EP0148023B1

EP0148023B1 - Process for the purification of mercury

Info

Publication number: EP0148023B1
Application number: EP84309076A
Authority: EP
Inventors: Giorgio Gavelli; Marino Gramondo; Gianni Donati; Giuseppe Faita; Gian Lorenzo Marziano
Original assignee: Montedipe SpA
Current assignee: Montedipe SpA
Priority date: 1983-12-30
Filing date: 1984-12-21
Publication date: 1989-03-15
Also published as: ES8707568A1; IT8324439A0; EP0148023A2; IT1170081B; DE3477205D1; EP0148023A3; ES539137A0

Description

The present invention relates to a process for the purification of mercury, coming from a NaCI or KCI electrolysis plant, and more particularly to the purification of the mercury of electrolytic cells of chlorine-sodium hydroxide and chlorine-potassium hydroxide plants.
In chlorine-sodium hydroxide and chlorine-potassium hydroxide plants, one of the main factors that influence the reliability and safety of the mercury amalgam process is the purity of the NaCl or KCI brine introduced into the cell. Impurities such as iron, calcium and magnesium are normally present in the sodium chloride in quantities varying from 0.01 to 0.3% by weight, while other heavy metals such as chromium, vanadium, molybdenum and magnesium are often present in quantities of the order of 0.01 ppm.
These impurities must be removed from the brine because they tend with time to accumulate in the mercury, where, if they exceed certain threshold concentrations, they catalyze the cathode development of hydrogen which, mixing with the chlorine developed at the anode, may give rise to explosions.
The impurities present in the mercury also have other adverse effects on the electrolytic process. There may, for instance, form an amalgam foam (called "mercury butter") which disturbs the regular flow of the mercury, as a result of which the cell voltage rises and there may occur short-circuits which will damage the anodes.
Moreover, the wettability between the bottom of the cell and the mercury is reduced, with consequential frequent rupture of the continuity of the amalgam layer, and consequential corrosion of the bottom which has remained uncovered.
The accumulation of impurities in the mercury also causes unbalances in the distribution of the current in the various longitudinal sections as well as cross sections of the cell.
In order to limit the introduction of impurities in the mercury, the brine, before its conveyance to the cells, is subjected to a costly process of chemical and physical purifications. Since there remain, however, possible accidental pollutions of the brine, the mercury tends nonetheless to grow rich in impurities, as a result of which it is necessary to frequently carry out periodical washings of the cells and purification of the mercury itself, by means of distillation.
It has been suggested to purify the mercury by electrolytic treatment after its dis-amalgamation. To this end the mercury, contacted with an aqueous acid solution, is anodically polarized, whereby the impurities dissolve in the aqueous solution.
This process involves, however, constructional complications in particular because the mercury which is connected with the negative pole in the electrolysis cell, must be connected to the positive pole in the purification apparatus.
US―A―3895938 discloses a process for the purification of mercury wherein mercury flowing out from an amalgam decomposer is turbulently admixed by a rotor with an aqueous acid solution. In such a process the mercury will be dispersed in the aqueous phase in the form of minute droplets and will be intensively mixed with the aqueous phase, whereby the impurities contained in the mercury will pass into the aqueous phase.
One aim of the present invention is to provide a simple, cheap and effective process for the purification of mercury from the metal impurities contained in it.
The present invention provides a process for the purification of mercury, coming from a NACI or KCI electrolysis plant and containing metal impurities, wherein the impure mercury is brought into contact downstream of the disamalgamation step with an aqueous solution containing an inorganic acid inside a purification reactor (1), whereby a liquid dispersion isformed therein, a gas being present in the upper portion (2) of said reactor, characterized in that a stream of said liquid dispersion is taken from the bottom of said reactor and recycled to said upper portion through a pump (11) and through an ejector (3, 4), from which ejector said stream is so ejected as to suck in said gas and redisperse mercury together with said gas in the form of very small droplets in the liquid dispersion contained in the reactor, whereby said metal impurities are transferred into said aqueous phase without addition from outside of any mercury salt.
The invention will be further described, by way of example only, with reference to the accompanying drawing, which is a schematic view of an apparatus suitable for carrying out the process of the invention.
The drawing shows a reactor 1 which contains in its upper part 2 a gas. Fixed to the upper end of this reactor is arranged a nozzle 3. Furthermore there is present a converging-diverging tube 4 which is so arranged with respect to the nozzle 3 as to form with this latter a liquid-gas ejector.
The converging-diverging tube 4 is maintained in the reactor 1 in the most suitable position with respect to the nozzle by means of any mechanical device suited for this purpose, for instance by means of fixing spokes fixed to the nozzle itself. The mechanical device is chosen in such a way as not to hinder the passage of the gas from the upper part 2 of the reactor to the inside of the converging-diverging tube 4 itself. The lower part of the tube 4 is immersed in the liquid phases present in the reactor.
