JPS5850785B2 - How to dry nitrate solution - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for drying a nitrate solution such as a calcium nitrate aqueous solution that requires disposal, or a nitrate-based solution such as a calcium nitrate aqueous solution produced as a by-product during wet denitration treatment.

Exhaust gas emitted from industrial furnaces such as boilers and combustion furnaces contains several hundred PP of nitrogen oxides (hereinafter referred to as NOx).
This NOx is extremely harmful and must be removed from the exhaust gas.

Conventionally, methods for removing NOx from exhaust gas include the so-called equimolar method, in which NO is oxidized to NO2 and absorbed into an alkaline absorption liquid so that the molar ratio of NO and NO2 is approximately 1, or NO is removed using ozone. There is the so-called ozone method, which oxidizes to NO2 and N2O5 and absorbs them in an alkaline absorption liquid or sulfite absorption liquid, but these methods produce nitrates and nitrites as by-products, so they are discarded as they are. They must be reused as raw materials for fertilizers or explosives, or treated to render them harmless.

In all of these cases, several percent of the nitrate waste liquid must be concentrated and dried.

The inventors of the present invention have already proposed a method of thermally decomposing nitrates to generate a high concentration of NOx, and reducing the NOx to make it harmless as nitrogen, with the aim of making nitrate waste liquid harmless.

In this method, when spray drying is used as a drying method for calcium nitrate waste liquid, the particle size of the dried and anhydrous calcium nitrate Ca (NOs) becomes too small, and the sample is not transferred to a pyrolysis device, especially a direct heating rotary kiln. Powder scatters violently when charging, and this scattered powder adheres to and accumulates on the raw material supply hood, often causing operational troubles.

Therefore, in order to prevent a large amount of dried material from scattering, it is necessary to increase the particle size of the dried material to some extent.

A spray drying apparatus for drying the nitrate solution is shown in FIG.

1 is a spray dryer, this spray dryer 1
A spray nozzle 3 connected to a nitrate solution input pipe 2 is provided at the upper part of the chamber.

On the upper side of the spray dryer 1, there is a hot air supply pipe 4 for supplying hot air of 300 to 400°C, and on the lower side of the spray dryer 1, there is a hot air discharge pipe 5 for discharging hot air of 150 to 200°C. A nitric acid explosive powder uncollected container 6 is connected to the lower part of the dryer 1.

The hot air discharge pipe 5 is connected to a cyclone 8 having a container 7 for unrecovered nitric acid explosives at its lower part.

10 is an exhaust gas outlet, and 11 is an air supply pipe for two-fluid spray (described later).

In order to increase the particle size of the dried material, spray nozzle 3
Although it is possible to increase the size of the droplets sprayed, there is the disadvantage that the drying properties become worse.

In order to improve drying performance, the drying temperature may be increased or the residence time may be increased.

However, there is a limit to how high the temperature can be raised.) There is a concern that introducing hot air over 400'C may decompose calcium nitrate, and some concentrated liquid contains magnesium nitrate (Mg), which is an unmetathesis product, in the equimolar denitrification process. A small amount of NO3)2 is mixed in, and this decomposes even if hot air at 400°C is introduced, so it is desirable that the temperature of the hot air is 350°C or lower.

Furthermore, increasing the residence time means increasing the tower length of the soufflé dryer 1, which is not preferable because it increases the size of the equipment and increases the amount of fuel used.

In the spray drying apparatus shown in FIG. 1, if only drying performance is emphasized, the liquid nozzle 13 communicating with the liquid inlet 12 and the air nozzle 15 communicating with the compressed air port 14 as shown in FIG. A two-fluid nozzle may be used, but in this case, a dried material of 8 to 60 μm (average 20 μm) is obtained, which is too fine powder.

