HU180340B - Process for the regulation of the dosage of carbamide applied for the reduction of nitrogen oxide flowing out from nitric acid solutions and measuring system for performing the regulation - Google Patents

Abstract

The method for reducing the emission of nitrogen oxides from the nitrogen-containing solutions according to the invention is used in the process of obtaining mineral fertilizers in the so-called nitro-phosphate process and consists in the application of a continuous current between the electrode of the solution of the acid solution nitric, where the voltage is lower than the potential for water decomposition, the electrical current between the electrodes being approximately proportional to the nitric acid concentration, this amperage being usable for a linear measurement of the relative concentration of the nitric acid, thus the measured amplitude depends first of all the surface of the anode and so the system can be simplified by using the platinum electrode as the anode and the vessel walls as cathode.

Description

The present invention relates to a process for controlling urea dosing by reducing urea nitric oxide effluents from nitric acid solutions. The effluent nltrogen oxlds are formed by reacting phosphate ore or other minerals with nitric acid, or nltrogen oxlds in the early absorption of acidic solutions ·

The invention also relates to a measuring system for carrying out the process.

The so-called ntrophosphate process, one of the first steps in the production of fertilizers, involves the acidification of mineral or sedimentary phosphate ores in concentrated nitric acid. During this step of the process, a well-known problem arises, namely the formation of nitric oxides, which are generally associated with readily oxidizable impurities of organic or sulfuric origin in phosphate ore. Discharges of Nitrogen Oxides require certain measures to avoid air pollution. We need to purify the gases either in a purification unit or add additional chemicals when phosphatase is added.

According to U.S. Pat. No. 3,528,797, the outflow of nitric oxides, during dissolution or acidification of phosphates in nitric acid, can be suppressed by the addition of an arm to the acid used in the molten molten liquid or gas. These methods have proven to be very effective.

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Usually urea is added in solid form to the acid vessel while phosphoric acid and nitric acid are added continuously, and the added urea is added in a fixed proportion to the phosphate.

The disadvantage of the known procedures is. that the outflow of nitrogen oxides varies considerably due to inaccurate urea feed. If a large excess of urea is added, the outflow of nitrogen oxide can be maintained at a low level, but the resulting liquid will have an unfavorably high amount of urea. This may lead to some problems in the next steps of the process and unwanted tu * in the final product. It can cause specifics.

As mentioned above, the amount of urea required depends on the composition of the phosphate ester. The ratio between the amount of urea and the amount of phosphate is generally determined ex post and then maintained constant for each quality of the phosphate. This method has proved inadequate in practice, for several reasons. Above all, phosphatic ore feed! there will be unwanted fluctuations in speed. Secondly,. even within the same type of phosphate ore from the same consignment, there may be differences in ore contamination. In addition, it is often necessary to mix different types of phosphate ores, which makes the above problems even more pronounced. The above can quite easily lead to an overdose of urea to keep nitric oxide outflows low.

Another method, in which carbamyl is added to a saline scrubbing solution, is the purification of the waste gases containing the nitric oxides in absorption towers. The requirements for regulation and dosing are quite similar to those for urea dosing with phosphate ore. In order to avoid the precipitation of urea nitrate at the same time with satisfactory purification and to obtain a usable solution as a product, the addition of urea - and thus its concentration in the solution - must be constantly corrected and controlled.

The urea-induced reduction of nitrogen oxide outflow can be explained as follows *

When phosphate ore or similar minerals such as limestone or olivine are dissolved in concentrated nitric acid, oxidizable impurities such as sulfides reduce nitric acid to nitric acid * / 1 / HHOj + HgS ^ HNOg + HgO ₊ θ

The nitric acid thus formed liberates nitrogen oxides according to the following reaction equations * / 2 / HN0 ₂ + HN0 ₅ ................... ^ HgO + 2N0 _2/3 / 2HN0 ₂ - ...........> H ₂ 0 + N0 ₂ + NO

Because nitric acid is present in high concentrations, which is common in the acid vessel, the a / 2 / reaction will dominate. The release of nitrogen oxides will increase approximately with the square of the concentration of nitric acid. When urea is added to the pot, the urea is relatively fast ¹¹

-3180.340 reacts with nitric acid according to the following reaction equation:

/ 4 / Ιβί0 ₂ + / Hllg / ₂ C0 + IINOj _......

