JPS598411B2 - Method for scrubbing cyanide-containing furnace gas generated from a melting furnace or ferrochrome furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for contacting cyanide-containing furnace gas generated in melting furnaces or Huekou chrome furnaces in the metallurgical industry with a circulating aqueous scrubbing solution, and after removing solids and dissolved impurities from this aqueous scrubbing solution. The present invention relates to a method for scrubbing cyanide-containing furnace gas by returning an aqueous scrubbing solution to the scrubbing process.

In particular, the present invention relates to a method for burning off combustible components of furnace gas after scrubbing.

Melting furnaces for steel production and closed Huet chrome furnaces are characterized by the generation of gases containing cyanides, mainly alkali cyanides, in addition to carbon monoxide, carbon dioxide and hydrogen under operating conditions.

Such furnace gas usually contains CaO and M in addition to the above components.
slagging agents such as gO
It also contains in the form of fine dust alkaline compounds originating from the minerals ent) or from the ore to be treated, such as ZnO, as well as sodium and potassium compounds.

Since such furnace gases therefore contain a significant amount of solids, it is necessary to purify the gases before their use or further processing.

Wet scrubbers, such as Venturi scrubbers, which scrub the solids and water-soluble gases present in the gas with water are commonly used to purify the gas.

As a result, the aqueous scrubbing solution contains cyanide which pollutes the environment in addition to solids, and various attempts have been made to reduce the cyanide content in the aqueous scrubbing solution.

According to one known method, the acidity of the cyanide solution is reduced to about p
H10-11 and the cyanide is then oxidized with a suitable oxidizing agent such as chlorine, hypochlorite, hydrogen peroxide, persulfuric acid or ozone.

This oxidation converts cyanide to cyanate or, if the oxidation is complete, to nitrogen and water.

According to another known method, cyanide is converted to a form that is more harmless to the environment by precipitating it as Berlin blue or Berlin white with ferrous salts.

Although Berlin Blue gradually decomposes into cyanide under alkaline conditions, the precipitation as Berlin White is not quantitatively complete, resulting in a relatively large amount of cyanide remaining in the cyanide solution.

Another method for treating cyanide-containing aqueous scrubbing solutions is to catalytically oxidize the cyanide in gaseous form using a platinum/rhodium catalyst or activated carbon.

This oxidation can also be carried out by electrolysis of concentrated cyanide solutions, in which case the final concentration is usually about 1 cyanide.
It will be 9/11.

Disadvantages of the known processes mentioned above include the expensive chemicals used, the complexity of the process, the need for large investment capital, and the inadequacy of the treatment of the dilute cyanide solutions, especially those produced in the scrubbing of the furnace gas. .

Furthermore, a common feature of such known processes is that they primarily concern the removal of cyanide already present in the water.

It is an object of the present invention to eliminate the disadvantages associated with the known processes mentioned above and to establish an equilibrium between the furnace gases generated in the metallurgical industry and the scrubbing aqueous solution, so as to reduce the cyanide concentration in the scrubbing aqueous solution. It is an object of the present invention to provide a method for scrubbing cyanide-containing gases that maintains cyanide-containing gas at such a low level that it does not require separate removal.

The method of the present invention involves retaining as high a proportion of cyanide as possible in the furnace gas and burning the furnace gas after scrubbing to decompose the cyanide into its more harmless components.

In the present invention, the cyanide equilibrium between the furnace gas and the scrubbing solution is adjusted by adding acid to the circulating scrubbing solution, so that the acidity of the scrubbing solution is adjusted so that as large a proportion of cyanide as possible is present in the furnace gas. Adjust the level to such a low level that the combustible components inside will be eliminated during combustion.

The method of the present invention has the advantage that the equipment used for its implementation is simple, requires less capital investment, and has lower operating costs.

The furnace gas contains such a large amount of combustible components that it is beneficial to utilize its heat content by combustion of the furnace gas after scrubbing.

Combustion decomposes the cyanide in the furnace gas into much more harmless combustion products.

The pH of the circulating scrubbing aqueous solution is below 9.5, e.g. 7
．． It is preferable to adjust it to 5 or less.

By keeping the pH below normal, various impurities, such as zinc, are dissolved and concentrated in the scrubbing solution.

The impurity concentration in the circulating scrubbing aqueous solution is adjusted by removing a portion of the aqueous solution from the circulation system and replacing it with fresh water.

Impurities are precipitated from the scrubbing solution and removed from the circulation system by increasing the pH value of the solution.

Since the cyanide concentration in the aqueous scrubbing solution is low in the method of the invention, the solution can be processed to remove impurities without any risk.

