NL9500269A - Electrochemical removal of metals from an aqueous waste stream. - Google Patents

Description

ELECTROCHEMICAL REMOVAL OF METALS FROM AN AQUEOUS WASTE STREAM

The present invention relates to a method of electrochemically removing metals, especially heavy metals, including mercury, from an aqueous waste stream, especially when the metals are present therein in low concentrations.

The outflows of industrial operations usually contain certain amounts of metals. Such metals, which are considered a major environmental hazard, should be removed from said streams. That is why a large number of processes have been developed in the past ten years to tackle this problem.

Of these metals, especially mercury, a lot of attention is paid, even if it is present in very small quantities.

It is known from the literature that such currents can be treated in electrolysis units. Highly selective removal of certain metals, such as mercury, has been described as having been found to be highly toxic if released into the environment. In the article by G.H. Kelsall, "Fluidized bed electrodes", Chem. Eng., Feb. 1983, pp. 63-64, for example, the mercury content of a mercury containing stream is reduced to less than 2 ppb (parts per billion) of mercury by depositing the mercury on lead particles that form a fluidized bed in the cathode compartment of an electrolysis -unit. At low current densities, a current efficiency of up to 90% is achieved for mercury.

In the article by G. van der Heiden et al., "Fluidized bed electrolysis for removal or recovery of metals from dilute solutions", Chem. and Ind., July 1978, pp. 465-468, describes how ionic as well as metallic mercury is removed to no more than 50 ppb.

A method of purifying mercury-containing gases while simultaneously recovering mercury in metallic form is described in US 4,640,751. As a product of waste streams from thematic processes, such as drying, calcination and roasting, the waste gases produced often contain small amounts of mercury contained in the processed raw materials and which have become volatile during processing. In order to purify such waste gases, they are washed in a washing tower or passed through a venturi and cooled. Calomel is obtained by the reaction of metallic mercury and mercury ions. After the calomel has been oxidized with chlorine, the mercury chloride thus obtained is electrolyzed so that mercury precipitates on the cathode in the metallic form. Although this is a reliable method of mercury recovery, it does require an intermediate oxidation step, with extensive construction of tubing and monitoring equipment.

Furthermore, no metals other than mercury are removed in the same electrolysis version. Since the said processes are directed to mercury removal under operating conditions that provide economically attractive flow efficiency, a fairly low current density is used.

An important object of the present invention is therefore to remove as much as possible metals, in particular heavy metals, including mercury, from aqueous waste streams.

Another object of the invention is to remove metals, including mercury, from aqueous waste streams by electrolysis.

According to the present invention, the aforementioned method therefore comprises the removal of metals from an aqueous waste stream, among which metals is at least mercury, which method comprises the following steps: passing the waste stream into an electrolysis unit containing anode and cathode supplies and as electrolyte contains at least the waste stream, the cathode having a large specific surface area.

electrolyzing the electrolyte such that hydrogen gas develops at the cathode and changing the electrolyte after the mercury concentration is significantly reduced, removing mercury in the form of a metal deposit on the cathode and turning the remaining metals into powdered hydroxides knocked down.

In a preferred embodiment of the invention, the pH increases from acid to alkaline, preferably from less than 4 to more than 8, more preferably from less than 3 to more than 9, measured near the cathode. The pH can rise from 1 to a maximum of 13, measured near the cathode, advantageously removing about 95% (m / m) of the metals present in the waste stream.

Preferably, the electrolysis unit used comprises a large surface cathode, with a specific surface area of 40 to 20 · 10 µm · * ·, in a packed or fluidized bed. By using such electrodes, current densities can advantageously be set and controlled very accurately.

In another preferred embodiment of the present invention, the cathode is an amalgam cathode; rather the bed is formed by lead particles.

The invention will now be described in more detail and the method according to the invention is explained by means of an example.

In the field of electrochemical removal of metals, in particular heavy metals, in particular mercury, from aqueous solutions, cathodes forming amalgam, for example copper and copper cathodes, may be used for electrolysis units or electrolysis cells. lead. Such specific use is highly dependent on the type of metal to be removed and the method by which such a cell functions electrically.

First, the cathodic limit of the above-mentioned cathode metals, being the electric power at which hydrogen gas develops at a given electric current and which is often referred to as overvoltage, must be compared with the normal voltage of the metals to be removed. It will be apparent to those skilled in the art that a higher current efficiency can only be achieved if the span of the cathode metal used for deposition is not exceeded. For the aforementioned copper and lead, the hydrogen span is -0.24 V and -0.69 V, respectively. Thus, the largest electrochemical working range and thus the lowest achievable mercury level for the removal of mercury is achieved when using a lead cathode.

Second, careful consideration should be given to the incorporation of the metal to be removed into the cathode metal. In the present case, it is to be avoided as much as possible that mercury droplets are formed which can disperse in the flow of the electrolyte. Mercury-lead alloys with a mercury content of up to 31% (m / m) form a solid single phase at room temperature.

In view of the foregoing, lead is the preferred cathode material when removal of especially mercury is contemplated.

Electrolysis using a current density greater than the limiting current for lead produces hydrogen at the cathode generating an excess of hydroxyl ions. As a result, the pH at the cathode rises, and as a result, other metals in solution which have lower solubility at the higher pH value precipitate in the form of the corresponding hydroxides. Therefore, in the present invention, for the deposition of mercury on lead, a current density greater than and more preferably much greater than the limit current is advantageously used.

Depending on the type of current and the charge used, the pH value can rise from very acidic to very alkaline, for example in the range of 1-13.

In a preferred embodiment of the invention, the pH of the electrolyte rises to a value at which the solubility of the hydroxides of the heavy metal is lowest (generally 8-12), advantageously about 95% (m / m) of the heavy metals present in the waste stream are removed. Preferably, the electrolysis unit used comprises a large surface cathode, with a specific surface area of more than 1,103 m -1, preferably from 40 to 20,103 m -1, in a packed or fluidized bed.

