The invention relates to a process for the recovery of a heavy metal from a solution of a complex of the heavy metal and a complexing agent.

A complexing agent is understood to mean a compound, in particular an acid or salt, the anion of which is capable of forming a complex with the heavy metal.

Such a process is known from US-A-3,749,761, in which an aqueous solution of a polysulphide-mercury complex and sodium sulphide is treated with activated carbon, which adsorbs the mercury complex.

A drawback of this known process is that the mercury-containing carbon, as chemical waste, must be carefully stored and always remains a potential environmental hazard.

The object of this invention is to provide a process that lessens or obviates this drawback.

According to the invention, this object is achieved in that the solution is electrolysed in an electrolytic cell containing an anodic compartment and a cathodic compartment separated from the former by a membrane selectively permeable to cations, the solution being present in the cathodic compartment, and wherein the anion of the complexing agent and of the complex is a halogenide or a sulphide.

In this electrolysis the heavy metal is separated in the metallic form. The metal can be re-used, obviating the need for storage. Thus, no metal is lost and environmental pollution is avoided. This has the added advantage that no resources are lost.

A further advantage of the process according to the invention is that by using a cell with separate anodic and cathodic compartments the solution in the cathodic compartment can be kept free of substances not originally present therein, allowing this solution to be re-used as such. Again, this avoids the loss of resources and potential environmental pollution. At the same time the presence of the membrane prevents formation of harmful gases at the anode, because the cations that may potentially lead to such formation, for example Cl ^" -ions, cannot reach the anode.

The process according to the invention can be applied to any solution suitable for use as an electrolyte. Preferably, aqueous solutions are used. The process is essentially suitable for removing any metal present in complex form in a solution, but it is particularly suitable for removing heavy metals. In general, and in the context of this invention, heavy metals are understood to be those metals that occur in the waste or purification streams of chemical processes and have as a common characteristic the fact that they are more or less toxic to human beings and animals. Because of their toxicity, contact with the environment is to be avoided wherever possible. At the same time many heavy metals are relatively scarce and from an economic point of view their re-use is therefore desirable. Heavy metals which can be effectively removed are given in: "Electrochemical Methods, Fundamentals and Applications", page 382, by Bard and Faulkner (John Wiley & Sons, 1980). In particular, the category of heavy metals includes nickel, tin, lead, cadmium, arsenic, antimony, mercury and chromium.

The presence of such metals in the feed streams of chemical processes is often harmful. For example, heavy metals may poison certain catalysts and they may attack plant. For this reason it is frequently endeavoured to remove these heavy metals, particularly mercury and

arsenic, from such feed streams, for example cracker feed consisting of naphtha or gas condensate. To this end, use is made of an adsorbent or ion exchanger, for example. A process of this kind is described in US-A-4,950,408. To permit the adsorbent or ion exchanger to be re-used, whenever possible, the adsorbed metal is then removed by washing with a regeneration liquid that absorbs the heavy metal. Because of the generally poor solubility of heavy metal salts, a concentrated solution of a complexing agent is frequently employed for regeneration, with the heavy metal forming a stable anionic complex.

The process according to the invention is particularly suitable for removing heavy metals from such complex-containing regenerates. The heavy metal may be present in such a solution in a very wide range of con¬ centrations, from the ppb-level to a saturated solution. The complexing agent may likewise be present in the solution in a very wide range of concentrations. The higher the concentration of the complexing agent, though, the greater the quantity of metal that can be bound in complex form and here, too, the upper limit is determined by saturation of the solution with the complexing agent. In practice, concentrations of the heavy metal complex in regeneration solutions vary from 0.001-1 mole/litre and those of the complexing agent from 0.01-10 mole/litre, but the process according to the invention is also suitable for removing heavy metals from a solution down to less than the ppb-level.

To achieve separation of the heavy metal in the metallic form rather than separation of the cation of the complexing agent in the electrolysis step, the cation of the complexing agent should have a lower standard reduction potential than the heavy metal ion that is to be removed. A list of the value of the standard reduction potential of virtually all known cations can, for example, be found under the title 'Electrochemical Series' in Section D 151-160 of the 70th edition of the "Handbook of

Chemistry and Physics", published by CRC Press.

The complexing agent and metal-complex- containing solution are electrolysed in a cell in which the anodic and cathodic compartments are separated by a membrane selectively permeable to cations. Such a membrane is permeable to positively charged ions only and is essentially impermeable to negatively charged ions. The presence of such a membrane separating the anodic and cathodic compartments in an electrolytic cell prevents negative ions from reaching the anode and forming toxic products there. In this way the formation of harmful chlorine gas at the anode is avoided in cases where the solution in the cathodic compartment contains chloride ions. Such membranes are known per se and may be manufactured from perfluorosulphonic acid, for example.

