invention can be easily obtained by oxidizing ammonia with air, and usually has a pressure lower than 10 kg/cm2 and a temperature higher than 1 500C, preferably a temperature of 190--3000C. It is important to keep the molar ratio of NO/NO2 in the gas within the range of 1.6:1 to 2.5:1.

When the molar ration of NO/NO2 is less than 1.6:1, the formation of the calcium nitrate byproduct is increased in the step (1) of contacting the gas with the aqueous slurry of calcium hydroxide to absorb it, whereby the aqueous solution of calcium nitrite obtained is not of sufficiently high purity.

When the molar ratio of NO/NO2 is greater than 2.5:1, the rate conversion of nitrogen oxides in the step (1) is decreased whereby the efficiency of the reaction is decreased.

It is not sufficient however, only to define the molar ratio of NO/NO2 in the nitrogen oxidescontaining gas used in the step (1). The total nitrogen oxides concentration in the gas must also be within specific range defined above.

When the nitrogen oxides concentration is lower than 5 vol.%, the rate of conversion is decreased making the process uneconomical from the viewpoint of economical use of apparatus.

When the nitrogen oxides concentration is higher than 5 vol.% the reaction proceeds at a satisfactory rate. However, the total nitrogen oxides content must not exceed 1 2 vol.%.

It is preferable to use a nitrogen oxidescontaining gas having a high concentration within the above range in the step (1) of the process of the invention.

The nitrogen oxides containing gas is fed at a temperature of 190 to 3000 C. When the temperature is lower than 1 500C, moisture in the gas is liquefied to give nitric acid and calcium nitrate is produced as a by-product. The liquefaction of the moisture in the gas can be prevented almost entirely at a temperature higher than 1 900C. However, when the temperature is higher than 3000 C, the amount of heat to be removed from the reaction zone according to the step ( 1 ) increases.

In the process of the present invention, it is necessary to maintain the temperature of the aqueous slurry at 75-11 00C in the step (1) of contacting the nitrogen oxides-containing gas with the aqueous slurry of calcium hydroxide under an elevated pressure such as 2.0 to 10 atm.

When the temperature is high, the formation of the complex can be prevented. However, the steam partial pressure of the aqueous slurry is higher than the steam partial pressure of the nitrogen oxides containing gas at temperatures higher than 700C under atmospheric pressure, whereby concentration of the aqueous slurry occurs and the process of contacting and absorbing nitrogen oxides gas does not proceed smoothly, so the reaction according to the step (1) is carried out at a temperature of 75--1 100, preferably under a pressure of 2-1 0 kg/cm2 in order to prevent concentration of the slurry.

In order for the reaction to proceed at a temperature higher than 750C, a molar ratio of NO/NO2 is required as high as 1.6 to 2.5. Since the reaction temperature is high as 75 to 11 00C, the rate of production of calcium nitrate as a byproduct is increased at a molar ratio of NO/NO2 less than 1.6:1. However, the conversion of nitrogen oxides is lower at a molar ratio of NO/NO2 of greater than 2.5:1.

In the process of the present invention, it is necessary to separate continuously the unabsorbed and unreacted nitrogen oxidescontaining gas from the reaction mixture in the step (1) and to stop the feeding of the nitrogen oxides-containing gas when the residual calcium hydroxide content is reduced to within a range of 2. 10 wt.% to stop the reaction.

When the residual calcium hydroxide content is greater than 10 wt.% the reaction in the step (3) takes a long time and gives a low conversion even through a large volume of the nitrogen oxidescontaining gas having a low concentration is contacted with the reaction mixture.

When the reaction has reduced the residual calcium hydroxide content to less than 2 wt.%, the formation of the by-product of calcium nitrate is increased in the step (1), making it difficult to obain an aqueous solution of calcium nitrite having high purity.

With the process described above an aqueous solution of calcium nitrite can be obtained with high efficiency and high processibility while preventing the formation of calcium nitrate and the precipitation of the complex in the step (1).

However, the resulting slurry of the reaction mixture in the step (1) contgains 2 to 10 wt.% of calcium hydroxide. Accordingly, if the residual calcium hydroxide is separated from the aqueous slurry, there is a loss of calcium hydroxide and moreover, a concentration step is required to obtain an aqueous solution of calcium nitrite having the required concentration process.

In the process of the present invention, the aqueous slurry of the reaction mixture containing calcium nitrite and having low calcium hydroxide content obtained in the step (1) is further contacted with a nitrogen oxides-containing gas in the step (3) so as to overcome the abovementioned disadvantages.

It is necessary, however, to use a specific nitrogen oxides concentration in the gas used in the step (3).