/ N ₂ + co ₂ + nh ₄ no ₅ + h ₂ o

The nitric acid concentration will then decrease and the release of nitric oxide will decrease accordingly. Controlling the release of nitric oxide involves controlling the concentration of nitric acid.

In a urea solution containing nitric acid, when absorbing nitric oxides, when excess urea is used, the reactions of equations (2) and (5) take place from right to left, and up to 90% of the oxides of nitrogen are reacted in this direction. will play. After the formation of nitric acid in the solution, the process proceeds according to the reaction equation / 4 /. As the solution decreases with nitric acid, the absorption of nitric oxides will be faster (5 reactions), so there will be a specific relationship between the concentration of nitric acid and the rate of absorption.

After studying the kinetics of the reaction between urea and nitric acid, the inventors came to the conclusion that the decrease in reactive nitric acid concentration over time can be accepted as being proportional to the product of reactant concentrations:

/ 5 / - ° HN0 ₂ = k. ° HNO ₂ . ^ urea

The rate constant k depends, inter alia, on the concentration of nitric acid and the temperature.

Since the operating conditions of a pottery for a particular type of phosphorus are constant, as are those of an absorption tower into which a given concentration of gas is introduced, the amount of nitric acid formed - which, after a small outflow of nitric oxide, . This means that the final concentrations of urea and nitric acid will be reversed.

Control of urea dosing according to this model can thus keep nitric acid concentrations at a desired level and nitrogen oxide release at an acceptable low level. The instantaneous concentration of urea in the acidification unit will be approximately proportional to the amount of readily oxidizable impurities present in the phosphate ore feed. It is very difficult to determine this instantaneous final concentration by known analytical methods. However, the inventors found that the final concentration was only a small fraction of the total amount of urea added. There are various ways of controlling the dosage of urea. For example, the amount of nitrogen oxides from the phosphate acid pot can be measured and the urea dosage can be controlled using these measurements, but the equipment can easily be damaged by fluorine and chlorine gas in the gas. The inventors also found that it was impossible to achieve accurate dosing when urea had to be added at a constant rate relative to the amount of phosphate used. Certain analytical problems and phosphate ore replaced it

-4180.340 Composition © discouraged them from this way of controlling their urea dosage.

However, further studies have shown that by applying a DC voltage to two electrodes placed in solutions containing nitric acid and lower than the decomposition voltage of water, the electrical current flowing between the electrodes is approximately proportional to the concentration of nitric acid. Accordingly, it was believed that this current could be used to linearly measure the relative concentration of nitric acid. It has been found that the measured current is primarily dependent on the surface of the anode and therefore the system can be simplified by using a platinum electrode as the anode while the cathode is applied directly to the wall of the vessel used.

In this case, it has been found that the current is approximately proportional to the surface of the anode. Therefore, it is assumed that the current is determined by the diffusion of nitric acid to the anode, when nitric acid is oxidized to the nitric acid by the direct current.

The measuring system can be further simplified by wiring a suitable resistor in series with the power source and electrode and measuring the voltage drop across the resistor using a millivoltmeter (see Fig. 5). I-Ia the concentration of nitric acid increases, the voltage drop at the electrode decreases and the amplitude is no longer proportional to the concentration (see Figure 6). The measuring interval can be varied by using resistors of different sizes to provide maximum sensitivity within the given measuring interval.

Characteristic features of the invention are defined in the claims.

The invention will now be illustrated by the following figures and examples.

