Electro-Chemical Method In An Enrichment Process
The invention relates to a method that inhibits the formation of a deposition layer on metal surfaces in liquid submersion in a process susceptible to deposition, e.g. in an enrichment process. In the method, anodic current is supplied to the metal surface (11) to be protected against deposition via an external power source (100) and a cathode (12) immersed in the liquid, producing in the composition of the deposition adhering to the metal surface a change causing the adherence of the deposition to the surface (11) to be substantially weakened or the formation of deposition on a clean surface (11) to be substantially retarded.
The present invention relates to a method as defined in the preamble of claim 1.
Certain ore enrichment processes comprise dissolution of solid matter under high temperature and pressure conditions and a subsequent crystallization stage, from which the desired pure concentrate is obtained. For example, in the Bayer process generally used in the production of aluminum oxide, the ore is dissolved in a hot alkaline solution under high pressure. A problem with these processes is fast formation of deposition on the surfaces of process equipment. Deposition is typically formed on the interior walls of dissolution reactors, concentrators and crystallizers because they are at a temperature lower than in the rest of the process: The solution close to the cooler surface becomes over-saturated with respect to a solids particle and the formation of deposition begins.
In the Bayer process, the hard deposition formed on the surfaces mostly contains gibsite and various silicate compounds and is very difficult to remove. Due to the formation of deposition, pipes and valves get blocked, the efficiency of heat exchangers is reduced and the volume of tanks and containers and consequently the production capacity are reduced. For example, the volume of a crystallizer may be reduced by as much as one third, which in a crystallization process based on dwell time increases the delivery cycle time in a corresponding proportion, reducing the production quantities.
Periodic cleaning shutdowns are required, and therefore a stand-by reserve of process equipment is needed; some of the equipment is always excluded from production due to cleaning. Thus, the highest costs result from production losses during shutdowns. Coatings more easily cleanable have been developed for process equipment. In addition, to restrain the formation of deposition, large quantities of different chemicals for preventing deposition are supplied into the process. Electro-chemical deposition inhibiting methods have also been proposed; for example, patent CA2495957 describes an application of cathodic polarization.
The object of the present invention is to achieve an improvement in currently known deposition preventing methods used in mining industry. A more particular object of the invention is to create a method that will make it possible to alter the structure of especially the deposition formed in a Bayer process so as to prevent or considerably reduce its adherence to the surface of process equipment. A further object of the invention is to create a method that will retard the formation of deposition so that the length of the period between cleaning shutdowns can be substantially increased. In addition, the objects of the invention have to be achieved using an electro-chemical treatment that will not impair the corrosion resistance of the structural material of the process equipment to be protected against deposition.
The objects of the invention are accomplished by a method characterized by the features presented in the characterization part of claim 1.
The method of the invention is based on the insight of using the effects resulting from the electro-chemical reactions taking place on and near the surface to alter the structure of the deposition so as to weaken its adherence to the surface.
In a preferred embodiment of the invention, the surface of a process device in a process susceptible to deposition is polarized in the anodic direction by using an external power source and a current supply electrode placed in the solution so that the electro-chemical reaction products thus produced alter the structure of the deposition present or accumulating on the surface in such a way that the adherence of the deposition to the surface is substantially weakened. The change in the potential of the metal surface in the anodic direction caused by the anodic polarization is adapted to be such that the metal surface of the process device will not suffer corrosion damage due to the raised potential. The highest anodic potentials that can be used are determined by commonly known corrosion analysis methods and they depend on the properties of the electrolyte, the properties of the metal and the reference electrode used. Therefore, real-time measurement of the electro-chemical potential of the process device to be protected against deposition is important in respect of risk-free application of the method.
In the method of the invention, the current/voltage supplied by the power source is controlled on the basis of an external measurement signal and/or an internal measurement signal. Alternatively, the current/voltage can be controlled on the basis of a control signal obtained from a data file.
In a preferred embodiment of the method of the invention, the current/voltage produced by the power source is controlled by means of a timed control signal so that the current/voltage is changed at desired time intervals t1, t2, . . . , tn to a pre-desired magnitude.
In a second preferred embodiment of the invention, the surface of a process device in a process susceptible to deposition is polarized in the anodic direction by using an external power source and a current supply electrode placed in the solution so that the potential of the surface of the process device is brought to a range where the electro-chemical reactions provoked prevent the formation of deposition or substantially retard it. In this case, too, the potential has to be in a range where the structural material of the process device will not undergo corrosion damage.
In the method of the invention, it is possible to use anodic direct current, pulsed anodic direct current or a combination of an alternating-current component and an anodic direct-current component to produce the desired electro-chemical reactions.
