GB2043611A - Process for scrubbing cyanide- bearing furnace gases produced in the metallurgical industry - Google Patents

Abstract

It has been a problem that when these gases are scrubbed to remove other contaminants, the cyanide enters the scrubbing solution and thereby creates problems of subsequent safe treatment of the scrubbing solution. The solution to this problem put forward by the present invention involves making the scrubbing solution more acid than usual, typically so as to prevent the cyanide ion concentration in the solution from exceeding 100 mg/litre, and then burning the scrubbed gas to remove the cyanide.

Description

SPECIFICATION A process for scrubbing cyanide-bearing furnace gases which are produced in the metallurgical industry The present invention relates to a process for scrubbing cyanide-bearing furnace gases produced in the metallurgical industry, by contacting these cyanide-bearing furnace gases with a circulating scrubbing water solution and by removing solid and dissolved impurities from the scrubbing water solution before it is returned to the scrubbing. This invention relates in particular to a process in which the combustible constituents of the scrubbed furnace gases are eliminated by burning.

It is characteristic of both smelting furnaces used for steel production and closed ferrochromium furnaces that, under the conditions prevailing in these processes, gases are generated which, in addition to carbon monoxide, carbon dioxide and hydrogen, also contain cyanides, primarily alkali metal cyanides. Since the.

furnace gas also contains a considerable amount of solid matter, it is necessary to purify the gas before its use or further treatment. In general, wet scrubbers, e.g. venturi scrubbers, in which the solids and water-soluble gases present in the gas are scrubbed with water, are used for the purification of the gases.

Consequently, the scrubbing water solution contains, in addition to solids, cyanide which pollutes the environment. Various attempts have been made to reduce the cyanide content in the scrubbing water solution.

According to one prior known process the acidity of the cyanide solution is raised to approx. pH 10-11, whereafter the cyanide is oxidized with a suitable oxidizing agent, e.g. chlorine, hypochlorite, hydrogen peroxide, persulfuric acid or ozone. As a result of the oxidation, cyanide is converted either to cyanate or, if the oxidation is carried out until complete, to nitrogen and water.

According to another process known per se, the cyanide is converted to a form less hazardous to the environment by precipitating it by means of ferrous salts, either as Berlin blue or as Berlin white. The former gradually decomposes under alkaline conditions to cyanide, whereas precipitation as Berlin white is not quantitatively complete and consequently a relatively high amount of residual cyanide is left in the cyanide solution.

Other processes for the treatment of cyanide-bearing scrubbing water solutions include the catalytic oxidation, in gas form, of the cyanides, using a platinum/rhodium catalyst or activated carbon. The oxidation can also be performed by electrolysing concentrated cyanide solutions, in which case the final concentration is usually approx. 1 g cyanide/liter.

The disadvantages of the prior known process mentioned above include high cost of chemicals and high investment, a complicated process, or the unsuitability of the process for the treatment of dilute cyanide solutions, which are produced especially in the scrubbing of furnace gases. Furthermore, a characteristic common to these known processes is that they primarily relate to the removal of cyanide which is already in water.

The object of the present invention is to eliminate the disadvantages involved in the prior known processes mentioned above and to provide a process for the scrubbing of cyanide-bearing gases generated in the metallurgical industry, wherein the equilibrium between the furnace gases and the scrubbing water solution is controlled so as to maintain the concentration of cyanide in the scrubbing water solution at such a low level that no separate cyanide removal from the scrubbing water solution is necessary. In the process according to the invention, as large a proportion of the cyanide as possible is retained in the furnace gases, which are burned after the scrubbing, whereby the cyanides decompose to less hazardous constituents.

According to the invention, the cyanide equilibrium between the furnace gases and the scrubbing water solution is controlled by adding acid to the circulating scrubbing water solution in order to adjust the pH of the scrubbing water solution to such a low level that as large a portion of the cyanide as possible or at least a major portion is eliminated during the burning of the combustible constituents of the furnace gases. When this process is employed, the apparatus used for carrying out the process is simple, the investment costs are low and, furthermore, the operating costs are low. The furnace gases contain such a large quantity of combustible constituents that it is profitable to exploit its heat content by burning the furnace gases after the scrubbing.Thereby the cyanide present in the furnace gas decomposes to combustion products which are far less hazardous.