Figure 1 shows a known arrangement of an acid vessel with feeders for feeding urea, phosphate ore and nitric acid. The figure also shows the waste gas treatment unit.

Fig. 2 shows the phosphorus ejection with the urea dosage according to the invention.

Figure J shows a tower for absorbing nitric oxides, wherein urea is added to the nitric acid absorption solution. Figure 4 shows the absorption of nitrogen oxides according to the present invention.

Fig. 5 shows a measuring system for carrying out the procedure.

Figure 6 shows the relationship between nitric acid concentration in the treated solution and the recorded voltage.

Figure 7 shows the relationship between the voltage recorded in the treated fluid and the nitric oxide released / kg N / t P / from the phosphate ore acidification unit. ............. ~

Figure 8 shows the amount of urea in the solution leaving the acidification unit as a function of the recorded voltage.

Figure 1 illustrates a conventional apparatus for co-purifying phosphate- ^{1 with} nitric acid, with the addition of urea.

-5180.340 for driving. Nitric acid is passed through a line 5 to an acid vessel with mixing belts. Phosphate ore comes from the 3-hopper tank with the help of the measuring conveyor 4. The carbamate comes from the hopper container 6 with the aid of the measuring conveyor 7 in proportion to the amount of phosphate ore supplied. Connected to the whey vessel 1 is a waste gas purification unit equipped with a scrubber 10 to which a weak acid solution 11 is fed. The gases are removed with an exhaust fan 9. The whey fluid leaves the vessel 1 through line 8.

Figure 2 shows the same unit, but here urea is added according to the invention.

A portion of the solution leaving the reactor flows through a vessel containing a measuring system of the arrangement shown in Figure 5 ·. The measuring electrode 13 is located in the conduit 8 which controls the speed of the measuring conveyor 7 through the regulator. The control can be done directly, as shown in the figure, or by controlling the metering ratio between the metering conveyors 7 and 4. This figure does not show the purification unit of the waste gas, but it is the same as that shown in Figure 1.

Fig. 3 shows an absorption tower 15 in which nitric oxides are removed from the gases with nitric acid. The gases arrive on the conduits 16, 60% nitric acid on the conduits 17, the amount of which is controlled by the pH regulator 18. The urea solution 20 arrives at line 19.

Figure 4 shows the same unit as Figure 3, but urea is administered according to the invention. The measuring electrode 21 is placed in space 22 below the charge in the absorption tower. The signal from the electrode 21 controls a valve in the conduit 19 for adding the urea solution. The acid is added via line 19 as shown in Figure 3.

Figure 5 is a schematic representation of a measuring system for carrying out the invention. The measuring electrodes 13 and 21 are located in the fluid stream in line 8/2. and in space 22 in the absorption tower 15. and the resistor shield 23 is connected in series with the DC source 24. One of the electrode terminals of the power source 24 is vessel 1/2. / wall or the tower 15/5. / wall. The voltage measured through the resistor 23 will be a function of the concentration of nitric acid in the treated fluid 25 and the voltage value will be recorded by the voltage meter 26.

Fig. 6 shows the fixed voltage values (in volts) measured in the treated liquid at different concentrations of scarf and romic acid across the resistor in line 8/2. and / or the tower 15 in the space 22 in the liquid mixture. figure/. The calibration curve was obtained using an electrode having a surface area of ² x 2.5 mn ² and 1 M nitric acid, connected in series with a 100 kilohm resistor and a 0.7 volt current source.

Figure 7 illustrates the change in the amount of nitric oxide released in the acid vessel - which is determined by the scrubber / l. and Fig. 2 (a) as a function of the voltage drop (volts) of a 13 electrode / 2 in the acidic solution 8 leaving the vessel. /

-6180.340 up. The release of nitrogen oxides is given as kg N / t P / phosphate / unit.

