EP0452178A1 - Process for preparing mercury from calomel - Google Patents

Abstract

Process and apparatus for the production of mercury by reduction of calomel by using a process for the preparation of metallic mercury. <??>This plant comprises:   - a reactor 2 with a sloping bottom 4 for the reduction, provided with a stirrer 3, connected via a pipeline 5 to the said phase separator 1 and provided with means for feeding water 7 and sulphuric acid 8; and   - a mercury recovery vessel 11 connected to the bottom part of the reactor. <IMAGE>

Description

The subject of the invention is a process for the production of metallic mercury from calomel, an installation serving for the implementation of such a process, as well as the mercury metal obtained.

We know that the mercury contained in sulphide ores, such as blende or pyrites, is volatilized during roasting of these ores and that it is found in its elementary form in the sulfurous gases from roasting. Sulfur gases are then used for the manufacture of sulfuric acid, it will contain the mercury thus entrained by volatilization, which will then be found in fertilizers, or other products entering the food chain, in the manufacture of which between l sulfuric acid thus produced. Therefore, the presence of mercury in sulfur gases is undesirable and its content must be limited in the roasting gases.

The most widely used process for washing gases is that known under the name of "Boliden-Norzink process", which is the subject, for example, of US patents 3,849,537 and 4,233,274 and described in a large number of publications. . According to this process, the gas to be purified is washed in a tower where a reaction takes place between the metallic mercury of the gas and the mercuric ions Hg⁺⁺ in solution in the liquid washing phase; this produces a very poorly soluble mercury compound Hg₂Cl₂. The calomel thus produced can be decanted and extracted from the system. This process allowing a purification of 99.9% gives satisfactory results, but it has the disadvantage of giving rise to the production of calomel, whose outlet is extremely limited and which, in addition, because of its volatility, is not an ideal material for further processing in conventional mercury production units. In addition, the storage of calomel is limited in quantity by regulations.

American patent US Pat. No. 3,039,865 describes a process for recovering metallic mercury from saline solutions, such as calomel previously cited, using a reducing metal, such as iron. However, the concentrations of the solutions treated are low and this process requires a treatment step with a base, for example sodium hydroxide, in order to make the initially acidic pH alkaline.

The object of the present invention is, by a simple process, which can be directly integrated into an ore roasting process, to produce mercury metal of purity greater than 99.999%, the market of which presents appreciable outlets and which has the advantage of be able to be stored in a small volume with suitable packaging.

The present invention relates to a process of the aforementioned type in which the preparation of metallic mercury from calomel Hg₂Cl₂, is characterized in that the calomel in acidic aqueous solution, preferably in the form of a suspension, is reduced by the action of a powder of a reducing metal which does not form an amalgam with the mercury and in which the mercury formed precipitates.

According to an object of the present invention, the pH of the solution is maintained at a value of the order of 0.5 by addition of sulfuric acid.

According to another object of the present invention, the suspension is agitated during the reduction operation.

According to another object of the present invention, the reducing metal powder is an iron powder and the quantity of reducing iron used varies between 1.5 and 1.7 times the stoichiometric quantity corresponding to that of the reaction: $$\text{2 Hg⁺ + Fe → 2 Hg + Fe⁺⁺}$$

The conditions for a good process yield include the quality and quantity of the metal powder to be used, as well as the operating conditions to respect, in particular the acidity, which lead to a correct coalescence of the mercury formed and avoid the formation of residue or amalgam or foam at the start of the reaction.

According to yet another object of the present invention, the stirring time is at least 8 hours.

The power of the stirring and its duration must be carefully determined, taking into account the suspension used and the size of the industrial plant, since the effectiveness of such stirring is a preponderant condition for the proper functioning of the process.

The present invention also relates to a process for the preparation of metallic mercury from Hg₂Cl₂, characterized in that the calomel is in suspension in an acidic aqueous solution, in that the reducing metal is added in the form of powder, in that the suspension is stirred and left to stand to allow the mercury to coalesce and precipitate.

The present invention also relates to a process characterized in that an aqueous solution containing the precipitate of mercury is covered with a film of nitric acid, in that compressed air is bubbled through this solution, then in that the mercury thus treated is collected then passed through a film of nitric acid in water or an acid solution and, finally, caused to trickle, in the finely divided state, into a column with filled baffle diluted nitric acid before being collected in a purified state. The nitric acid used is preferably 20% nitric acid.

• un réacteur à fond incliné pour la réduction muni d'un agitateur et relié par une canalisation audit décanteur et pourvu de moyens d'alimentation en eau et en acide sulfurique; et
• une cuve de récupération du mercure, reliée à la partie basse du réacteur.