Into the upper part 2 of the reactor are fed in the mercury to be purified via a line 5, and the aqueous solution via a line 6. Starting from the bottom 7 of the reactor, there is provided a recirculation line 8―9―10 which leads first to a pump 11 and thereafter to an optional heat exchanger 12 and returns back to the reactor, entering the nozzle 3. From the upper end of the reactor, the gas produced by the purification reaction (hydrogen) flows out through a line 13.
From the part 8 of the recirculation line, between the reactor 1 and the pump 11, there is a branch line 14, in which is installed a valve 15, which allows the discharge of the liquid phases.
The pump 11 ensures the circulation of the two liquid phases (the mercury and the aqueous solution) in the reactor and in the external recirculation line. The nozzle 3 causes an agitation (or stirring-up) of the liquid phases and the consequential dispersion of the mercury in the aqueous solution, in the form of droplets of small diameter.
The liquid-gas ejector sucks gas from the top of the reactor, still further increasing in this way the turbulence in the jet and in the reactor, and dispersing gas into the system.
This dispersed gas reduces the coalescence of the mercury particles and provides an extensive exchange surface for the removal of the gas produced by the reaction.
The aqueous solution is a mineral acid solution, for example a sulphuric acid solution containing from 0.5% to 10% by weight of H₂S0₄. In fact, in the presence of an acid solution, the impurity dissolving reaction proves to be faster. Using an acid solution, the chemical reaction that takes place is, for instance in the case of iron:
There may also be added to the acid solution an oxidizer, for instance H₂0₂.
The gas present in the reactor may be at atmospheric pressure; however, it may instead be at a reduced pressure; that is, the process may be operated under a vacuum, as a result the only gas present being the gas freed by the reaction (H₂). However, it is also possible to work at a pressure higher than atmospheric pressure. The pressure of the gaseous phase is for example from about 10 mmHg to about 5 atmospheres absolute.
When working at atmospheric pressure or above atmospheric pressure, the nature of the gas present in the reactor is not critical; there may be used various types of gases, for example air or nitrogen. The use of air is convenient in as much as it may help to dissolve the impurities because the oxygen contained in the air acts as an oxidiser.
The ratio between the volume of liquid phases sent back into the reactor each hour and the volume of the reactor itself is in general at least 10, more preferably from 50:1 to 150:1. The delivery pressure of the pump is in general at least 0.3 atmospheres, more preferably from 0.7 to 3 atmospheres.
The temperature at which the purification reaction takes place is usually from 20°C to the boiling temperature of the aqueous solution. The purification reaction obviously proceeds faster when the temperature increases, but at high temperatures, in an acid medium, there may arise problems of corrosion of the equipment.
It is useful and convenient to use a pump which will ensure a dispersion of the liquid phases in each other. The mercury droplets tend, in fact, to coalesce in the part 8 of the external recirculation line.
If the pump re-disperses the liquid phases, new mercury droplets form again. The reforming of the mercury droplets accelerates the purification reaction. In fact, if the size (dimensions) of the droplets remains stable in time, there will form concentration gradients and the diffusion of the impurities towards the mercury-water interface will slow down. On the contary, if the droplets re-coalesce and are formed again with a new surface, the negative effect of the concentration gradients will be appreciably reduced.
Pumps suitable for ensuring an effective dispersion of the liquid phases in each other are for example gear pumps and rotary pumps as well as particular types of centrifugal pumps.
The liquid phases may be discharged through the valve 15 which leads, for instance, to a tank where the purified mercury and the exhausted aqueous solution are separated by decanting.
When purifying the mercury of electrolytic cells, the mercury is subjected to the purifying process after its dis-amalgamation.
In the operation of the cells, the mercury purification system in which the process of the present invention is carried out may be coupled to the conventional purification of the brine, or the latter purification may be dispensed with, as a result of which the only purification carried out is that in accordance with the invention.
It is also possible to carry out a simplified purification of the brine, thus reducing the costs of this operation.
According to one specific embodiment of the process of the invention, the whole flow of mercury which flows out of the dis-amalgamator (disamalgamation apparatus), passes into the purification apparatus shown in the drawing, during all the time of operation of the electrolytic cell or at suitable time intervals. The mercury is separated by decanting from the aqueous phase and then flows back into the cell.
Instead of passing the whole flow of mercury into the purification apparatus, it is possible instead to pass only part of the flow either during the whole time of operation of the cell or at suitable time intervals.
In these two kinds of operation, the purification apparatus is provided with a circulating pump of its own. Thus, the plant will have two pumps, one for the circulation of the mercury in the cell, and one for the purification apparatus. There may instead be used in the plant only one pump which, besides circulating the mercury in the cell, will convey part or the whole of it to the purification apparatus. In this latter case, the mercury will pass only once through the purifying apparatus before being sent back into the cell.
The purification process of the present invention may also be used for purifying the mercury butters gathered during the washing of the cells.
The invention will be further described with reference to the following illustrative Examples.