As stated above, the inventors have found that when a nitrogen oxides-containing gas is absorbed into an aqueous slurry having a 2 to 10 wt.% calcium hydroxide content and containing the resulting calcium nitrite, the formation of calcium nitrate is increased depending upon the increase of a nitrogen oxide concentration in the nitrogen oxides containing gas whereas the formation of the by-product of calcium nitrate is decreased and calcium nitrite is produced in high efficiency depending upon the decrease of a nitrogen oxide concentration. The nitrogen oxides concentraion in the gas used in the step (3) is preferably lower than 5 vol.%. However, it is also not preferable to be too low concentration, because of slow speed of the production of calcium nitrite. It is suitable to be 1 to 5 vol.% of the nitrogen oxides concentration.

The molar ratio of NO/NO2 in the gas having a low nitrogen oxides concentration used in the step (3) is preferably in a range of 1.6 to 2.5 on the same reason of the step (1).

The temperature of the slurry in the step (3) is preferably in a range of 75 to 1 100C on the same reason of the step (1). The reaction according to the step (3) is carried out under a pressure of 210 kg./cm2 in order enough to prevent concentration of the slurry at a high reaction temperature of 75-11 1 OOC.

In the step (3), calcium hydroxide is converted to calcium nitrite. In order to completely convert all of calcium hydroxide, it takes a long time to be unefficient as an industrial process. When nitrogen oxides gas is fed into a slurry having low concentration of calcium hydroxide for a long time, a by-product of calcium nitrate tends to be produced by reacting nitrogen oxides gas .with the resulting calcium nitrite.

It is effective to stop the reaction under the condition remaining less than 3 wt.% preferably about 1 wt.% of calcium hydroxide in the reaction mixture ontained in the step (3). The unabsomed and unreacted gas is discharged continuously from the reaction zone according to step (3). The discharged gas may, if desired, be used to manufacturing of the nitric acid, since the gas still has a high pressure sufficient to use for the manufacturing of the nitric acid.

The loss of nitrogen oxides can be prevented by utilizing a discharge gas containing unabsorbed nitrogen oxides discharged from the step (1), as the gas having a low nitrogen oxides concentration used in the step (3).

The step (2) is provided for this purpose.

The molar ratio of NO/NO2 in the discharge gas which is not absorbed in the step (1) is usually higher than about 4. In order to control the molar ratio of NO/NO2 in a range of 1.6 to 2.5, it is necessary to oxidize the unabsorbed discharge gas.

The oxidation can be easily carried out by using an oxidizing tower with 4 to 5 vol.% of oxygen contained in the unabsorbed discharge gas.

The oxidation can be easily performed by retaining the unabsorbed discharge gas in the oxidizing tower for a satisfactory time.

The concentration of nitrogen oxides can be easily attained by feeding nitrogen gas as desired.

However, when a gas having 5 to 12 vol.% of a nitrogen oxides concentration is used as the gas having high nitrogen oxide concentration in the step (1), the nitrogen oxides concentration in the unabsorbed discharge gas in the step (1) is in a range of about 1 to 5 vol.% whereby the discharge gas can be used without a specific controlling treatment. As the gas discharged from the reaction zone according to the step (1) usually has a high pressure and temperature i.e. about 90-1 300C, it is convenient to maintain the gas at the high temperature and high pressure for using to the subsequent oxidation step (2), so as to be directly used to the reaction step (3) subsequent to the step (2). Moreover, the high pressure of the gas makes it possible to use an oxidation tower of small type or a pipe instead of the large tower formerly used.

In the control of the discharge gas, the oxidation is carried out at 80 to 1 500C and the gas is controlled to give the nitrogen oxides containing gas having a concentration of 1 to 5% and a molar ratio of NO/NO2 of 1.6 to 2.5 so as to be used in the step (3).

In accordance with the process of the present invention combining the steps (1), (2) and (3), an aqueous solution having high calcium nitrite concentration can be obtained. However, the solution obtained in the step (3) contains a small amount of calcium hydroxide and insoluble impurities included in the starting materials.

These insoluble materials are separated to obtain the aqueous solution of calcium nitrite having high purity and high concentration.

The solution obtained in the step (3) can be easily filtered whereby it is preferable to provide a filtering step as the step (4) for separating the insoluble impurities.

The process of the present invention can be a batch system and a semi-continuous system and a continuous system.

In a batch system, two large reactors are used.

In the first reactor, the first stage of conversion of calcium hydroxide to calcium nitrite is performed and then in the second reactor, the unabsorbed discharge gas is fed to perform the second stage.

Since the calcium hydroxide content in the first reactor is varied in the batch system, the balance of the reactions is not suitable.

Accordingly, it is preferable to use the continuous process.

Since it takes a long time for converting calcium hydroxide with nitrogen oxides to calcium nitrite, pt is preferable to use two or more reactors in the continuous process, as follows, though a pipe line continuous process reacting through two pipe systems can be used, if desirable.