The present invention further relates to an installation for the production of mercury by reduction of calomel by implementing the method described above comprising:

• an inclined bottom reactor for reduction provided with an agitator and connected by a pipe to said decanter and provided with means for supplying water and sulfuric acid; and
• a mercury recovery tank, connected to the lower part of the reactor.

• un premier bac muni de moyens pour l'introduction d'air comprimé de barbotage;
• un deuxième bac relié au premier par une conduite d'alimentation; et
• une colonne de ruissellement munie de chicanes, pourvue au sommet de moyens d'introduction du mercure finement divisé en provenance du deuxième bas et pourvue au fond d'une conduite de soutirage du mercure lavé et dont le compartiment du bas rempli de mercure sert de garde hydraulique.

According to one embodiment of the above-mentioned installation, it also comprises:

• a first tank provided with means for the introduction of compressed bubbling air;
• a second tank connected to the first by a supply line; and
• a runoff column provided with baffles, provided at the top with means for introducing the finely divided mercury coming from the second bottom and provided at the bottom with a pipe for withdrawing the washed mercury and whose bottom compartment filled with mercury serves as a guard hydraulic.

As already mentioned, the process according to the invention can constitute a phase occurring after the roasting operation of the ores, followed by a decantation phase of the washing solution of the roasting gases; the decanted calomel is directly introduced into the reduction reactor.

• la figure 1 montre une installation pour la réduction du calomel; et
• la figure 2 montre une installation pour la purification du mercure obtenu dans l'installation précédente.

Other objects, advantages and characteristics will appear on reading the description of an embodiment of an installation for the reduction and purification of mercury according to the invention, given without limitation and with reference to the appended drawing, or:

• Figure 1 shows an installation for the reduction of calomel; and
• Figure 2 shows an installation for the purification of mercury obtained in the previous installation.

The installation for producing mercury metal or mercury in metallic form shown diagrammatically in FIG. 1 is associated with a decanter 1 and comprises a reactor 2 provided with an agitator 3 and an inclined bottom 4. The reactor has inputs d supply of calomel at 5, metal powder at 6 and, respectively, sulfuric acid and water or acid solution at 7 and 8.

Calomel from a gas washing installation, for example according to the Boliden-Norzink process, is extracted from the settling tank 1 and is introduced directly into the agitated reactor 2, maintained under vacuum.

After preliminary stirring, a representative sample of the suspension is taken to allow the amount of metal powder to be introduced to be calculated.

The pH of the solution is maintained at around 0.5 by injection of sulfuric acid and the stirring is continued for 8 hours, after which the stirring is stopped in order to allow the mercury to coalesce and decant. After decantation, the mercury is extracted at the bottom of the tank at 9 to be stored in the container 11, while the supernatant solution is drawn off at 10 to be neutralized by lime.

The mercury extracted from the reactor at 9 can be purified by washing with nitric acid in a washing installation, an embodiment of which is shown diagrammatically in FIG. 2.

The washing installation comprises a first bubbling tank 20 filled with water or an acid solution, the surface of which is covered with a film 23 of nitric acid. A compressed air line 22 terminating in a bubbling ramp is immersed in the tank.

The first tank 20 is connected by a pipe to a second tank 30 filled with water or an acid solution, the surface of which is covered with a film of nitric acid 31.

The tank 30 is followed by a runoff column 40 with cascade or baffles 41, the bottom compartment 42 of which remains filled with mercury to serve as a hydraulic guard.

A container 50 is used to store the purified mercury.

After having introduced the raw mercury into the tank 20, via line 21, the compressed air is bubbled through to thoroughly wash the mercury and rid it of impurities such as residues or amalgams. Then, the mercury from the first tank is introduced into the second tank 30 after passage through the nitric acid film 31.

Finally, the finely divided mercury from the tank 30 is introduced at the top of the baffled runoff column 40, filled with nitric acid. Mercury accumulating in the lower compartment 42 is gradually withdrawn to be stored in the container 50.

The mercury thus purified reaches a purity greater than 99.999%.

The following examples are given by way of illustration and in no way limit the present invention.

EXAMPLE 1

The manipulation is carried out in a glass laboratory reactor having a total volume of 2.2 liters. Calomel is a sample taken from a roasting gas treatment plant and is in the form of a white mud.

The various products and reagents are introduced at ambient temperature (i.e. 19 ° C) and the masses used are the following:

The reaction medium is kept under stirring, so that the calomel is in suspension in the reaction medium. Stirring is continued for 8 hours.