Example 1

There was used the apparatus shown in the drawing, equipped with a liquid-gas ejector 3-4, a heat exchanger 12 and a gear pump 11.
The total volume of the reactor and of the recirculation circuit was about 5 litres. The process was conducted discontinuously, at room temperature.
There were introduced into the apparatus 5 litres of a 5% by weight solution of H₂SO₄ which was recirculated at a flow-rate of 500 litres/hour. The delivery pressure of the pump equalled 1.1 atmospheres.
Into the reactor were then introduced 500 g of mercury containing 20 ppm of sodium and 15 ppm of iron. After 30 seconds there was drawn a sample of mercury; the content of sodium was found to be less than 1 ppm while that of the iron was less than 3 ppm.

Example 2

There was used the same apparatus as in example 1 and there were followed the same procedures, except as otherwise specified.
At room temperature, there were treated 500 g of mercury containing 42 ppm of iron with 5 litres of an aqueous solution containing 5% by weight of H₂SO₄ and 0.1% by weight of H₂0₂. After less than one minute, the content of iron was found to be below 3 ppm.

Example 3

There was used the same apparatus as in example 1 and there were followed the same procedures, except as otherwise specified.
500 g of mercury, containing 430 ppm of iron, were treated at 70°C with five litres of an aqueous solution containing 2% by weight of H_ZSO₄. After one minute, the content of iron in the mercury had dropped to 10 ppm, and after a further 2 minutes it was found to be below 3 ppm.

Examples 4-7

There was used the same apparatus as in example 1 and there were followed the same procedures, except as otherwise specified.
The mercury to be treated came from end boxes (examples 4 and 5) and from feed boxes of electrolytic cells of a chlorine-sodium hydroxide plant.
500 g of mercury were treated at room temperature with 3 litres of an aqueous solution containing 2% by weight of H₂SO₄. The initial composition of the mercury and its composition after 5 and 10 minutes of treatment are given in the following table.

Claims

1. A process for the purification of mercury, coming from a NACI or KCI electrolysis plant and containing metal impurities, wherein the impure mercury is brought into contact downstream of the disamalgamation step with an aqueous solution containing an inorganic acid inside a purification reactor (1), whereby a liquid dispersion is formed therin, a gas being present in the upper portion (2) of said reactor, characterized in that a stream of said liquid dispersion is taken from the bottom of said reactor and recycled to said upper portion through a pump (11) and through an ejector (3, 4), from which ejector said stream is so ejected as to suck in said gas and redisperse mercury together with said gas in the form of very small droplets in the liquid dispersion contained in the reactor, whereby said metal impurities are transferred into said aqueous phase without addition from outside of any mercury salt.

2. A process as claimed in claim 1, characterized in that said gas is an oxidising gas, preferably air.

3. A process as claimed in claim 1 or 2, characterized in that said aqueous solution contains an oxidizing agent, preferably hydrogen peroxide.

4. A process as claimed in any of claims 1 to 3, characterized in that the space velocity is at least 10m³/h of recycled liquid per m³ of reactor, and in that the delivery pressure of said pump is at least 0.3, preferably from 0.7 to 2, atmospheres.

5. A process as claimed in claim 4, characterized in that said space velocity is from 50 to 150 h-¹ (m³/h per _m3).

6. A process as claimed in any of claims 1 to 5, characterized in that the purification temperature is from 20°C to the boiling point of said aqueous solution.

7. A process as claimed in any of claims 1 to 6, characterized in that said mineral acid is aqueous sulphuric acid, the concentration of which is from 0.5 to 10% by weight.