The anions in the complex and in the complexing agent, which are preferably the same, should have a high solubility. According to the invention, a halogenide or, more preferably still, a sulphide is used as anion. The cation of the complexing agent should likewise have a high solubility. Preferably a complexing agent should be used in which the cation is ammonium or is derived from an alkali metal or alkaline earth metal. Because of the generally very high solubility of their salts, alkali metals, particularly sodium, are preferable. Moreover, the standard reduction potential of these metals is lower than that of most heavy metals.

The solution to be electrolysed is present in the cathodic compartment. In itself, the cathode material is not critical. Resistance to the solution environment is an important requirement, and on this basis one skilled in the art will know what materials are suitable for application. Preferably, from those materials suitable on the basis of their resistance, a material will be selected with a high overvoltage for H ₂ formation, in particular Hg, Pb and graphite. Iron or steel, tin, aluminium, platinum and nickel are also very suitable cathode

- 6 -

final concentration of the heavy metal also play a role. The required cell voltage can be readily determined by one skilled in the art by means of standard electrochemical procedures. A very suitable empirical process for determining the most appropriate cell voltage is based on the current-voltage diagram of the cell, hereafter referred to as the I-V-diagram, filled with the desired electrolytes. This I-V-diagram is determined by measuring the current through the cell with increasing cell voltage. Above a certain threshold voltage, below which no current flows through the cell, the current increases exponentially as a function of the voltage. In this range reduction of the heavy metal ion is all that takes place. As the cell voltage is increased further, at a given moment there is no further increase in the current, indicating that a saturation value has been reached. The relationship between I and V is thus in the form of an S- curve, ending with a plateau value for the current through the cell. If the cell voltage is increased still further, at a given moment there may again be an exponential increase in the current, indicating activation of another electrolytic process in addition to reduction of the heavy metal ion. As the standard reduction potentials of the two processes approximate one another more closely, the associated S-curves follow one another in ever quicker succession and it is even possible that a second process is activated before the saturation value of the first active process is reached. The cell voltage is now chosen to advantage such that competing side-reactions, electrolysis of water for example, cannot yet occur to any substantial degree, which would reduce the efficiency of the cell. Some minor degree of water electrolysis need not be unfavourable, however, because this causes the pH of the solution to rise, hindering further reduction of the water to H ₂ . The cell voltage is therefore preferably set at a value on the plateau of the I-V-curve for the reduction of the heavy metal ion, up to a maximum of 10%

- 5

materials. Because of its relatively low price, steel is very suitable for use in larger-scale electrolysis units. Preferably, use can be made of Hg as cathode material, either as metal as such or deposited on a cathode carrier material. In such a mode, the heavy metals present in the solution can amalgamate with the Hg. As a result hereof a higher removal-efficiency of those heavy metals is obtained. The amalgamate can be removed from the cathodic compartment for further treatment (like a destillative work-up).

In principle the electrolyte applied in the anodic compartment can be any, preferably aqueous, solution of a dilute acid, salt or base, and even water is suitable. In order that the solution containing the complexing agent retains its potential for re-use, however, preferably no cations are introduced into the system other than the cation of the complexing agent. The anode may consist of platinum, carbon in the form of graphite or glassy carbon, lead, polymeric sulphur nitrides or dimensionally stable anodes consisting of a metal oxide on a carrier metal. An example of this last category is ruthenium oxide on titanium. The environment in the anodic compartment is preferably basic and, more preferably, the pH is at least 13. In that case the total voltage required for electrolysis is lower than that necessary at a lower pH. At the same time in the event of damage to the membrane, or under other circumstances whereby the liquids from the anodic or cathodic compartments may unintentionally come into contact with one another, formation of gaseous acids such as HCl or H ₂ S can be avoided.

The voltage applied across the anode and cathode, hereafter referred to as the cell voltage, is chosen as a function of the cations present and the nature of the solution in the anodic compartment. The standard reduction potentials of the cations involved, the concentration of the complexing agent and the desired

higher. If this plateau does not occur, as a result of a second process having been activated earlier, the cell voltage is preferably set at a value deviating by no more than 10% from the initial voltage at which the second process is activated.