Therefore, the inventors of the present invention conducted experiments in order to obtain dried calcium nitrate with larger particle size using a spray dryer with a lower temperature and smaller equipment (spray dryer).
As a result of repeated research, they discovered that when a small amount of fine particles of calcium carbonate were added to a concentrated liquid and the resulting mixture was spray-dried, the particles of the dried product became larger because the calcium carbonate formed the nucleus.

That is, the present invention involves dispersing and suspending a small amount of calcium carbonate powder, preferably 3% by weight or less, in a nitrate solution or a solution containing nitrate as a main component, and then spray-drying the powder to adjust the particle size. The purpose is to obtain a large amount of dry anhydrous nitrate.

At this time, even if a small amount of calcium carbonate is mixed into a nitrate, such as calcium nitrate, if the following thermal decomposition reaction formula is taken into consideration, the mixing of calcium carbonate will not cause any problem.

In other words, when pyrolyzing calcium nitrate, in order to prevent the pyrolysis product, quicklime CaO, from turning into carbon dioxide, the calcination is carried out at around 900°C, which is the pyrolysis temperature of calcium carbonate, so the mixed calcium carbonate is This is because it thermally decomposes and changes into quicklime CaO.

Next, Examples and Comparative Examples of the present invention will be shown.

Example Concentrated solution with calcium nitrate concentration 20% (specific gravity 15/4℃1
．． 164) with 2% by weight of calcium carbonate fine particles dispersed and suspended therein and spray-dried continuously for 22 hours using the spray-drying apparatus shown in FIG.

The operating conditions are a hot air temperature of 400℃ at the spray dryer, and a hot air temperature of 200℃ at the outlet of the spray dryer.
, the throughput of concentrated liquid was 401/I (the amount of compressed air supplied from the two-fluid nozzle (see Figure 2) was 11 ol/MIN).

As a result, the recovery rate of calcium nitrate was 98% (value including the recovery rate from containers 6 and 7 where nitric acid bomb powder was not recovered), and the moisture content of dried calcium nitrate was 2.3% (the dried material was heated at 200°C until it reached a constant weight). It was determined by repeating heat treatment.

), particle size average 110 μm (measured by light transmission particle size distribution measuring method).

) good results were obtained.

Comparative Example The same concentrate as that used in the example was continuously operated for 20 hours using the spray drying apparatus used in the example without adding or dispersing calcium carbonate.

The results showed that the recovery rate of calcium nitrate was 86.2% (adding the recovery rate from containers 6 and 7 where nitric acid bomb powder was not collected), and the moisture content of dried calcium nitrate was 3.0% (the dry matter was heated at 200°C until it reached a constant weight). It was determined by repeating the heat treatment.

) Particle size average 30 μrrL (measured by light transmission particle size distribution measuring method).

), the particle size was small and the recovery rate was poor.

A particle size distribution diagram measured by a light transmission particle size distribution measuring method is shown in FIG.

As explained above, the present invention has the effect that dry anhydrous nitrate with large particle size can be obtained at a high recovery rate from a nitrate solution or a solution containing nitrate as a main component.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet of a spray drying apparatus used in Examples and Comparative Examples of the present invention, FIG. 2 is a cross-sectional explanatory diagram of a two-fluid nozzle, and FIG. 3 is a particle size distribution diagram of dried calcium nitrate. 1... Spray dryer, 2... Nitrate solution input tube, 3... Spray nozzle, 4...
... Hot air supply pipe, 5 ... Hot air discharge pipe, 6
, 7... Nitric acid explosive powder uncollected container, 8...
Cyclone, 10... Exhaust gas outlet, 11...
...Air supply pipe, 12...Liquid inlet, 13...
...Liquid nozzle, 14°°°°°゛Compressed air inlet, 1
5...Air nozzle.

Claims (1)

[Claims] 1 In a nitrate solution or a solution containing nitrate as the main component,
A method for drying a nitrate solution, which comprises dispersing and suspending a small amount of calcium carbonate powder in advance, and then performing spray drying to obtain dried anhydrous nitrate having a large particle size.