JPS61275132A - Dissolving method for iron by nitric acid - Google Patents

Abstract

PURPOSE:To dissolve iron in the quick velocity without increasing the amount of NOX to be generated and the amount of hardly-filterable iron to be settled by showering continuously the prescribed amount of HNO3 having the preferable concn. on the surface of iron. CONSTITUTION:50-72wt% HNO3 aq. soln. si showered on iron in the amount of >=200ml/min preferably 600ml/min per 100g iron. A passive film momentarily formed on the surface of iron is destroyed with O2 contained in air dissolved in NHO4d3 aq. soln. which is thinly spread on the surface of iron by a reaction shown in a reaction formula and the reaction dissolving iron is performed almost continuously and therefore the reaction temp. reaches 60->=70 deg.C and the effective iron treatment is performed and iron can be dissolved in 2-3 times velocity of a conventional method without increasing the amount of NOX to be generated and the amount of hardly-filterable iron to be settled. Also when making an apparatus a closed type, the environmental pollution due to NOX can be prevented and the operation efficiency is increased becauses HNO3 denser than the conventional method can be used.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an improved method for dissolving iron (in an aqueous nitric acid solution).

[Conventional technology]

Iron nitrate has been used for a long time in tanning leather and the pharmaceutical industry, but has recently attracted attention as a raw material for magnetic ferrite. Conventionally, iron is dissolved in nitric acid as follows: 1) Iron is added and dissolved in an aqueous solution of nitric acid with a specific gravity of 1.29 (45.95% by weight) until the initially green liquid turns dark red.

2) Dissolve iron with a specific gravity of 1.25 to 1.33 (38,92 to 5237 weight-11) and excess nitric acid.

3) A mixed solution of 100 - concentrated nitric acid and 30111 water (specific gravity 1.31, 49.0% by weight) may be prepared by adding K2O2 iron shavings little by little and heating at below 70°C. Described in the literature.

However, in the above method, when the nitric acid concentration is high (specific gravity 1.30 or higher > VCId), when iron is added to the nitric acid aqueous solution, a passive film is formed on the surface of the iron, making it generally difficult to dissolve the iron.

Part 1. When this passive film is destroyed by stirring or the like during the iron melting operation, the iron dissolution reaction proceeds all at once. The reaction at this time was 1. Since this is an exothermic reaction, the liquid m rises rapidly and generates 17 tatami of NOx.

Originally, the dissolution reaction of iron in nitric acid was carried out at a temperature of 40°C.
It does not progress at temperatures below 70°C, and iron nitrate decomposes at temperatures above 70°C.
．． As a result, a difficult-to-filter iron precipitate (mainly 1-Fe203) is formed.

Therefore, when dissolving iron with nitric acid, the liquid temperature should be 40°C or higher and 70°C.
It is necessary to keep the temperature below ℃, but as mentioned above, the reaction does not start until the passive film is destroyed for some reason, and if the passive film is broken, the iron melts. Since the reaction is carried out intermittently, it is necessary to repeat heating and cooling in order to control the liquid temperature within a desirable range), and the dissolution efficiency is also poor. In addition, when dissolving iron with nitric acid with a specific gravity of about 1.25, the dissolution reaction proceeds smoothly at a constant rate, but on the other hand, divalent iron nitrate is produced12,
When this decomposes as shown in the following equation, the formation of iron precipitate with poor filterability is unavoidable.

6Fe(N03)2+5H20→ 3Fe203+2NO+ 1oHNo3 [Invention solved 1
7. Problems to be Solved] The object of the present invention is to provide a method for dissolving iron with nitric acid that does not have the above-mentioned drawbacks.

[Means for Solving the Problems] The method of the present invention is applicable to, for example, 5 to 110×
10 seconds of chips or lumps of iron are filled into a dissolution tank that has a chamber and an outlet in which the solution accumulates through a perforated plate at the bottom, and a 50 to 72% by weight aqueous nitric acid solution is added to the top of the nuclide per 100 g of iron. ) It is characterized by continuous showering of 200 d or more, preferably 600 au or less per minute. The aqueous solution showered into the dissolution tank and discharged from the bottom of the nuclide is then introduced into the storage tank and fed back into the circulation pump to shower the same amount of nitric acid aqueous solution. It is said that it dissolves in.

