FI62040C - PROCEDURE FOR THE FIELD OF THE METALLURGICAL INDUSTRY - Google Patents

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Patent · och registerstyrelsen '' Amok »utkgdoeh uti.ikriften pubikand 30.07.82 (32) (33) (31) Pyydetty etuolkeui — Begird prloritet (71) Outokumpu Oy, Outokumpu, FI; Töölönkatu 4, 00100 Helsinki 10, Finland-Finland (Fl) y (72) Risto Johannes Honkala, Vanha-Ulvila, Matti Elias Honkaniemi,

Tornio, Jaakko Teijo Ilmari Poijärvi, Vanha-Ulvila, Frans Heikki Tuovinen, Ulvila, Matti Olavi Vattulainen, Kiviranta,

This invention relates to a process for the washing of cyanide-containing furnace gases from the metallurgical industry. In particular, the present invention relates to a process in which the combustible constituents contained in the washed oven gases are disposed of by incineration. both and by removing solid and dissolved impurities from the wash water solution before returning it to the wash.

Both blast furnaces and closed ferrochrome furnaces used in the production of steel are characterized by the evolution of gases under the conditions prevailing in these processes which, in addition to carbon monoxide, carbon dioxide and hydrogen, contain cyanides, mainly alkali metal cyanides. Because furnace gas also contains a significant amount of solids, the gas must be cleaned before it can be used or further processed. Wet scrubbers, such as venturi scrubbers, are commonly used to clean gases, where the solid contained in the gas as well as water-soluble gases are washed in water. As a result of this 2,62040, the wash water solution contains not only solids but also environmentally harmful cyanide. Various attempts have been made to reduce the cyanide content of the washing water solution.

According to a previously known method, the pH of the cyanide solution is raised to about 10 to 11, after which the cyanide is oxidized with a suitable oxidant such as chlorine, hypochlorite, hydrogen peroxide, sulfuric acid or ozone. As a result of the oxidation, the cyanide is converted either to cyanate or, if the oxidation is carried out to completion, to nitrogen and water.

According to another method known per se, the cyanide is brought into a form which is less harmful to the environment by mixing it with Ferro salts, either Berlin blue or white. The former decomposes over time under basic conditions back to cyanide, while the precipitation as Berlin white does not go quantitatively to completion, leaving a moderately high residual concentration of cyanide in the cyanide solution.

Other methods of treating cyanide-containing aqueous wash solutions include catalytic oxidation of cyanides as a gas with a platinum / rhodium catalyst or activated carbon. The oxidation can also be carried out by electrolysis of concentrated cyanide solutions, the final concentration usually remaining at about 1 g of cyanide / l.

The disadvantages of the above-mentioned known methods are either the high chemical and investment costs, the complexity of the process or the method is not suitable for the treatment of dilute cyanide solutions, which are especially formed when washing furnace gases. What these known methods also have in common is that they are mainly directed to the removal of cyanide already in the water.

The object of the present invention is to obviate the drawbacks of the above-mentioned prior art methods and to provide a method for washing cyanide-containing furnace gases from the metallurgical industry, in which the balance between furnace gases and wash water solution is adjusted so that the cyanide content in the wash water solution remains so low. In the process according to the invention, the aim is to keep as much of the cyanide as possible in 3 62040 oven gases, which are burned after washing, whereby the cyanides also decompose into less harmful constituents.

The main features of the invention appear from the appended claim 1.

According to the invention, the cyanide balance between the furnace gases and the wash water solution is adjusted by adding acid to the circulating wash water solution to adjust the pH of the wash water solution so that as much cyanide as possible remains hydrogen cyanide in the gas and can be disposed of in combustion. By doing so, the equipment used to apply the method is simplified and the investment costs are low, in addition to which the operating costs are low. Oven gases contain so many flammable ingredients that it is profitable to utilize its heat content by burning oven gases after washing. In this case, the cyanide contained in the furnace gas decomposes into much safer combustion products.

The pH of the circulating wash water solution is preferably adjusted to a maximum of 9.5, for example to a maximum of 7.5. Due to the aim of keeping the pH lower than usual, various impurities, such as zinc, are dissolved and enriched in the washing water. The impurity content of the circulating wash water solution is adjusted by removing part of the wash water cycle and replacing it with fresh water. Impurities are precipitated from the wash water solution removed from the cycle by raising its pH. Because the cyanide content of the wash water solution is low, it can be safely treated to remove contaminants therefrom.

