Process for Purifying Metallurgical Gases Containing Sulphurous Anhydride by Extracting Mercury

Although mercury in its inorganic forms, especially in its elemental state, cannot be considered to be seriously toxic, the possibility that it can be transformed into methyl-mercury, toxicity of which is extremely high, makes it advisable to decrease the mercury content of the residual gases from the plants carrying out the metallurgical beneficiation of cinnabar.

Until now, the greater part of the efforts directed towards the purification of gases containing mercury has been applied to the treatment of the gases produced in the chlorine-alkali plants. With respect to the residual gases in metallurgical plants, where mercury is accompanied by a greater or lesser content of sulphurous anhydride (and even sulphuric acid), up to the present time we do not know of any other patented process except that of the Finnish firm Outokumpu-Oy.

The process of this invention consists of putting the gases proceeding from a plant for the pyro-metallurgical treatment of mercury-bearing ores into contact with a solution of a soluble thiocyanate and an acid, utilizing the sulphurous anhydride that accompanies the gases (in the absence of which sulphurous anhydride it will be necessary to add it) as an oxydising agent for the mercury.

The gases to be treated are those proceeding from the plant and in which, in addition to mercury (inany of its forms) the presence of sulphurous anhydride, sulphuric acid, water vapour, dust and any other compound of the mineral treated and of the fuel used for the calcination may be expected to exist.

Likewise, this process can be applied to any other gas containing mercury provided that it is accompanied by a given concentration of sulphurous anhydride.

Likewise, the process which is the subject of this invention, and under the conditions specified below, collects the mercury present in the gas, leaving the said gas with a detectable content of less than 5 mg of mercury per cubic meter of gas.

The result of this process will be improved, to the extent that there is a greater or better contact between the solution and the gas to be purified.

It has been observed that the temperature of the solution of soluble thiocyanate and acid that has to retain the mercury from the gases has little influence on the result of the process since it is possible to work with the solution between freezing and boiling points, it being most suitable to carry out the process at the atmospheric temperature of the plant, since this will make the process more economical.

Irrespective of the temperature at which the gas is introduced, the process with which we are concerned will take place, since if the temperature is higher than, or of the order of 100.degree.C, the corresponding vapourization of water will occur, and consequently the concentration of the solution that will have to be corrected; on the other hand, if the temperature of the gases is lower than that of the solution, it will have as a limit the freezing temperature of the solution. The most suitable temperature range for working is indicated as being between 10.degree. and 100.degree.C.

The range of concentration of the components of the solution is wide, the lower limit being 2 g/l of soluble thiocyanate and the upper limit being the saturation of the liquid with soluble thiocyanate at the temperature at which the process is to be carried out.

Likewise, for the acid, the concentrations are found between the lower limit of 2 g/l and the upper limit consisting of that concentration which does not affect the stability of the solution in the conditions under which the process is carried out.

The most suitable conditions for the carrying out of this process are: 150 g/l of soluble thiocyanate and 50 g/l of acid.

With respect to the concentration of sulphurous anhydride in the gas, the most suitable value lies between 1 per cent and 6 per cent in volume (m.sup.3 N of SO.sub.2 /m.sup.3 of gas). No improvement in yield in the recovery of mercury has been noted with values higher than 6 per cent of SO.sub.2, as will be indicated below in the examples.

As far as the concentration of mercury in the gases to be purified is concerned, this can have a very wide margin, which may range from 5 mg Hg/m.sup.3 of gas to about 20 g Hg/m.sup.3 of gas.

The mercury content of the gas after its treatment is lower than 5 mg Hg/m.sup.3 of gas, if the concentrations of sulphurous anhydride in the gas, on the one hand, and the concentrations of acid and thiocyanate in the solution, on the other hand, are the suitable concentrations indicated in this descriptive memorandum of the present invention.

The maximum concentration that mercury can reach in the solution that treats the gases will depend on the concentrations of thiocyanate and acid in the said solution, up to that value at which a slowing-down is observed in the process. A normal value for this concentration would be between the values comprised between 0 g Hg/l and 5 g Hg/l.

The recovery of mercury from this solution may be effected in various ways. By adding a solution of sodium sulphide the mercury is precipitated in the form of a sulfide.

Once part or all of the mercury retained in the solution has been recovered from it, the solution may be returned to the process, adjusting to the extent necessary, the concentrations of soluble sulphur cyanide and acid.

As the gases produced in the roasting of cinnabar always contain greater or lesser quantities of sulphurc acid, in principle, this acid has been spcifically selected for the tests on the solution that is going to be used for retaining the mercury encountered in the gases.

We explain below, by way of example, the most characteristic of a series of tests carried out.

EXAMPLE 1

A solution with 50 g/l of sulphuric acid and 150 g/l of potassium thiocyanate. Gases with 6 percent of sulphurous anhydride and mercury contents of between 8 mg/m.sup.3 and 1 g/m.sup.3. The residual gases have a mercury content lower than 5 mg/m.sup.3 of gas.

EXAMPLE 2

A solution with 50 g/l of sulphuric acid and 110 g/l of potassium thiocyanate. Gases with 6 percent sulphurous anhydride and 150 mg/m.sup.3 of mercury. The residual gases have a mercury content of less than 5 mg/m.sup.3.

EXAMPLE 3

When the concentration of potassium thiocyanate fell to 90 g/l, with concentrations of sulphurous anhydride of 3 per cent and 150 mg/m.sup.3 of mercury in the gases, the concentration of mercury in the residual gases was 6 mg/m.sup.3 : a value that rises to about 8.5 mg/m.sup.3 when the content of sulphurous anhydride in the gases falls to 1 percent.

In all the examples cited, for the contact between liquid and gas a column 3 meters in height full of 10 mm. Raschig rings was used and a mass ratio between the flows of gas and liquid in the column of the order of 5. When this ratio is increased, it has been observed that the content of mercury in the residual gas increases slightly, and likewise when the height of the column is decreased.

The temperature of the solution in the column in all the tests varied between 10.degree. and 40.degree.C.