GB1584288A - System for producing direct current for electrolytic processes - Google Patents

Description

(54) SYSTEM FOR PRODUCING DIRECT CURRENT FOR ELECTROLYTIC PROCESSES (71) We, EMPRESA NACIONAL DEL ALUMINIO, S.A. of C/General Sanjurjo, 4, Madrid-3, Spain, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention refers to a system for producing direct current for electrolytic processes, the main field of application of which is to anodize aluminium, in process for electrtolytically obtaining metals, in the refining of metals by electrolysis, in the obtainment of chemical products by electrolysis, and, generally, in processes for aqueous electrolysis and igneous electrolysis.

The power consumed during electrolytic processes is determined by two factors, one of which is a constant which is determined by the nature of the electrolyte used, and the other by a combination of losses, of which the most important is the polarization effect which establishes a potential barrier in opposition to the electrical field applied. Thus, the voltage necessary to carry out an electrolytic process has to be higher to overcome the effect of this barrier.

This over-voltage is transformed into waste energy whose magnitude, from the point of view of an industrial installation, is very considerable.

Since the theoretical power to obtain an electrolytic process constitutes a constant, the only possible action to achieve a saving in energy is to minimise the losses produced by the polarization effect.

The saving of energy dissipated in electrolytic processes has been the subject of studies for a long time, and reference may for example be made to French patent No.

1,399,797 and U.S. patent Nos. 3,977,948 and 4,011,152.

By way of illustration, the example on page 5 of the aforementioned French patent can be cited, wherin according to the improvements of the invention described therein, a consumption of 0.8 KWh/m2 is obtained in comparison with a consumption of 1.4 KWh/m2 represented by the conventional methods then used.

A known fact is the use in electrolytic pro- cesses of direct current, superimposed on an alternating current, thus obtaining a reduction in the electrode potential, a reduction in any irreversibility of the reaction, a reduction in the overvoltage of the hydrogen, and a reduction in the overvoltage of chloride, and there is also facilitated the anodic dissolution of the metals and a higher density of direct current in the electrode is permitted. In any' case, it can readily be verified that in this field a,sub- stantial saving in energy cannot be achieved since, although a series of advantages is obtained during the time in which the polarity is inverted, these advantages are the result of the contribution of a certain amount of energy.

The present invention provides a system for producing direct current for an electrolytic process wherein to each of the phases of a three-phase alternating current network there is connected a thyristor rectifier the thyristors of which are controlled by a programmed control unit to start the conduction of the thyristors at an angle close to zero degrees and greater than this value, the thyristors remaining conducting until an angle close to 90 degrees and smaller than this value is reached, and are then non-conducting for the remainder of the half period, thus obtaining within each half period a voltage having an 'increasing value followed by a zero value for the remainder of the half period, the thyristors further being controlled so that after a predetermined number of half periods a pause lasting another predetermined number of half periods is produced, which number will depend on the electrolyte used in the electrolytic process.

The system according to the present invention uses a continuous increasing voltage, whose value is adapted at all times to the polarization voltage present, for a determined period of time, the passage of the current then being completely interrupted. This correspondence betwen the necessary depolarization voltage and that applied at every moment, implies a considerable saving in energy due to the fact that at each instant the applied voltage is maintained lower than the maximum depolarization voltage which, as previously stated, is that which is constantly applied in conventional systems. During the periods of time in which the applied voltage is cancelled, a natural depolarization takes place consequently without a consumption of energy.

Since the energy consumed is a direct function of the applied voltage and of the intensity with which it circulates, the consumption of energy will be reduced at the rate at which the applied voltage is reduced. The estimated saving in energy, when using this system, is calculated to be about 20%, a highly im portant amount to be taken into account in any industrial electrolytic process.

The field of application of the system according to the present invention is very wide, for example the application thereof in the electrolyte industry with respect to aspects such as electroparting, electroextraction, electrorefining of metals, electrodeposition, and galvanoplastics, in the electro-furnace industry the processes of obtaining for example aluminium and magnesium. The system can likewise be utilized for obtaining thermoelectric products, charging of batteries, obtention of oxydation-reduction products, in the gas electrochemical industry, and in electrophoretic processes.

The obtainment of the applied voltage and the control thereof can be carried out in different ways. One possible embodiment con sists in starting from an industrial three phase electrical network, transforming the in put voltage into three single-phase output voltages, corresponding to the three phases, each one of these single-phase voltages being rectified with the help of a full-wave rectifier made from thyristors, the control of the gates of which is carried out by a programming device to establish the conduction pauses of the thyristors for the obtainment of the pre viously described voltage.

Another embodiment consists in arranging in each phase of the secondaries of a transformer three independent and equilibrated tappings, each one of which is controlled by a thyristor with its corresponding programming device for the gate control, so that a rectified voltage resulting from the partial sum of three voltages out of phase with each other in a certain angle is obtained, all of which within each phase are obtained from the secondary of the transformer.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a graph of the theoretical geometry adopted by the wave voltage obtained with the system according to the present invention; Figure 2 illustrates schematically one embodiment of the system according to the invention; Figure 2 shows the wave obtained by the embodiment shown in figure 2; Figure 4 illustrates schematically another embodiment of the system according to the invention and Figure 5 illustrates the voltage graph obtained by the embodiment shown in figure 4.