The pH of the circulating scrubbing water solution is preferably adjusted to a value of 9.5 at maximum, e.g.

to 7.5 at maximum. Since efforts are made to maintain the pH at a level lower than usual, various impurities, e.g. zinc salts, dissolve and concentrate in the scrubbing water. The concentration of impurities in the circulating scrubbing water solution can be controlled by removing a portion of the scrubbing water from circulation and by replacing it with fresh water. The impurities are precipitated out from the scrubbing water solution removed from circulation by raising its pH value. Since the concentration of cyanide in the scrubbing water solution is low, the solution can be treated without risk in order to remove impurities from it.

Balance calculations of a furnace gas which contains carbon dioxide, carbon monoxide and nitrogen indicate that, in the presence of water vapor, cyanide is present in the form of hydrogen cyanide, the partial pressure of which decreases when the temperature and the oxygen pressure increase. This offers a possibility for decreasing the concentration of cyanide in the gases by increasing the oxygen pressure in these gases at a high temperature before the scrubbing.

In the presence of potassium, the cyanide is in practice completely in the form of potassium cyanide. When the oxygen pressure decreases, cyanide is increasingly in the form of hydrogen cyanide, and at a low temperature and a low oxygen pressure (CO 80%), the bulk of the cyanide is present in the form of hydrogen cyanide. If the gases contain potassium, it can be noted in general that the oxygen pressure has little effect on the presence of cyanide, i.e. cyanide is always present in the equilibrium mixture.

In the scrubbing of furnace gases, the concentration of cyanide depends on the pH of the scrubbing water solution as follows: From the reaction equation CN- (aq) = H + (aq) = HCN (g) we obtain: aGTO to15500 - 108 T cace in Joules/mol, T is K) and therefore 810 log PHCN t T - pH = log [CN-] + 5.62 where PHCN = partial pressure of cyanide (bar) [ CN- ] = concentration of cyanide in scrubbing water (mol/1) T = scrubbing temperature (K) At a constant temperature the cyanide level and pH of the scrubbing water determine the partial pressure of hydrogen cyanide in the scrubbed furnace gas.In other words, the concentration of cyanide in the output gas is adjusted by controlling the pH of the scrubbing water, to the same value as the partial pressure of cyanide in the entering gas. The lower the level of cyanide desired in the scrubbing water, the lower the pH must be in order that a certain cyanide load entering in the furnace gas pass entirely through the scrubbing.

The following theoretical example will illustrate the point: If a furnace gas having a cyanide concentration of 300 mg/m3 (00C, 101.325 kPa) (0.024% by vol., 2.4 x 10-4 bar), is scrubbed at a temperature of T = 300 K (27 C) and it is desired that the entire cyanide content of the gas remains substantially unchanged through the scrubbing, so that the concentration of cyanide in the scrubbing water will just not surpass 10 mg/l (more precisely 3.6 x 10#4 mol/l), the pH of the scrubbing water must in principle be adjusted as follows:: pH = log [CN-] - log PHCN + T + 5.62 + 810 pH = log (3.6 x 10-4) - log (2.4 x 10-4) + 300 + 5.62 300 pH = 8.50 If the cyanide level in the scrubbing water in circulation is allowed to increase to 100 mg/l, the pH may be 9.5. The concentration of cyanide in the furnace gas and the partial pressure of hydrogen cyanide in the scrubbing water will be within the same range within a pH range of 7.5-9.5, if it is desirable to maintain the cyanide level in the circulating scrubbing water at a moderate level (~10 mg/l), in which case the treatment of the circulating scrubbing water, for example in the separation of solids, will not constitute a health hazard.

In this case the cyanide load present in the furnace gas, within the range of 30-3000 mg/m3, would pass through the scrubbing process; Figure 2.

