THE ECF TECHNOLOGY (ElectroCoagulationFlotation)ElectroCoagulationFlotation process is based on the electrolytic treatment of an aqueous solution, in which a series of chemical and electrochemical reactions are triggered, associated with physical processes capable of favoring the transformation and subsequent removal of contaminants. The geometry of the cell and that of the electrodes, their number, the material of which they are composed and the sequence of placement depend on the particular characteristics of the solution to be treated and therefore these are specifically calculated and designed according to each individual case as well as the system of power supply (RDD).
THE PROCESSThe process develops continuously in an electrolytic cell in which some metal plates acting as anode or cathode are placed and in which the solution to be treated continuously flows. The metals used for the electrodes are usually aluminum or iron for the anode (more rarely titanium or zinc) and iron for the cathode. The electrodes are connected to a direct current generator with the anode acting as a positive pole and the cathode as a negative pole. In the electrolytic cell, the potential difference produced by the current generator produces an electric field. The passage of electric current causes the anode to release (dissolving) part of the metal: in the case of the aluminum anode, the atoms of the metal pass into solution as aluminum ions and hydrate. Similarly, ferrous ions oxidize and dissolve:
The behavior of aluminum ions and ferrous ions is very similar, so from this point on we will assume the aluminum ions as representative of the behavior of the ions produced at the anode (Al and / or Fe). The solvation mechanism is not exactly known in all details, but the final result, generally accepted, is that the Aluminum ions are surrounded by a group of hydroxyl ions, in turn surrounded by an outermost layer of water molecules of solvation. These Aluminum ions, pushed by the electric field, move towards the cathode, always surrounded by hydroxyl ions. The latter are instead attracted to the anode and continually jump away from the electronic cloud covering the aluminum ion to 'land' on another aluminum ion (encountered in the movement towards the anode), being immediately replaced by other hydroxyl ions coming from the cathode . These moving ions are the carriers of the electrons through the solution and are, ultimately, the real players in the electrical conductivity of the solution. Often the hydroxyl ions already present in the water are not sufficient to hydrate all the aluminum ions that are oxidized and which tend to pass into solution. In this case, water molecules are also oxidized on the anode, in which the oxygen atoms release electrons to the electrode releasing hydroxonium cations, forming the oxygen molecules which develop in form of gaseous bubbles:
At first, the oxygen bubbles and hydrated aluminum ions cover the anode surface and, acting as a shield to the passage of other hydroxyl ions (and / or other water molecules) hinder the subsequent dissolution of other aluminum atoms. In this way the reaction would tend to stop, but since the voltage is sufficiently high, aluminum atoms and hydroxonium ions, even if hydrated, begin to move towards the cathode, pushed by the electric field. Other water molecules can therefore approach the anode to yield electrons to it. At the opposite electrode (cathode), the electrons made available in large quantities (by the generator) are captured by the hydroxonium cations in which the hydrogen is reduced and develops in gaseous form, creating a barrier of bubbles adhered to the surface of the cathode. In this case too, in absence of H3O+, the water molecules that release the hydrogen contained are directly reduced, releasing hydroxyl ions according to the following reaction:
In this way, a current of negatively charged ions is formed, which move from the cathode towards the anode, investing in their movement all the substances contained in the solution, both those in solution (hydrated) and those in suspension, solid or liquid (colloids). Similarly, the hydrated aluminum atoms move from the anode, which generates them, towards the cathode that attracts them, also coming into contact with all the various particles contained in the solution. Even on the cathode, the bubbles produced create a barrier in contact with water and positive ions (hydroxonium ions and metal ions) and would tend to stop the reaction. But since the potential difference (the voltage) is sufficiently high, the water molecules still manage to overcome the bubble wall and reach the electrode, receiving the electrons and releasing gaseous hydrogen and hydroxyl ions. Therefore, the hydrogen bubbles merge, growing in size and, detaching from the cathode, begin their upward movement (towards the surface). In an electrolytic cell, usually the positive ions are also reduced at the cathode, especially the metallic ones (hexavalent chromium, manganese, copper, etc.), which, pushed by the difference in potential, move in the solution towards the cathode, on which they are discharged:
In our cell, on the other hand, the solvated metal atoms (i.e. surrounded by the cloud of solvation water molecules), in their movement towards the cathode, are intercepted by the hydrogen bubbles produced by the cathode, are incorporated into the foam and pushed upwards (flotation). Therefore, pollutants are not usually deposited on the cathode of our cells. We observe that, to pass current through the solution, it is not sufficient to apply a potential difference. When electrons accumulate at the cathode, ready to enter solution, in order to travel to the anode they must find a carrier atom (or ion) on which they are able to bind and on which they are transported in the solution towards the cathode. In pure water (pH = 7) both hydroxonium ions and hydroxyls are in very small quantities (10 to minus 7) and electrons do not find carriers, so the current does not flow and the reaction practically does not develop. On the contrary, in the polluted waters to be treated, the abundant presence of pollutants usually provides consistent quantities of positive and negative ions that act as carriers for the electrical charges and the reaction develops with sustained yields. In fact, it is a common experience for us who operate in electroflotation, that in waters that are not very loaded with pollutants, the reaction develops with difficulty, while in waters that are very full of dissolved substances (i.e. solvated) the reaction proceeds smoothly and the substances are removed. In reality, a small part of electrons still flows in the solution (which behaves like a conductor), but this contribution to the electrical conductivity is so small that it is negligible for practical effects.