The present invention preferably uses a two compartment cell separated by a porous membrane or an ion selective membrane, as known in this art. In that case, the anolyte circuit, the anode material and the anode reaction can be freely selected.

The development of oxygen from a sodium sulfate solution is particularly suitable by using a dimensionally stable anode (dsa, dimensionally stable anode), as is known in the art.

For example, waste streams containing heavy metals such as Fe, Mg, Mo, Nb, Ta, V, W and Zn can be processed. In principle, all metals which yield insoluble hydroxides and which do not fall into the group of the alkali metals can be advantageously removed by the method of the present invention.

The process of the present invention has been found to be particularly suitable for treating waste streams with metal concentrations of not more than 1000 ppm (parts by weight in parts) in total, including a mercury concentration of not more than 50 ppm. Especially mercury, both metallic and ionic mercury, with a valence of 1 and 2, can advantageously be removed to a residual value of at most 10 ppb.

The (waste) stream can be treated continuously through a single passage through the bed or a series of beds or through multiple passages through the bed or beds and with a predetermined return and drain. The flow can also be treated batchwise. Suitable electrolysis times are in the range of 0.01-1 hours.

The invention is now illustrated by the following Example.

EXAMPLE

A stream from cooling gases in a venturi containing a total of 300 ppb of mercury (ionic and metallic), as well as other metals, as indicated in the first column of Table 1 below, was electrolyzed using a flow-through system in which a packed bed of lead shot with a diameter of 1.0-1.18 mm served as a cathode. The anode and cathode compartments that formed the electrolysis cell were separated from each other by a porous polypropylene membrane with an area of 80 cm 2. A 0.5 M sodium sulfate solution was used as the anolyte. The anode consisted of a lead plate. The packed lead shot bed placed in the cathode compartment had the following dimensions: deep, 3 cm, wide, 8 cm, high, 11.5 cm. Power was supplied to the bed through a copper current feeder with a surface area of 70 cm 2. The cooling water temperature was kept at 50 ° C. The flow of liquid passed through the cathode consisted of 4 l of cooling water to be treated and was circulated through the bed at a flow rate of 50 l / h. The total volume of the cell was 0.35 l. The specific surface area of the packed bed was 2.5.10 µm.

Table 1

* mercury content in ppb + magnesium content outside measuring range (max. 25 ppm)

In a first test, a current density of 63 A / m2, based on the membrane surface, was supplied to the cell. Hydrogen evolution was clearly visible on the cathode bed, indicating that the current density exceeded the limit current for mercury deposition. In 10 minutes the mercury level decreased from 300 ppb to <5 ppb. At the same time as the mercury precipitation, hydrogen developed and as a result of the pH increase near the cathode bed, a not very adherent precipitate could be observed in the bed.

A second test was performed with a sample of the same composition at a current density of 13 A / m2. Again, hydrogen evolution was observed and the mercury content dropped to 25 ppb in 20 minutes and further decreased to <5 ppb after 90 minutes.

In a third test run on a similar sample, the current density was 125 A / m2. The mercury content decreased to <5 ppb at a flow rate of 50 l / h, that is, with a single pass through the bed. Hydrogen developed in the packed bed and a precipitate was formed at the same time.

All three tests were performed at a cell voltage of 3.5 V.

The cell operating conditions used in the experiment are listed in Table 2 below, while the results in Table 1 above are given as removal efficiency.

Table 2

* for mercury precipitation

From the above results, it will be clear that according to the method of the present invention an advantageous reduction of the metal contents in a waste stream is obtained, both with regard to mercury and other metals.

Furthermore, the precipitate can be removed from the bed by intermittent fluidization. After prolonged electrolysis, the pH of the cooling water rose from 5.8 to 12. However, such a large increase in pH is not necessary for the precipitation of the other metals. Due to the hydrogen development, the pH in the vicinity of the electrode is high and precipitation occurs even though there is hardly any change in the pH in the majority of the water sample. Redissolving of the precipitated hydroxides proceeds slowly and therefore they can still be removed, even at relatively low pH values.

Claims (10)

A method for removing metals from an aqueous waste stream, including at least mercury metals, which method comprises the following steps: passing the waste stream into an electrolysis unit containing anode and cathode supplies and, as an electrolyte, at least waste stream, where the cathode has a large surface area, electrolyzing the electrolyte such that hydrogen gas develops at the cathode and changing the electrolyte after the mercury concentration has decreased significantly, removing mercury in the form of a metal deposit on the cathode and where the remaining metals are precipitated as powdered hydroxides. The method of claim 1, wherein the specific surface area is in the range of from 40 to 20 · 10 µm -1. The method of claim 1 or 2, wherein the cathode device is a cathode that forms an amalgam. The method according to any of the preceding claims, wherein the electrolysis unit consists of an anode compartment and a cathode compartment which are separated from each other by means of a membrane or a selective membrane. The method of any preceding claim, wherein the cathode supply is a packed bed of electrically conductive Pb particles. The method of any one of claims 1-4, wherein the cathode supply is a fluidized bed of electrically conductive Pb particles. A method according to any one of the preceding claims, wherein during the electrolysis of the electrolyte the pH increases from 1 to at most 13. The method of any preceding claim, wherein the metals removed from the aqueous waste stream additionally contain at least one of the metals Fe, Mg, Mo, Nb, Ta, V, W, and Zn. The method of any preceding claim, wherein the metals have a total element concentration of no more than 1000 ppm, including a mercury concentration of no more than 50 ppm. A method according to any preceding claim, wherein the mercury concentration is reduced to a maximum of 10 ppb.