Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Reexamination Certificate
2002-12-16
2004-10-05
Lawrence, Frank M. (Department: 1724)
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
C210S748080, C210S243000, C204S555000
Reexamination Certificate
active
06800206
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to water treatment and relates particularly, though not exclusively, to treating polluted water using electroflocculation and/or electrocoagulation reactions.
BACKGROUND OF THE INVENTION
The ready availability of clean water is a key requirement for health and economic prosperity. Accordingly, the treatment of polluted water assumes a new importance with increasing population and consequent demands on fresh water supplies.
Electrolysis-based water treatment techniques represent one kind of method for treating polluted water. In this respect there are three different types of electrolysis-based water treatment processes:
(i) electroflotation;
(ii) electroflocculation; and
(iii) electrocoagulation.
Electroflotation involves the use of inert electrodes in conjunction with a coagulating agent which has been added to the polluted water to be treated. A voltage is applied to the electrodes, and liberated gas bubbles capture coagulated particles, floating them to the surface. This is similar to a process known as dissolved air flotation (DAF) for treating industrially polluted water—except that the gas bubbles are supplied by electrolysis of the water and not by compressed air. The process has been commercially available since the mid 1970s, and is generally described by Dr Anselm Kuhn in
Electroflotation—the technology and waste water applications
, Chemical Processing, IPC Industrial Press Ltd, London.
Electroflocculation uses sacrificial electrodes to generate the coagulating agent, but also uses the bubbles liberated at the electrodes to float the contaminants to the surface. A few electroflocculation processes have been attempted before but they have not been successful for various reasons. These limitations include:
the clogging of the electrodes before the metal has been adequately sacrificed; and
the requirement to purpose build each unit for a particular application because of inability of a single set of electrodes to handle a wide array of water conductivities.
Electrode clogging considerably increases the cost of the treatment process due to the high cost of electrode replacement, typically making the process prohibitively expensive. The latter limitation has meant that it is necessary to tailor existing electroflocculation systems for particular water qualities. This has not been found to be a practical solution for commercial viability.
Electrocoagulation also involves the use of sacrificial electrodes to generate the coagulating agent—usually aluminium or iron ions. Once the water has been treated, it is either filtered, allowed to settle or sent to a gas or air flotation unit to remove the contaminants. Electrocoagulation process offers a number of potential advantages, if they can be realised. However, as with electroflocculation, electrocoagulation similarly suffers from problems of electrode clogging and the need for purpose building each system to the particular water to be cleaned.
The electrocoagulation process has been tried for many years but has not been made to work satisfactorily. For example, U.S. Pat. No. 4,872,959 to Herbst et al describes a tubular system in which the water passes between inner, outer and central electrodes, and is designed to add flocculating ions to settle the pollutants to the bottom of the settling tank into which the water will be passed. It uses iron and aluminium anodes, though suggests other metals as well, for example copper cathode, iron anode. It is not very successful because the electrodes have a tendency to clog. Further, the system is designed for specific conductivities of the water, and controlling the dose of coagulating ions required for a particular reaction is not easy.
U.S. Pat. No. 5,372,690 to Gardner-Clayson et al acknowledges that replacement of electrodes is a big problem and uses metallic, preferably aluminium, steel (alloy) or magnesium as method of overcoming the problem. Uses a layer of metal balls, shot, irregular shaped particles to be consumable, instead of sheet. Current is passed from metal anode to these balls etc., which then pass cations into the water and the process repeats itself. It has the problem that large voltages are required to drive the electric current between the anode and cathode. Also there is no guarantee that the metal particles in this shape are well suited for the passage of an electric current. Also, it does not address the problems of variations in electrical conductivity of the water.
U.S. Pat. No. 5,558,755 to Gardner-Clayson et al is based upon a continuation application of the patent noted directly above. The discussed apparatus includes a fluidised bed of metallic particles through which the medium is flowed and through which an electric current is applied by electrodes for agglomerating contaminants in the medium. In order to allow the electrodes to be non-consumable so that they do not require frequent replacement, the particles are consumable.
In both electrocoagulation and electroflocculation, there has been a tendency to use electrodes of the same material, for example, an aluminium anode and an aluminium cathode. In this case, a significant problem is that the electrodes have clogged because of the build up of an oxide type layer across the surface of the electrodes. Aluminium is deliberately oxidised in a process known as anodising, and it is generally thought that the aluminium went into solution at the anode. As a result, it has been considered difficult to overcome this problem of aluminium going into solution at the anode without forming an oxide layer at the anode.
In essence, existing techniques generally suffer from one or more limitations, such as:
The electrodes continually clog up, long before the metal had been adequately sacrificed into solution. This meant that the metal electrodes must be replaced at a high cost, making the process uneconomic.
Variations in the conductivity of the water meant that the process was often difficult to control.
Variations in the amount of pollutant present meant that installations had to be configured for a particular water quality. This again made commercial viability difficult.
It is an object of the present invention to at least attempt to address one or more of these and other limitations associated with existing techniques.
SUMMARY OF THE INVENTION
It has been recognised that aluminium goes into solution at the cathode as well as at the anode, and that the reaction rate at the cathode is around 2.5 to 3 times faster than at the anode. The cathode is also the electrode at which hydrogen gas is liberated, generating hydroxide groups and raising the pH of the water. Aluminium is an amphoteric metal and reacts with alkaline radicals (OH
−
) as well as with acidic conditions (H
+
). At the cathode, OH
−
reacts with water and aluminium to form an insoluble AL(OH)
3
, which adheres to the cathode, while at the anode, aluminium only goes into solution without forming an oxide layer.
The invention recognises that a water treatment process can be advantageously provided by using the electroflocculation/electrocoagulation principle, in which two electrodes are used in combination. A voltage is applied across the electrodes, with a positive voltage being applied to a sacrificial anode and a negative voltage applied to an inert cathode. This voltage can be either direct current (DC) or rectified alternating current.
The polarity of the electrodes can be periodically reversed to mitigate surface clogging of the aluminium electrode. Reversing the polarity causes the material attracted to one polarity to be driven off by repelling the material which was attracted. Repulsion can occur in a short time interval compared to that which results in clogging due to electric attraction. Reversing the polarity for small periods of time, between a broad duty cycle range of 0.1% and 30% of the time, reduces the build up of electrically deposited material, thus extending effective electrode lifetime.
Using a predetermined amount of charge as an indication of the degree to which the
Ladas & Parry
Lawrence Frank M.
Ozcent PTY Ltd.
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