Electro-chemical method in an enrichment process

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 ti, t ₂ , ..., t _n 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.

Fig. 1 represents a preferred embodiment of the method of the invention in diagrammatic side view.

Fig. 2 represents a second preferred embodiment of the method of the invention in diagrammatic cross-sectional side view.

Fig. 3 illustrates an advantage achieved by using the method of the invention in obviating deposition in a Bayer process.

Fig. 4 presents a flow diagram representing the regulating function of a preferred embodiment of the method of the invention.

Fig. 5 presents a flow diagram representing the regulating function of a second preferred embodiment of the method of the invention.

In Fig. 1, a process solution exposing equipment to the formation of deposition is indicated by reference number 10. In this embodiment, the process device's metallic inner wall to be protected against deposition and functioning as one of the electrodes in an electric circuit is indicated by reference number 11, a current supply electrode by reference number 12, a reference electrode by reference number 13 and a galvanic anode current supply contact by reference number 14. In magnified detail B, the deposition layer in the immediate vicinity of the surface 11 is indicated by reference number 20 and an outer deposition layer by reference number 21.

In the embodiment according to Fig. 1, a power source 100 is fitted to supply the electrode pair 11, 12 either with direct current modified by an AC component, direct current or pulsed direct current, as visualized in magnified detail A. In this embodiment, the current supply

electrode 12 functions as a cathode while the inner wall 11 of metallic structure of the process device to be protected functions as an anode. The current supplied by the power source 100 is conducted via the cathode 12 through the process solution 10 serving as an electrolyte to the anode 11 to be protected against deposition and further via the galvanic contact 14 back to the power source 100. Placed near the object 11 to be protected against deposition is also a sensor, in this embodiment e.g. the reference electrode 13. The sensor 13 may be any detector that can provide the measuring and control unit 101 with information regarding the current state of the process susceptible to deposition. The measuring and control unit 101 controls the operation of the power source 100 via a control signal 106 so that a current of a desired magnitude flows between the electrode pair 11, 12, so that the oxidation reactions occurring on the surface of the object 11 to be protected are of a quantity and quality such that the reaction products formed as a result of the chemical reactions on the surface 11 and in its immediate vicinity inhibit the formation of deposition or substantially reduce the adherence of deposition to the object 11 to be protected. Functionally, this appears e.g. as a process whereby the deposition layer 20 closest to the surface 11 is softened sufficiently to weaken its adhesion to the surface 11 and the weight of the outer deposition layer 21 causes the deposition layers to come off the wall, as illustrated in magnified detail C.

In the embodiment illustrated in Fig. 1, the current/voltage produced by the power source 100 is controlled by means of an external measurement signal 104 or an internal measurement signal 105. The current/voltage produced by the power source 100 can naturally also be controlled by means of an external measurement signal 104 and an internal measurement signal 105.

In the embodiment illustrated in Fig. 2, the intensity of the current supplied by the power source 100 can also be controlled by only using a data file 107. The data received into the data file 107 includes e.g. information regarding a property of the process section preceding or following the process section susceptible to deposition that has been found to bear a correlation to the velocity of the formation of deposition or to the quality of the deposition. Information may also be supplied to the data file 107 from laboratory analyses. Such information includes e.g. the pH, solids content, temperature, concentration of a deposition component, viscosity, etc. The data file 107 also contains information regarding earlier current/voltage values produced by the power source 100 that have been found to be advantageous in respect of workability of the invention. From the data file 107, a control signal 108 is applied to the measuring and control unit 101.

Fig. 3 presents a measurement result empirically obtained by applying a preferred embodiment of the method of the invention in a Bayer process. The duration of the experiment is represented as a number of days on the x-axis, while the y-axis represents the test sample area covered by deposition as a percentage of the total area of the test sample. Both the cathodically treated test sample and an untreated reference sample were completely covered by a deposition layer within 13...25 days from the beginning of the experiment. Of the surface area of the anodically treated test sample, about one third was covered by deposition after a month from the beginning of the experiment, but after that the situation stabilized as the deposition began to detach spontaneously from the surface. The current densities with the test samples were about 0.5 - 20 A/m ² , a pulsed direct current being used.

Fig. 4 presents a flow diagram representing the regulating cycle performed by the control unit 101 to control the supply of current. When a sensor 13 measuring the electro-chemical potential of the surface 11 is in use, the first step in the regulating cycle is measurement of the potential, whereupon it is possible to check whether the maximum potential which is of essential importance regarding the risk of corrosion has already been reached. Information regarding this corrosion risk potential may be pre-supplied, or it may be a value based on earlier measurements, or the sensor 13 may be provided with a system that together with the control unit 101 is able to perform on-line measurements for determining the corrosion risk potential. If it is established that the measured potential has already reached the allowed maximum limit, then the regulating cycle is terminated without further action. If the allowed maximum limit of the potential has not yet been reached, then the next step is to check whether a target current or target potential defined in the regulating parameters has already been reached. If the target current or target potential has already been reached, then the regulating cycle is kept waiting until the end of the pulse duration, whereupon it is terminated. If the target current or target potential has not yet been reached, then the current or voltage supplied by the power source 100 is increased by a step of desired magnitude and the new potential is measured, whereupon the above-described checks are repeated. After the end of the regulating cycle, a new regulating cycle is started upon the lapse of a desired length of time.

Fig. 5 presents a flow diagram representing the regulating cycle performed by the control unit 101 to control the supply of current when the control signal 106 received by the power source 100 can be calculated on the basis of the information contained in the data file 107. At the

beginning of the regulating cycle, in case this information concerns on-line conditions, a measurement of the values to be fed into the data file 107 is performed. Next, a calculated regulating parameter applicable to the situation is obtained from the data file 107 and the voltage or current supplied by the power source 100 is increased as desired. After this, the regulating cycle is kept waiting until the end of the pulse duration and then terminated. After the termination of the regulating cycle, a new regulating cycle is started upon the lapse of a desired length of time.

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.