Fig. 8 shows the changes in the final concentration of urea in the acidic solution 8 leaving the vessel as a function of the voltage drop, the latter of which is shown in Figs. measuring electrode with a given phosphate content and a given oxidation contaminant content. Figures 7 and 8 were obtained by direct measurement at a NKP fertilizer factory. For the measurements, an electrode with a surface area of 70 µm was used as the anode, while a stainless steel vessel was used as the cathode. The applied voltage is 0.7 V and the resistance is 4 kilohms.

Example 1

In this example, the acidification of phosphate ore in the unit shown in Figure 1 is shown. Phosphate from silo 3 was added to the whey vessel at a rate of 20 t / h with the aid of metering conveyor 4, while the phosphate feed was determined from the velocity of the conveyor. At the same time, 60% nitric acid was added to the acid vessel at 35a '/ hour. The acid was added in proportion to the amount of phosphate fed. With the help of the measuring conveyor 7, 90 kg / h of solid urea from the hopper container 6 was mixed with the phosphate while the dosing was controlled at the speed of the conveyor. Dosage of carbamyl was manually adjusted at a given ratio relative to the feed phosphate. The reaction mixture was removed from the vessel via line 8 for further processing.

The vent was connected to a vent that sucked out the reaction gases. The waste gases were treated in a scrubber tower to which 11 5% nitric acid was added. Due to differences in the mixture of acid, phosphate and urea, and due to the varying amount of impurities in the phosphate, nitrogen oxides released in the effluent gas range from 0.1 to 1.5 kg N / t P, the urea in the and concentrations ranging from 0 to 800 mg urea / liter.

Example 2

This example illustrates the acidification of phosphate ore carried out in the unit of Figure 2. Phosphate feed was adjusted to a maximum of 15 t phosphate / hour while 60% nitric acid was added at a rate of 26 m '/ hour. The signal from the measuring electrode 15 directly controlled the speed of the meter conveyor for feeding the urea. The nominal value of the controller is set to 150 mV. The nitrogen oxide outflow in the waste gases is 0.20 -0.1 kg N / t P. The final concentration of urea in the 8 acidic liquids is 250-100 mg of urea / liter of liquid and the average feed of urea is 65 kg / h. This control system is appropriate for units that have fluctuations in phosphate feed.

Example 3

This example illustrates the acidification of phosphate ore in the apparatus of Figure 2. however, using a slightly modified regulatory system. Phosphate feed in the range of 15 to 25 t phosphate / hour was manually controlled ·

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Accordingly, the 5θ% nitric acid feed ranged from 25 to 45 d '/ h. The amount of urea added to the pot was automatically controlled to be proportional to the amount of phosphate added to the pot. The signal from the measuring electrode controlled the proportionality factor between the phosphate feed and the urea feed so that the signals were as close as possible to the manually preset 200 mV nominal value.

The nitrogen oxide outflow in the waste gases ranged from 0.25 to 0.05 kg / t P and the final concentration of urea ranged between 100 and 250 mg urea / liter acidic liquid. Urea feed ranged from 65 to 115 kg urea per hour. This control system is suitable for equipment in which the phosphate supply is variable.

Example 4

This example illustrates the absorption of nitrogen oxides in the apparatus of Figure 5. Absolute gas pressure of 3.0 bar at 10 ° C, and nitrogen in addition to 325 ppm nitrogen dioxide, 1045 ppm nitrogen oxides and oxygen-containing gas is 5%, 30,000 Nm '/ hr with a 3 »5 ^m diameter, 20 m treated with a high absorption tower with a charge area of 12,000 n *. The gas was flushed through the tower counter to the absorption solution circulating at a volume of 150 µl / h. 60% nitric acid was added to the absorption solution via line 17 and the addition was controlled by means of a pH regulator. the excess acid in the circulating solution

2.5%. The acid was added in an amount of 89 liters per hour. · A 40% urea solution was added through line 19. This addition was manually controlled by periodic analysis of the circulating fluid and also checked for urea nitrate to settle. At a rate of 100 liters of urea per hour, the urea concentration in the circulating fluid was 2%. · The process produced ammonium nitrate in such a way that the circulating solution contained 38.4% ammonium nitrate. The waste gases from the absorption tower contained 400 ppm NO + NO ₂ gas under constant operating conditions. If the urea in the absorption fluid was in the range of 0.5 to 3%, the concentration of nitrogen oxides released ranged from 600 to 375 ppm.