It is therefore the ions, (especially those of aluminum) that flow through the wastewater which, by investing both the substances in solution and those in the colloidal state (i.e. in suspension or emulsion), that destabilize the surface charge, approaching the point where Van der Waals forces begin to be effective and, by binding, create clots. These then begin to grow and form flakes which progressively compact and begin to settle (precipitation). The mechanism described above is not yet well known and there are even more doubts. In any case, it is generally accepted that the destabilization of suspensions and emulsions is the determining factor that allows the growth of the flakes (floculation) and the formation of small 'solid' formations (more precisely colloidal) within the solution. The growth of the flakes, however, is not such as to lead in a short time to such dimensions as to allow a fast decantation (i.e. separation of the solids from the liquid). It is known that, for a solid colloidal particle suspended in a liquid, the force of gravity is often secondary to other forces (solvation, Brownian, etc.), due to its small size. In the case of a colloidal particle, therefore, the decantation speed is often negligible even in the presence of considerable density differences and this leads to too low separation speeds (of the clots and flakes from the liquid) and the time needed for decantation is too long and economically unacceptable. In the case of electroflotation, on the other hand, the separation mechanism (of clots and flakes) that prevails is the dragging upwards by the hydrogen and oxygen bubbles which, in their ascent, meet, capture and drag towards the surface all the micro particles present in the solution and therefore these clarify, in a short time, the treated water. Furthermore, the action of the oxygen bubbles produces a violent oxidation that quickly degrades most of the oxidizable substances (especially organic) then reducing the COD (and especially the BOD). One of the accelerating factors is the very high reaction surface, given that the bubbles generated in situ (i.e. those that detach from the electrodes) have initial dimensions of a few tens of nanometers and their specific surface (and therefore the reaction rate) is millions of times greater than that of bubbles with a diameter of one millimeter. In conclusion, in the reaction compartment (the electrolytic cell) an upward current of bubbles is created. That drags the pollutants upwards to form a consistent foam, much more compact than the foams that are usually found on the surface of water in systems of dissolved air flotation. The gas contained in the bubbles (of the foam) is initially composed of oxygen and hydrogen. After a few minutes, the hydrogen escapes (migrates through the walls of the foam bubbles), while the oxygen remains contained for a few hours, thus keeping the foam floating and therefore allowing separation by skimming and subsequent filtration. In conclusion, we observe that the electrolysis of water plays a great role in the electroflotation process. It requires a potential difference greater than that necessary for the dissolution of the metal of the anode, but produces the bubbles of hydrogen and oxygen, necessary both for the flotation (both hydrogen and oxygen) and for the degradation by oxidation (only oxygen) of many of the pollutants contained in the water to be treated. |
THE MAIN RESULTS
Wastewater polluted by hydrocarbons
Wastewater after ECF treatment
Closed vessel ECF plant
Open tanks ECF plant
ECF plant placed in container
ECF overview in service of a drilling site
Combined vessel + tank ECF plant
Interior of a containerized plant
ECF tank detail
Detail of ECF tank in operation
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PRACTICAL APPLICATIONS
The first practical applications of electroflotation were developed towards the end of the 1800s, but the first uses in (purifying) water treatment date back to the 90s of the last century. They were intended to remove metals and hydrocarbons from wastewater. Today we apply electro-flotation successfully in wastewater treatment, especially when heavy metals have to be broken down, emulsions to be broken (by reducing oils and hydrocarbons), suspended solids do be intercepted, reducing COD and BOD, in addition to often significantly reduction of most of the other pollutants.
A PROJECT OF A TREATMENT PLANT
During the electrocoagulation treatment of a wastewater, tens, sometimes hundreds, of competing reactions usually take place in reaction cells. All these reactions are in equilibrium in the (unique) liquid phase of the reaction module, with equilibria determined by the respective redox potentials, concentrations, temperature, activation energies, electrode overpotentials, etc. The current supplied by generators (in practice the electron flow), is therefore splitted in the various possible uses (hydrolysis of water, reduction of cations, development of aluminum ions, etc.) as a function of all the reaction parameters, which are always different from wastewater to wastewater. Therefore, when you are faced with a wastewater to be treated with electro-flotation, it is first of all necessary to conduct an initial analysis of the type of pollutant concentrations to be abated, assuming the reactions that are supposed to take place and calculating the reaction parameters (the specific current to be supplied, the necessary voltage, the time of residence, etc). However, it is good to admit that it is not possible to predict at the table, on the basis of the documentary analysis alone, the yield to equilibrium (of the abatement reactions, the development of oxygen and consequent oxidation, etc.). It is therefore necessary to verify the hypotheses with an experimental practical test. On the basis of the experimental results it will then be possible to predict with good approximation the abatement yields of the various pollutants obtainable with an industrial electro-flotation plant for a given wastewater to be treated.
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Via Dante Severgnini 2 | 24043 Caravaggio (BG) | +39 392 4174000
Company information | Legal Notes | Copyright | Cookies | Privacy | Events | Partita Iva 09238950969