In the first reactor, the aqueous slurry of calcium hydroxide is fed from the upper part and the nitrogen oxides containing gas is fed from the lower part and the reaction mixture is discharged from the bottom.

The unabsorbed discharge gas can be separated in the reactor.

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

This continuous process can be shown as follows.

-Nitrogen oxides containing gas First reactorvSlurry of calcium hydroxide Oxidizing tower Second reactorDischarge gas FiltereSludge Aqueous solution of calcium nitrite In said continuous system, the aqueous slurry having high calcium hydroxide content (20 to 40 wt.%) is continuously charged from the top and the gas having high nitrogen oxides concentration is continuously fed from the bottom of the first reactor under maintaining the calcium hydroxide content in a range of 2 to 10 wt% to absorb the nitrogen oxide containing gas into the aqueous slurry of calcium hydroxide to react them.The reaction mixture is continuously transferred to the second reactor.

On the other hand, the unabsorbed nitrogen oxides containing gas discharged continuously from the first reactor is continuously fed to the oxidizing tower to oxidize nitrogen oxides to control the molar ratio of NO/NO2. The resulting gas is continuously discharged from the tower and fed from the bottom of the second reactor to contact with the reaction mixture transferred from the first reactor to the second reactor. The resulting solution is continuously discharged from the bottom and is fed to the filter to filtrate it whereby the aqueous solution of calcium nitrite having high purity and high concentration can be continuously obtained.

In other embodiment, the unabsorbed nitrogen oxides containing gas discharged in the step (3) is recovered and it is fed to the step (2') for controiling the molar ratio of NO/NO2 and if necessary the concentration of nitrogen oxides as the step (2). The reaction mixture obtained in the step (3) is contacted with the gas having low nitrogen oxides concentration obtained in the step (2') to absorb the nitrogen oxides in the step (3') and the solution obtained in the step (3') is filtered.

As the same manner, multi-steps (2")(2"')...

and (3")(3"')... can be added to perform the reaction of nitrogen oxides in the absorption until decreasing the calcium hydroxide content to less than 3 wt.%.

In the process of the present invention, the multi-steps can be combined, however, it is optimum to combine the steps (1), (2), (3) and (4) in said simple manner, in order to carry out the process in high efficiency on the apparatus. The process according to the present invention is especially suitable for an industrial massive production in a large scale through a compact apparatus.

Thus, in accordance with the process of the present invention, the steps (1), (2), (3) and (4) are combined to obtain the aqueous solution of calcium nitrite having higher than 95 wt.% of a calcium nitrite concentration and having high purity in a yield of greater than 95%. This aqueous solution can be used as anticorrosive agent and additive for cement without any treatment.

A further understanding can be attained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting in any manner, unless otherwise specified.

Example 1 In a first reactor having 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 equipped with a bottom of the tank, a gas containing nitrogen oxides of 9.4 vol.% obtained by an oxidation of ammonia with air (a molar ratio of NO/NO2 of 1.7) and having a temperature of about 2300C was fed at a flow rate of 1300 Nm3/hr. to react them under a pressure of 2.8 kg./cm2 for 8 hours. During the reaction, the reaction mixture was cooled to maintain at 78 to 830C and to maintain pH of higher than 11 whereby 8.5 tons of the reaction mixture containing 30.4 wt.% of calcium nitrite and 1.1 wt.% of calcium nitrate and 3.4 wt.% of calcium hydroxide was obtained.

The unabsorbed gas was continuously discharged. A concentration of nitrogen oxides in the discharged gas was 2.0 vol.%.

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

In the next batch reaction, all of the unabsorbed gas discharged form the first reactor was passed through a space tower to control a molar ratio of NO/NO2 to 1.7 and was fed through the porous nozzle equipped at the bottom. The reaction was carried out at 75 to 800 C, pH of higher than 11, under a pressure of 2.5 kg/cm2 for 8 hours. The unabsorbed gas was continuously discharged out of the reaction system, whereby 8.6 tons of the reaction mixture containing 34.0 wt.% of calcium, nitrite and 1.4 wt.% of calcium nitrate, 1.2 wt.% of calcium hydroxide and 1.4 wt.% of the other solid components was obtained.

The concentration of nitrogen oxides in the discharged gas was 0.4 vol.%.

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

Example 2 In the first reactor used in Example 1,8.5 tons of a slurry containing 29.5 wt.% of calcium nitrite, 1.1 wt.% of calcium nitrate and 4.0 wt.% of calcium hydroxide was charged and a slurry of slaked lime having a concentration of 29.8 wt.% was continuously fed at a rate of about 750 kg/hr.