Equilibrium calculations of carbon dioxide, monoxide and nitrogen-containing furnace gas show that in the presence of water vapor, cyanide is in the form of hydrogen cyanide, the partial pressure of which decreases with increasing oxygen pressure and temperature. Here, it is possible to lower the cyanide content of the gases, e.g. by passing air at a high temperature before washing.

In the presence of potassium, the cyanide is virtually entirely in the form of potassium cyanide. As the oxygen pressure decreases, the cyanide begins to appear more in the form of hydrogen cyanide, and at low temperature and oxygen pressure (CO 80%) the cyanide is predominantly in the form of hydrogen cyanide. If the gases contain potassium, it can generally be stated that the oxygen pressure has only a small effect on the presence of cyanide, i.e. cyanide is always present in the equilibrium mixture.

4 62040

When washing oven gases, the cyanide content depends on the pH of the washing water solution as follows:

From the reaction equation CN ~. , + H +,. jr * · HCN,.

(aq) (aq) * ^ (g) gives approximately Λ GT ° (J / mol)

^ Grp -15500 - 108 T

3a log PHCN * ^ - pH + log / CN "_7 + 5.62 where ΡΗςΝ = partial pressure of cyanide (bar) / CN J = concentration of cyanide in the washing water (mol / l) T = washing temperature (K)

At a constant temperature, the cyanide level and pH of the wash water determine the partial pressure of the cyanide hydrogen in the washed furnace gas. That is, the cyanide content of the effluent gas is adjusted, by adjusting the pH of the wash water, to equal the partial pressure of the cyanide of the incoming gas. The lower the cyanide level of the wash water is desired, the lower the pH must be in order for a certain cyanide load accompanying the furnace gas to pass completely through the wash.

The following theoretical example illustrates this:

If oven gas with a cyanide content of 300 mg / m 2 (0 ° C, 101,325 kPa) (0.024% by volume, 2.4 * 10 4 bar) is washed at a temperature of T = 27 ° C (300 K) and it is desired that the cyanide load goes completely through the wash and the cyanide content of the wash water does not exceed 10 mg / l (3.6-10-4 mol / l), the pH of the wash water should in principle be adjusted as follows pH = log / CN “_7 - log pHCN + ^ + 5.62 pH = log 3.6-10-4 - log 2.4-10-4 + + 5.62 pH = 8.50

If the cyanide level of the circulating wash water is allowed to rise to 100 mg / l, the pH must be 9.5. The cyanide content of the furnace gas and the partial pressure of hydrogen cyanide in the wash water come in the same range between pH 7.5 and 9.5 if the cyanide content of the circulating wash water is to be maintained at the current level (10 mg / l). health hazard. In this case, the cyanide load contained in the furnace gas, which would be between 30 and

O

3000 mg / m would go through the wash, Figure 2.

The invention will be described in more detail below with reference to the accompanying drawings, in which Figure 1 shows a schematic side view of an apparatus for applying the method according to the invention and Figure 2 shows a diagram showing the cyanide content of the washing water solution At C washing temperature.

In Fig. 1, which shows the CN level control system of the washing water, reference numeral 1 denotes a scrubber to which the gas coming from the furnace is led along a pipe 2. The pH of the wash water is adjusted by returning circulating wash water to the scrubber via line 3 to which sulfuric acid has been added in the mixing tank 4 to such an extent that the pH of the wash water leaving the scrubber via pipe 5 is at the desired control value. The pH of the outlet wash water is measured in the tank 6. The amount of concentrated sulfuric acid to be pumped from the tank 9 to the mixing tank 4 is controlled by means of a pH meter 7 and a valve 8. The water evaporating in the scrubber 1 and leaving the pipe 11 as steam with the exhaust gas via the fan 19 and the water leaving the thickener 14 with the solid are replaced by adding water to the circulating wash water along the pipe 12. The water leaving the scrubber 1 is led from the pH measuring tank 6 along the pipe 13 further along the pipe 16 to the thickener 14, where the solid is settled. The settling is carried out by passing a flocculant or pH adjusting and / or precipitating chemicals into the tank "17 via a line 15. The settled solid and precipitated salts are removed from the circuit via a pipe 18 to a waste area and more washers.

Example _

The gases from the ferrochrome furnace, the HCN content of which was 400 mg / Nin, were washed in the venturi scrubber 1 shown in Fig. 1. A 50 m 2 of circulating wash water solution per hour with a pH of 8.2 was introduced into the venturi scrubber 1. The pH adjustment was performed by adding 1.2 g of sulfuric acid / 1 of wash water to the aqueous wash solution. During the washing, cyanide was transferred to the washing water, so that the cyanide content of the washing water solution after washing was 5.1 mg / l of the washing water solution.