5 · Example

This example illustrates the absorption of nitric oxide in the equipment of Example 4. The amount of gas supplied, the pressure, the circulation rate, the concentration of acid and urea in the supplied solutions and the amount of acid and ammonium nitrate in the circulating solution were the same as in Example 4. The concentration of oxygen in the gas ranged from 4 to 8% of the difference in acid production. Due to differences in oxygen content, the concentration of nitrogen oxides ranged from 80 to 1000 ppm prior to purification.

In order to prevent the precipitation of urea nitrate and to minimize the unwanted urea content in the resulting solution, and in order to achieve optimum absorption, a measuring electrode was placed under the filler in contact with the solution. The signal from the measuring electrode 21 was directly controlled via the wire 19

-8180.340 feed urea by comparing the measured voltage signal with a manual preset of 200 mV.

Thus, the urea content in the resulting solution was 1.7-0.4% by weight. Leaving the absorption tower the waste gas' from 450 to 25 ppm of Ni-oxy TROG to criteria if the exhaust gas oxygen content of 4%, and comprising 15 PP 285 * ^m nitrogen oxides when the exhaust gas has an oxygen content of 8%. Urea consumption ranged from 85 to 130 liters / hour, and acid consumption ranged from 70 to 11 liters / hour.

As shown in FIGS. 6-8. 1 through 2, the inventors have recognized that the relationship between the concentration of nitric acid and the signal of the electrode, recorded as a voltage drop (in volts), can be used to control the nitric oxide content of waste gases and the urea storage limit for solutions in which nitric acid concentration can change. By controlling the urea feed using the relationships shown, the process of the present invention achieves significantly better control of both the nitric oxide outflow and the urea content of the product.

Comparative Examples 1 and 4 clearly show that the prior art procedures result in unwanted fluctuations and high urea utilization which adversely affect the urea content of the product.

Examples 2 and 6 show that in a given acidification unit, the release of nitric oxide per phosphate according to the invention is about 1.4 to 0.2 kg N / t P and the fluctuation of urea in the acid leaving the vessel is 800 Can be reduced to -150 mg urea / liter.

Comparison of Examples 4 and 5 also shows that the process according to the invention also reduces the amount of variation in nitrogen oxides emitted from an absorption unit. The examples also illustrate the changes in the waste gases where, in the optimum case, the outflow is approx. From 50% to approx. Decreases to 10%.

Claims (3)

Patent claims A method for controlling the dosage of urea for reacting phosphatic or other minerals with nitric acid or reducing nitric oxide effluent from nitric acid solutions formed in acidic solutions of nitric oxide, wherein the concentration of the solution is less than 1.5 V by nitric acid concentration. between the polarized electrode is determined by measuring the current flowing through the solution and the addition of urea is controlled by the electrical signal obtained from the measurement of nitric acid concentration. 2. The process of claim 1, wherein the urea addition is controlled according to the nitric acid concentration of the solution leaving the acid vessel and the electrodes are Ο, Ι-Ι, Ο V, preferably 0.4-0.8 V switching on DC. 180 340 A measuring system for carrying out the method according to claim 1 or 2, characterized in that it has a DC source for use in the 0-1.5 V measuring range, which has an electrode terminal connected to a wall of a tube or vessel containing nitric acid and another It has an electrode output connected to a measuring electrode - has a resistance connected in series between the measuring electrode and the DC voltage source, and has a potentiometer connected to the resistor by measuring the voltage across the resistor ·