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which Figure 1 depicts a schematic side elevation of an apparatus intended for carrying out the process according to the invention, and Figure 2 depicts a diagram showing the concentration of cyanide in the scrubbing water solution, in mg/l, as a function of the pH of the scrubbing water solution and the concentration of cyanide in the furnace gas (mg/m3) at a scrubbing temperature of 27 C.

In Figure 1, which depicts the control system for the CN level of the scrubbing water, numeral 1 indicates the gas scrubber, into which the gas from the furnace is directed through the pipe 2. The pH of the scrubbing water is adjusted by returning to the scrubber, through pipe 3, circulating scrubbing water to which sulfuric acid has been added in the mixing tank 4 to such a degree that the pH of the scrubbing water removed from the scrubber through the pipe 5 is at the set value. The pH of the outlet scrubbing water is measured in the container 6. The rate of concentrated sulfuric acid to be pumped from the container 9 through the pipe 10 into the mixing tank 4 is controlled by means of the pH-meter 7 and the valve 8.The water vaporizing in the scrubber 1 and removed as vapor together with the outlet gas through the pipe 11 and the blower 19 and the water removed from the thickener 14 together with the solid are removed by adding water to the circulating scrubbing water through the pipe 12. The water leaving the scrubber 1 is directed from the pH-measuring tank 6 through the pipe 13 further through the pipe 16 to the thickener 14, where the solid is settled. The settling is carried out by feeding through the pipe 15 into the container 17 a flocculating agent or pH-control and/or precipitating chemicals. The settled solid material and the precipitated salts are removed from circulation through the pipe 18 to a waste disposal area and the clear solution is recirculated through the pipe 19. When necessary, the gas scrubbing can be itensified by coupling two or several scrubbers in a series.

Example Outlet gases from a ferrochromium furnace were scrubbed in the venturi scrubber depicted in Figure 1.

Scrubbing water solution having a pH of 8.2 was fed into the venturi scrubber 1 at a rate of 50 m3/h. The pH was controlled by adding sulfuric acid to the scrubbing water solution at a rate of 1.2 g/l. During the scrubbing, cyanide passed into the scrubbing water with the result that after the scrubbing the concentration of cyanide in the scrubbing water solution was 5.1 mg/l.

Claims (8)

1. A process for the scrubbing of cyanide-bearing furnace gases generated in the metallurgical industry, by contacting the cyanide-bearing furnace gases with a circulating aqueous scrubbing liquid and by removing solid and dissolved impurities from the scrubbing liquid before it is returned to the scrubbing, the combustible constituents present in the scrubbed furnace gases being eliminated by burning, characterized in that in order to restrict the passing of cyanide from the furnace gases into the scrubbing liquid, the pH of the circulating scrubbing liquid is controlled by adding acid to it.

2. A process for the removal of cyanide from cyanide-bearing furnace gases generated in the metallurgical industry, which process comprises contacting the cyanide-bearing furnace gases with a scrubbing liquid having a pH low enough to prevent the concentration of cyanide ions in the scrubbing liquid from exceeding 100 mg/litre, burning the combustible constituents present in the scrubbed furnace gases, removing at least part of the solid and/or dissolved impurities from the scrubbing liquid and re-circulating at least part of the purified scrubbing liquid.

3. A process for the removal of cyanide from cyanide-bearing furnace gases generated in the metallurgical industry, which process comprises contacting the cyanide-bearing furnace gases with a scrubbing liquid having a pH of not more than 9.5, burning the combustible constituents present in the scrubbed furnace gases, removing at least part of the solid and/or dissolved impurities from the scrubbing liquid and re-circulating at least part of the purified scrubbing liquid.

4. A process according to claim 2 wherein the pH of the scrubbing liquid is low enough to prevent the concentration of cyanide ions in the scrubbing liquid from exceeding 10 mg/litre.

5. A process according to claim 3 wherein the pH of the scrubbing liquid is not more than 7.5.

6. A process according to claim 3, 4 or 5, wherein the concentration of cyanide in the gases is decreased at a high temperature by increasing the oxygen pressure in the gases before the gases are contacted with the scrubbing liquid.

7. A process according to claim 1 carried out substantially as herein before described with reference to the accompanying drawings.

8. A process according to claim 1 substantially as described in the Example.