In the system of the invention, the surface of the process device to be protected need not be treated or coated with any special coating or a different kind of metal. The system of the invention is operated by supplying anodic current through the oxide layer that is spontaneously created in alkaline conditions on the surface of the commonest structural material, carbon steel, used in process equipment. The oxide layer is an important factor contributing to the corrosion resistance of carbon steel, so eliminating it e.g. from a cathode by using polarization is hazardous.
Possible places where the system of the invention can be used include the inner walls, overflow chutes and process pipes of a process device, such as a precipitator, clarifier, crystallizer or reactor, in a process susceptible to deposition. Clogging of valves can also be obviated by the system of the invention. The current supply electrodes are mounted inside the process device so that they are in contact with the electrolyte and galvanically isolated from the process device to be protected.
Reference electrodes monitoring the intensity of anodic polarization are mounted near the surface to be protected so that they are in contact with the electrolyte and galvanically isolated from the process device to be protected. The material of the reference electrodes can be selected from metals that can withstand the conditions prevailing in the process and retain their characteristic potential at a level sufficiently constant with respect to reliability of the potential measurement. The choice of the reference electrode material does not affect the working principle of the system of the invention.
The method of the invention can be applied directly by installing in existing structures a system functioning according to the invention. If necessary, new structures can also be developed to allow application of the method of the invention.
In the following, the invention will be described in detail by referring to a few preferred embodiments presented in the figures in the attached drawings, but the invention is not exclusively limited to those embodiments.
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In the foregoing, only a few preferred embodiments of the invention have been described. It is obvious to a person skilled in the art that numerous modifications may be made in them within the scope of the inventive concept presented in the claims.
Claims
1. Method in ore enrichment processes taking place in water solutions, in which processes undesirable depositions (12) are formed in process containers, piping and equipment, characterized in that the surface (11) of an object to be protected in a process susceptible to deposition is polarized in the anodic direction by using an external power source (100) and a current supply electrode (12) placed in the solution so that the electro-chemical reaction products thus produced alter the structure of the deposition (20) present on the surface (11) of the object (11) to be protected or accumulating on said surface (11) in such a way that the adherence of said deposition (20) to said surface (11) is substantially weakened.
2. Method in ore enrichment processes taking place in water solutions, in which processes undesirable depositions (12) are formed in process containers, piping and equipment, characterized in that the surface (11) of an object to be protected in a process susceptible to deposition is polarized in the anodic direction by using an external power source (100) and a current supply electrode (12) placed in the solution so that the potential of the said surface (11) of the said object to be protected is brought to a range in which the electro-chemical reaction products formed inhibit the formation of deposition or substantially retard the formation of deposition on said surface (11).
3. Method according to claim 1, characterized in that the change in the potential of the said surface (11) of the said object to be protected resulting from the aforesaid anodic polarization is caused to be such that the said surface (11) does not undergo any substantial corrosion damage due to the raised potential.
4. Method according to claim 1, characterized in that anodic direct current is used in the method.
5. Method according to claim 1, characterized in that pulsed anodic direct current is used in the method.
6. Method according to claim 1 characterized in that a combination of an alternating-current component and an anodic direct-current component is used in the method to accomplish the desired electro-chemical reactions.
7. Method according to claim 1, characterized in that a current density value in the range of 0.5-20 A/m2 is chosen.
8. Method according to claim 1, characterized in that the current/voltage supplied by the said power source (100) is controlled on the basis of an external measurement signal (104) and/or an internal measurement signal (105).
9. Method according to claim 1 characterized in that the current/voltage supplied by the said power source (100) is controlled on the basis of a control signal (108) obtained from a data file (107).
10. Method according to claim 1 characterized in that the current/voltage produced by the said power source (100) is controlled by means of a timed control signal by changing the current/voltage at desired time intervals t1, t2,..., t, to a predesired magnitude.
11. Method according to claim 1 characterized in that the said surface (11) of the said object to be protected is selected from the group of: precipitators, clarifiers, crystallizers or inner walls of a reactor, heat exchangers, overflow chutes, process pipes and valves used in a process.
12. Method according to claim 1 characterized in that reference electrodes (13) monitoring the intensity of the anodic polarization are mounted near the surface (11) to be protected so that they are in contact with the electrolyte and galvanically isolated from the objects to be protected.
13. Method according to claim 12, characterized in that the material of the aforesaid reference electrode (13) is selected from metals that withstand the conditions prevailing in the process and retain their characteristic potential at a level sufficiently constant in respect of reliability of the potential measurement.
Type: Application
Filed: Mar 28, 2007
Publication Date: Sep 9, 2010
Inventor: Timo Laurila (Lahti)
Application Number: 12/294,771
International Classification: C23F 13/02 (20060101);