On the other hand, a gas containing nitrogen oxides at 9.4 vol.% obtained by an oxidation of ammonia with air (a molar ratio of NO/NO2 of 1.7) and having a temperature of about 2300C was fed at a flow rate of 1 300 Nm3/hr. to react them under a pressure of 2.8 kg./cm2.

During the reaction, the reaction mixture was cooled to maintain 78 to 830C and to maintain a concentration of calcium hydroxide in the reaction slurry to about 4 wt.%. The reaction mixture was continuously discharged from the first reactor at a rate of about 1,000 kg/hr. and was fed into the second reactor used in Example 1. The concentration of the unabsorbed nitrogen oxides discharged from the first reactor was about 1.9 vol.%.

All of the unabsorbed gas was passed through a space tower to control a molar ratio of NO/NO2 to 1.7 and was fed into the second reactor to react them at 75 to 800C under a pressure of 2.5 kg/cm2. The concentration of calcium hydroxide in the reaction mixture was maintained at 1.5 wt.%.

The reaction mixture was continuously discharged from the second reactor at a rate of about 1,000 kg/hr. and cooled and filtered to obtain an aqueous solution containing 32.4 wt.% of calcium nitrite and 1.6 wt.% of calcium nitrate.

Claims

1. A process for producing an aqueous solution of calcium nitrite of high purity and high concentration wherein a gas at a temperature of 1 90-3000C and having a nitrogen oxidesconcentration of 5 to 12 vol% and a molar ratio of NO/NO2 of 1.6:1 to 2.5:1 is contacted with an aqueous slurry containing 20 to 40 wt.% of calcium hydroxide at 75 to 1 100C under an elevated pressure sufficient to prevent condensation of said slurry until the calcium hydroxide content is reduced to within a range of 2 to 10 wt.% as a first stage; the unabsorbed and unreacted gas is separated and the gas is oxidized at 85-1 500C to form a gas having a nitrogen oxides concentration of 1 to 5 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5:1 and the resulting gas is contacted with the aqueous slurry separated from the first stage and having a 2 to 10 wt.% calcium hydroxide content and containing calcium nitrite, at 75 to 11 00C under an elevated pressure sufficient to prevent concentration of said slurry, to reduce the calcium hydroxide content to less than 3 wt.% as a second stage and the resulting solution is filtered.

2. A process according to Claim 1 wherein the nitrogen oxides-containing gas used in the first stage is obtained by oxidizing ammonia and has a pressure in a range of 2 to 10 kg/cm2.

3. A process according to Claim 1 or Claim 2 wherein the calcium hydroxide content of the aqueous solution obtained in the second stage is less than 1 wt.%.

4. A continuous process for producing an aqueous solution of calcium nitrite of high purity and high concentration by contacting a nitrogen oxides-containing gas with an aqueous slurry of calcium hydroxide, which process comprises: (a} feeding continuously an aqueous slurry containing 20 to 40 wt% of slaked lime and a nitrogen oxides-containing gas having a temperature of 1 90-3000C, a pressure of 210 kg/cm2, a nitrogen oxides concentration of 5 to 12 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5::1, obtained by oxidizing ammonia into a first reaction zone to contact them at a temperature of 75 to 11 00C and an elevated pressure sufficient to prevent concentration of said slurry until the calcium hydroxide content is reduced to within a range of 2 to 10 wt.%.

(b) discharging continuously the unabsorbed gas and the reaction mixture from the first reaction zone, separating and feeding the reaction mixture continuously to a second reaction zone and feeding continuously the unabsorbed gas into an oxidizing device at 85-1 500C; (c) oxidizing the unabsorbed gas in the oxidizing device at 85 degrees to 1 50 degree C to adjust the nitrogen oxides concentration therein to 1 to 5 vol.% and a molar ratio of NO/NO2 of 1.6:1 to 2.5::1, and feeding the resulting gas into the second reaction zone; (d) contacting continuously the resulting gas with the aqueous slurry separated from the first reaction zone and containing 2-10 wt.% of calcium hydroxide at a temperature of 75 to 11 00C and an elevated pressure sufficient to prevent concentration of said slurry to reduce the calcium hydroxide content in the second reaction zone to less than 3 wt.%; and (e) discharging continuously the unabsorbed gas and the reaction mixture from the second reaction zone while separating and feeding continuously the reaction mixture to a filter to separate the remaining calcium hydroxide.

5. A process according to Claim 4 wherein the calcium hydroxide content in the reaction mixture obtaied in the second reaction zone is kept below 1.5 wt.%.

6. A process according to any preceding Claim wherein the calcium hydroxide which is used as the starting material in the first stage is in the form of slaked lime.

7. A process according to Claim 1 substantially as herein described with reference to any one of the Examples.