[Effect]

In the present invention, the reason why the concentration of the nitric acid aqueous solution used is regulated is that although it is possible to dissolve iron even if the nitric acid concentration is 50% by weight or less, the amount of divalent iron ions produced increases.1. ．． If more than 72% by weight is used, perhaps because the oxidizing power of nitric acid is too strong, the generated immobile jI111K is not sufficiently destroyed even by the method of the present invention, and as a result, the dissolution rate of iron is slowed down, and the reaction temperature is low. This is because iron nitrate self-decomposes due to the increase in iron nitrate.

Next, nitric acid aqueous solution was added to iron 100%. 2001 g per minute
14 or more, preferably 600 or less, to continuously dissolve iron by showering.If the amount of nitric acid is less than 200 ILl/min, the heat of reaction during the dissolution reaction is too low, resulting in poor dissolution efficiency. This is because the destruction of the passive layer that occurs on the iron surface during iron melting is insufficient.

For the same reason, it is necessary to shower the iron continuously. In this way, when a predetermined amount of nitric acid of a suitable concentration is continuously showered on the surface of the iron, the nitric acid aqueous solution spreads thinly on the surface of the iron. Oxygen in the air dissolved in 1 destroys the immobile 8111 that is instantaneously formed on the iron surface according to the following formula.17 Since the iron dissolution reaction is carried out continuously, the reaction temperature is also 60 to 70.
It is estimated that iron processing is more efficient than C.

Fe, O+4HNO3+70□-+ 3Fe(No3)
3+2H20+NO? Normally, once the above temperature is reached, the iron nitrate that has been formed decomposes. However, in the method of the present invention, the flow rate of the nitric acid aqueous solution layer on the iron surface is fast and the iron surface is only exposed to high temperature for a very short time, so the produced iron precipitate is The amount of iron gradually added to nitric acid is 1. It is as low as when iron is melted under suitable conditions.

However, in order to prevent the decomposition of iron nitrate, it is desirable to take measures such as cooling with water so that the temperature of the aqueous solution collected in the liquid reservoir from the dissolution tank in the method of the present invention does not exceed 70'C.

According to the method of the present invention, the amount of NOx5 generated by the iron dissolution reaction and the treatment temperature can be controlled simply by adjusting the flow rate of the nitric acid aqueous solution.

The dissolution rate of iron by nitric acid is almost constant and extremely fast, about three times that of conventional methods.

〔Example〕

Examples will be described below.

Example 1 A 100# iron ingot measuring 5×10 mm and 10 μ thick was placed in a dissolving tank, and a 61% by weight aqueous nitric acid solution 11 was showered from above at a predetermined flow rate. Shower ring continuously until the iron ingot is completely dissolved. The average iron dissolution time was determined from the time required to complete dissolution. During operation, the liquid reservoir at the bottom of the dissolution tank was cooled with water so that the temperature did not exceed 70°C.

For reference, the gases generated during each treatment were absorbed in 20% by weight hydrogen peroxide solution, and the nitrogen concentration in this absorbed solution was determined by a colorimetric method to determine the amount of NOx generated, indicating the completion of iron dissolution. The amount of iron dissolved per hour was calculated from the time taken.

The results are shown in Table 1 in comparison with the conventional method.

As is clear from Table 1, all experiments except Na2, which was outside the scope of the method of the present invention, showed an iron dissolution rate more than twice that of the conventional method. Iron precipitation and NOx generation during iron dissolution tended to be lower (compared to conventional methods)17.

Example 2 Iron was dissolved in the same manner as in Example 1 except that the nitric acid concentration when dissolving iron was changed 12 and the flow rate of the nitric acid aqueous solution was kept constant at 2501117/min. I asked for it.

The results are shown in Table 2.

Table 2 As can be seen from Table 2, the dissolution rate of iron is faster in Experiment 7, which has a lower nitric acid concentration, but the amount of iron precipitate increases; #: t, the rate of iron dissolution rapidly decreases 1. Moreover, the amount of iron precipitate generated also increased1
-ta.

〔effect〕

By showering iron with a nitric acid aqueous solution under suitable conditions! l) Iron can be dissolved at a rate 2 to 3 times faster than conventional methods without increasing the amount of NOX generated or the amount of iron precipitation, which is difficult to filter.

If the device is sealed, it is also possible to prevent pollution caused by NOx.

Compared to conventional methods, it is possible to use nitric acid with a higher concentration, resulting in advantages such as better work efficiency.

Claims (1)

[Claims] A method for dissolving iron using nitric acid, which comprises continuously showering 50 to 72% by weight nitric acid aqueous solution onto iron in an amount of 200 ml or more per minute per 100 g of iron.