Balance calculations for furnace gases containing carbon dioxide, carbon monoxide and nitrogen show that in the presence of water vapor, cyanide exists in the form of hydrogen cyanide, the partial pressure of which can decrease with increasing temperature and oxygen pressure. shown.

The table below shows two types of water vapor concentrations at a temperature of 1500K, namely A = 5g/m (273K; 101,3 2 5
kPa), B=20.9/r. F (273K101
, 325 kPa) and for different carbon monoxide concentrations (CO~80-90 and 99% by volume): Vol. % CO CO2 N2 H20 HCNA~9
9 0.0067~1~O O. 0007
5B~99 0.0244~1~0 0.
00042A~90~ 5. ~ 5 0.00
4 0.00001B~90~ 5. ~50
,016 0.00002A~80~10,
~10 0.015~0.00001B~80~10
．． ~10 0.061~0.00001 As the temperature decreases, the equilibrium concentration of hydrogen cyanide increases, and the hydrogen cyanide concentration in the furnace gas decreases when the oxygen pressure is low (CO~99
% by volume), 0.012% by volume (for A) and 0.012% by volume (for A).
048% by volume (for B).

This indicates that the cyanide concentration in the furnace gas can be reduced by increasing the oxygen pressure in the furnace gas at high temperatures prior to scrubbing.

In the presence of potassium, cyanide is virtually completely converted to the form of potassium cyanide.

As the oxygen pressure decreases, cyanide gradually changes to the form of hydrogen cyanide, and at low temperatures and low oxygen pressures (coso%), cyanide exists entirely in the form of hydrogen cyanide.

If the furnace gas contains potassium, it can generally be said that the oxygen pressure has little effect on the presence of cyanide, ie cyanide is always present in the equilibrium mixture.

In scrubbing furnace gas, the cyanide concentration depends on the pH of the scrubbing solution as follows.

Reaction formula: CN(aq) +H+(aq)diHCN (g
), △GT.

The value of (J/mol) is approximately △GT0-15500-10
8T, which becomes .

where PHON-partial pressure of cyanide (bar), (C
N-) - cyanide concentration in the scrubbing aqueous solution (mol/13), T- scrubbing temperature (K).

At constant temperature, the cyanide concentration and pH of the scrubbing water
determines the partial pressure of hydrogen cyanide in the furnace gas after scrubbing.

In other words, the cyanide concentration in the output gas is controlled by controlling the pH during scrubbing to the same level as the partial pressure of cyanide in the input gas.

The lower the desired cyanide concentration in the scrubbing water, the lower the pF must be so that any amount of cyanide that enters the furnace gas passes completely through the scrubbing water.

This point will be explained using the following theoretical calculation.

The cyanide concentration is 300 mq/yrl (0'C,
1 0 1,325 kpa) (0.024 capacity%
, 2.4
If scrubbing is carried out at a temperature of 7°C (300°K), the total cyanide charge passes through the scrubbing water and the cyanide concentration in the scrubbing water is 101119/
l! If it is desired not to exceed (3.6 x 10-4 mol/l), the pH of the scrubbing water must in principle be adjusted as follows.

Reduce cyanide concentration in circulating scrubbing water to 100 Y/L
The pH may be 9.5.

Reduce cyanide concentration in circulating scrubbing water to intermediate levels (
cyanide concentration in the furnace gas and in the scrubbing water, if it is desired to maintain the cyanide concentration in the furnace gas and the scrubbing water at The partial pressure of hydrogen cyanide will be within the same range within the pH range of 7.5 to 9.5.

In this case, the amount of cyanide present in the furnace gas in the range of 30 to 3000 .mu./d will be passed to the scrubbing step (see FIG. 2).

The present invention will now be further described with reference to the accompanying drawings.

FIG. 1 shows a control system for the CN concentration in scrubbing water, in which furnace gas is fed into a gas scrubber 1 from a pipe line 2.

The pH of the scrubbing water is adjusted to such an extent that the pi of the scrubbing water taken out from the scrubber through the pipe line 5 is at a predetermined value.The circulating scrubbing water to which sulfuric acid has been added in advance in the mixing tank 4 is returned to the scrubber through the pipe line 3. It is adjusted by

The pH of the derived scrubbing water is measured in container 6.

The proportion of concentrated sulfuric acid to be pumped from vessel 9 through line 10 to mixing tank 4 is regulated by pH meter 7 and valve 8.

The water vaporized in the scrubber 1 and steamed off together with the discharged gas from the line 11 and the blower 19, as well as the water removed together with the solids from the thickener 14, is removed by adding water to the circulating scrubbing water from the line a12. .