The heavy metal ions present in the cathodic compartment transfer their charge to the cathode. The metal thus formed is initially deposited on the cathode. If the heavy metal is mercury, as time progresses it may fall off the cathode under the influence of its own weight and collect at the bottom of the cathodic compartment. Metals deposited on the cathode can be removed by means of techniques known per se for separating metals from mixtures and alloys. Owing to the presence of the membrane that is selectively permeable to positive ions, the anions are prevented from reaching the anode and remain in the cathodic compartment, which consequently becomes electronegative relative to the anodic compartment. Under the influence of the potential difference thus created, the cations of the acids, bases or salts present in the anodic compartment migrate through the membrane to the cathodic compartment to restore electroneutrality.

If H ⁺ ions from water or an acid in the anodic compartment migrate to the cathodic compartment, the pH there will decrease. In a preferred embodiment of the process according to the invention the hydroxide of the cation of the complexing agent is then added to the cathodic compartment in a quantity sufficient to bind the H ⁺ ions. If desired this can be effected after completion of the electrolysis treatment, although addition in the course of the process is preferable. Following electrolysis of such a solution, the cathodic compartment thus contains no other ions than the initial solution. Even if this solution contains a low concentration of heavy metal ions, it can be re-used as a regeneration liquid for removing the heavy metal from, for example, an

ion exchanger. In essence, therefore, no resources are lost. In this embodiment the heavy metal need not be completely removed from the solution. This is advantageous because removal of the heavy metal proceeds more slowly as the concentration of the ions in the solution decreases. The final, most time-consuming part of the electrolysis process, and that with the lowest yield, can thus be omitted. As a rule it is unnecessary to remove more than 95% or even 90% of the heavy metal present in the solution or regeneration liquid at the start of the electrolysis. A solution thus essentially freed of the heavy metal can be used for washing out an object laden with heavy metals such as an ion exchanger. In the process of washing, an equilibrium is established at a given concentration of the complex in the solution and the residual content of the heavy metal in the object being washed out. In practice the relationship between this residual content and the associated concentration of the complex in the regeneration solution can be readily established, enabling one skilled in the art to establish the maximum permissible quantity of complex initially present in a regeneration solution in relation to the initial heavy metal content of the object to be washed out and the desired residual content after washing. If a salt or base of the cation of the complexing agent is present in the anodic compartment, the cations in question will migrate to the cathodic compartment, thus ensuring that after electrolysis the cathodic compartment contains only those ions also present in the initial solution.

The invention will now be illustrated with reference to the following examples. In these examples use is made of an electrolytic cell consisting of two half- cells each of 250 ml capacity separated by a perfluorosulphonic acid membrane (NAFION, Reg. Trademark of Du Pont de Nemours) with a surface area of 28 cm ² . For both the anode and the cathode a platinum electrode with a

surface area of 18 cm ² is used. Both half-cells are stirred with a magnetic stirrer.

Example I Into the anodic compartment of the electrolytic cell described above 200 ml of an aqueous 1M Na ₂ S0 ₄ solution is introduced. Into the cathodic compartment of the cell 200 ml of a 10% Na ₂ S solution is introduced in which 0.02 M HgS has been dissolved to form a HgS ₂ ^~ complex. This corresponds to a mercury content of approx. 4000 ppm in the solution.

The whole set-up is at room temperature. Across the anode and cathode a voltage of -3.1 volts is applied. From weighing the cathode before and after electrolysis and the quantity of mercury introduced into the cell it follows that after 10 hours 55% of the mercury present has been separated out from the solution in metallic form. If a voltage of -4.0 volts is applied it is found that after 10 hours 95% of the mercury introduced into the cell has been separated out from the solution. The solution remaining in the cathodic compartment is suitable for re-use as regeneration liquid for a mercury- containing ion exchanger.

Example II

The procedure of Example I is repeated except that 200 ml of an aqueous 1M NaOH solution is introduced into the anodic compartment. At an applied voltage of - 2.6, -2.8 and -3.0 volts the quantity of mercury recovered from the solution is 60, 82 and 97%, respectively.

Example III

An ion exchanger, type Imac SMI®, used for removing mercury from a gas condensate and containing 0.9 wt.% mercury, is regenerated by washing it with a 10% aqueous Na ₂ S solution. After washing, the solution is found to contain 320 ppm mercury. The procedure of

Example I is repeated using 200 ml of the mercury- containing solution thus obtained. After 12 hours' electrolysis at a cell voltage of -3.0 volts the mercury concentration has been reduced to 9.5 ppm. The above procedure is repeated with a second ion exchanger loaded to a similar level as the first being washed out with the Na ₂ S solution obtained after electrolysis and still containing 9.5 ppm mercury. In this repeat procedure essentially the same results are obtained as the first time, both in the regeneration step and in electrolysis.