At the end of the reaction and 30 minutes of decantation, the operating conditions are as follows:

The mass of mercury collected at the bottom and drawn off is equal to 230.4 g. The mercury obtained is washed with nitric acid. At the end of this treatment, a qualitative spectrographic analysis, followed by a quantitative analysis by PKI of the elements detected is carried out. The results obtained on washed mercury and on unwashed mercury are as follows:

EXAMPLE 2

The operation is carried out in a pilot reactor having a volume equal to 500 1. The stirring of the reaction medium is carried out by means of a two-stage turbine, the tank being moreover equipped with four counter-blades. The maximum power of the engine used is 5 kW and stirring is sufficient to keep the calomel in suspension in the reaction medium. The reactor also has a flat bottom inclined so as to facilitate the recovery of mercury, thanks to a low point pipe provided with a sampling valve.

The calomel is introduced as sampled at the gas treatment plant for the roasting, that is to say in the form of a sludge containing calomel, water and traces of sulfuric acid. Its temperature is 16 ° C.

The other compounds are added at a temperature of 19 ° C. and the masses used are the following:

After 10 hours of reaction and 12 hours of decantation, the operating conditions are as follows:

The mass of mercury withdrawn is equal to 89.7 kg. Mercury is washed with nitric acid.

The results of the analysis of the elements most likely to be present in the washed mercury are as follows:

EXAMPLE 3

The operation is carried out in the industrial installation, the reactor having a volume equal to 1000 liters. The stirring of the reaction medium is sufficient to keep the calomel in suspension in the reaction medium.

The reactor also has a flat bottom inclined so as to facilitate the recovery of mercury, thanks to a low point pipe provided with a sampling valve.

The calomel is introduced directly from the decanter of the gas treatment plant of the roasting, that is to say in the form of a sludge containing calomel, water and traces of sulfuric acid.

The masses of the other compounds added at ambient temperature are as follows:

After 10 hours of reaction and 12 hours of decantation, the operating conditions are as follows:

The mass of mercury withdrawn is equal to 119 kg. The mercury is washed with nitric acid, according to the procedure described.

The results of the analysis of the elements most likely to be present in the washed mercury are as follows:

The minimum purity of the mercury thus obtained is 99.995%.

Claims (12)

1.- Process for the preparation of metallic mercury from calomel Hg₂Cl₂, characterized in that the calomel, in acidic aqueous solution, preferably in the form of a suspension, is reduced by the action of a powder of a reducing metal which does not form of amalgam with the mercury and in that the mercury formed precipitates. 2.- Method according to claim 1, characterized in that the pH of the solution is maintained at a value of the order of 0.5 by addition of sulfuric acid. 3.- Method according to claim 1 or 2, characterized in that the suspension is stirred during the reduction operation. 4.- Method according to claim 1 or 3, characterized in that the reducing metal powder is an iron powder. 5.- Method according to any one of claims 1 to 4, characterized in that the amount of reducing iron used varies between 1.5 and 1.7 times the stoichiometric amount corresponding to that of the reaction: $$\text{2 Hg⁺ + Fe ――> 2 Hg + Fe⁺⁺}$$

6.- Method according to any one of claims 1 to 5, characterized in that the stirring time is at least 8 hours. 7.- A method of preparing metallic mercury from Hg₂Cl₂, characterized in that the calomel is in suspension in an acidic aqueous solution, in that the reducing metal is added in the form of powder, in that the suspension, in that it is left to stand to allow coalescence and precipitation of mercury. 8.- Method according to any one of claims 1 to 7, characterized in that the aqueous solution containing the precipitate of mercury is covered with a film of nitric acid, in that is bubbled with compressed air in this solution, then in that the mercury thus treated is collected then passed through a film of nitric acid in water or an acid solution and, finally, caused to trickle, in the finely divided state, into a column with baffle filled diluted nitric acid before being collected in a purified state. 9.- Method according to claim 8, characterized in that the nitric acid used is 20% nitric acid. 10.- Installation for the production of mercury by reduction of calomel by implementing the method according to one of claims 1 to 9, characterized in that it comprises: - a reactor (2) with inclined bottom (4) for reduction provided with agitator (3), connected by a pipe (5) to said settling tank (1) and provided with means for supplying water (7) and acid sulfuric (8); and - a mercury recovery tank (11), connected to the lower part of the reactor. 11.- Installation according to claim 10, characterized in that it further comprises: - A first tank (20) provided with means (22) for the introduction of compressed bubbling air; - a second tank (30) connected to the first (20) by a supply line; and - a runoff column (40) provided with baffles (41), provided at the top with means for introducing the finely divided mercury coming from the second tank and provided at the bottom with a pipe for withdrawing the washed mercury and whose compartment of the bottom (42) filled with mercury serves as a hydraulic guard. 12.- Mercury obtained by implementing the method according to one of claims 1 to 9.

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