The water leaving the scrubber 1 is passed from the pH measuring tank 6) to the pipe line 1.
3 The solids are then sent through line 16 to thickener 14, where the solids are settled.

This sedimentation is carried out by supplying a flocculant or a pH adjusting agent and/or a sedimentation reagent from line 15 into container 17.

Sedimented solid material and precipitated salts are removed from the circulation system by line 18 to a waste area, while clear liquid is recycled via line 19.

If desired, the degree of gas scrubbing can also be enhanced by connecting two or several scrubbers in series.

Next, an embodiment of the present invention will be described.

EXAMPLE Exhaust gas from a Hue mouth chrome furnace was scrubbed in a Venturi scrubber shown in FIG.

A pH 8.2 aqueous scrubbing solution was fed into the Venturi scrubber at a rate of 50-m''/hr.

The pH of this aqueous scrubbing solution was adjusted by adding sulfuric acid at a rate of 1.2 g/II.

As a result of the passage of cyanide into the scrubbing water during this scrubbing process, the cyanide concentration in the scrubbing aqueous solution after scrubbing was 5.1 to 1/l.

Figure 2 shows cyanide-containing furnace gases with various cyanide (HCN) concentrations shown on the horizontal axis and various pH values shown on the vertical axis.
This figure shows the cyanide concentration in the scrubbing aqueous solution when scrubbing was performed at 27°C using the scrubbing aqueous solution having the following.

From the graph shown in Fig. 2, for example, HC of 300η/,1
Furnace gas containing N was scrubbed with a pH 10 aqueous solution (
The scrubbing aqueous solution usually has a pH of 10-12)
It can be seen that when scrubbing is performed using a scrubbing solution, the cyanide concentration in the scrubbing aqueous solution after use exceeds 100 μ/l.

Aqueous scrubbing solutions with such cyanide concentrations are associated with sanitary hazards during their treatment, ie settling of solids, separation of P filters and filter cakes.

On the other hand, if the same furnace gas as described above is scrubbed with aqueous scrubbing solutions having a pH of about 8.5 and 7.5, respectively, the cyanide concentration in the aqueous scrubbing solution after use will be 10/ l and 1 mp/
It is 7.

These results demonstrate that the p of the scrubbing solution according to the present invention
H) f: Clearly shows the effect of adjustment.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of the apparatus used to carry out the method of the invention, and FIG. 2 shows the cyanide concentration in the scrubbing solution (~/l) and the pH of the scrubbing solution at a scrubbing temperature of 27°C and the furnace gas. Cyanide concentration in (
It is a graph shown as a function of ~/m゛). In Fig. 1, 1 is a scrubber, 4 is a mixing tank, and 7 is a scrubber.
indicates a pH meter, 14 indicates a thickener, and 19 indicates a blower.

Claims (1)

[Scope of Claims] 1. A cyanide-containing furnace gas generated from a melting furnace or a Huet chrome furnace is brought into contact with a circulating aqueous scrubbing solution, and after solid impurities and dissolved impurities are removed from the aqueous scrubbing solution, the aqueous scrubbing solution is returned to the scrubbing process. In the method for removing combustible components present in the furnace gas after scrubbing by combustion, the acidity of the circulating scrubbing aqueous solution is adjusted by adding an acid thereto;
characterized in that the migration of cyanide from the furnace gas into the scrubbing aqueous solution is thereby limited so that as large a proportion of cyanide as possible is removed during the combustion of the combustible components present in the furnace gas. A method for scrubbing cyanide-containing furnace gas generated from a melting furnace or ferrochrome furnace. 2. The method of claim 1, wherein the pH of the circulating scrubbing aqueous solution is adjusted to below 9.5, preferably below 7.5. 3. The cyanide-containing furnace gas generated from the melting furnace or Huekou chrome furnace is brought into contact with the circulating scrubbing aqueous solution, and after removing solid impurities and dissolved impurities from this scrubbing aqueous solution, the scrubbing aqueous solution is returned to the scrubbing process, while after the scrubbing A method for removing combustible components present in a furnace gas by combustion, the acidity of the circulating aqueous scrubbing solution being adjusted by adding an acid thereto;
thereby limiting the migration of cyanide from the furnace gas into the scrubbing aqueous solution so that as large a proportion of cyanide as possible is removed during the combustion of the combustible components present in the furnace gas; A method for scrubbing a cyanide-containing furnace gas originating from a melting furnace or a ferrochrome furnace, characterized in that the cyanide concentration therein is reduced by increasing the oxygen pressure at high temperature before scrubbing.