The embodiment represented in figure 2 comprises a three-phase transformer 1 whose primary 2 will be connected in the most suitable manner and whose secondary has three single-phase windings 3, 4 and 5 to each one of which there is coupled a full-wave rectifier composed of two thyristors 6 and 7, whose gates 8 are controlled by a programming device 9, so that a rectified and controlled voltage is applied to an electrolytic chain 10 comprised of a plurality of tanks arranged in series.

The transformer 1 has means which permit the output voltage present in the secondary to be varied at will. There is provided an arrangement in series of the electrolytic tanks within each chain 10, corresponding to each one of the phases of the secondary of the transformer 1, to reduce to a maximum the losses in voltage which would be produced by adopting a parallel arrangement of the electrolytic tanks, which would make the use of much lower voltages in the secondary of the transformer 1 compulsory.

By controlling the thyristors 6 and 7 so that they conduct for an angle smaller than 90 and close to this value, a wave such as that represented in figure 3 is obtained, wherein T1 represents the conduction time of a thyristor and wave 11 is formed, while T2 corresponds to a pause having a zero voltage wherein a natural depolarization takes place without consuming energy. T, in turn represents the conduction time of the other thyristor connected in the full-wave rectifier. Since the depolarization times T2 could be insufficient for a full depolarization, it has been contemplated that after each determined number of semi-periods there is a further pause, likewise without a consumption of energy, lasting for another determined number of semi-periods, which number will depend on the electrolyte used.

The embodiment represented in figure 4 comprises a transformer 12, which, as in the prior case, has means to vary the output voltage of its secondary whose windings corresponding to each one of the three phases of the secondary are, in turn, divided into three like sections joined together in the form of a star, there being at the output of each section a thyristor 13, 14 and 15, each assembly of thyristors 13, 14 and 15 being controlled by a common programming device 16 which establishes the conduction periods thereof, within a determined angle, which conduction angles can be increased. Thus, the voltage applied to each electrolytic chain is the result of three partial voltages out of phase with each other and, therefore, partially superimposed, the resultant wave 17 being clearly shown in figure 5 wherein the reference numbers 18, 19 and 20 represent the three partial voltages which combine in the formation of the wave 17.

It can likewise be appreciated from figure 5 that the conduction time T5 is smaller than the zero conduction time T,, and in this case too there are provided after a number of semi periods some further pauses, wherein the thyristors 13, 14 and 1S do not conduct with the same end purpose as the further pauses T4 of the preceding embodiment.

In any case, and as previously mentioned, a certain correspondence is obtained between the necessary depolarization voltage and the voltage in fact applied, as can be seen from a mere observation of figures 3 and 5, comparing them with the theoretical graph of figure 1. The mentioned saving in energy, constituting the main aim of the present invention, is likewise reflected in the practical embodiments described above.

The invention will be further described with reference to the following illustrative Examples.

Example 1.

The following results were obtained from a copper deposition starting from a solution of copper sulphate and sulphuric acid: Using CuSO4.5H2O with a concentration of 100 g/l and H,SO, with a concentration of 25 g/l and using a stainless steel anode and a copper cathode, employing a direct current density of about 1.2 A/dm2 and a voltage of 2 volts, a consumption in energy of approximately 2.2 Wh/g of deposited copper was obtained.

With the pulsing current obtained by the system according to the invention and on the basis of this same data, a consumption of about 1.6 Wh/g is achieved.

Comparing the values obtained, it can be seen that the saving in energy achieved is calculated as more than 25%.

Example 2.

The following results were obtained from the deposition of zinc starting from a solution of zinc sulphate and sulphuric acid: Using a solution of ZnSO4 with a concentration of 250 g/l and H2SO4 with a concen tration of 125 g/l, and using a lead anode and an aluminium cathode with a current density of about 3.5 A/dm2 and a voltage of 3.7 volts, a consumption of energy, according to conventional methods, of about 3 Wh/g of deposited zinc was obtained, while with the system of the present invention the consumption was reduced to 2.5 Wh/g.

Comparing the values obtained in this case, it can be seen that a saving in energy achieved is calculated as more than 17%.

WHAT WE CLAIM IS: 1. A system for producing direct current for an electrolytic process wherein to each of the phases of a three-phase alternating current network there is connected a thyristor rectifier the thyristors of which are controlled by a programmed control unit to start the conduction of the thyristors at an angle close to zero degress and greater than this value, the thyristors remaining conducting until an angle close to 90 degrees and smaller than this value is reached, and are then non-conducting for the remainder of the half period, thus obtaining within each half period a voltage having an increasing value followed by a zero value for the remainder of the half period, the thyristors further being controlled so that after a predetermined number of half periods a pause lasting another predetermined number of half periods is produced, which number will depend on the electrolyte used in the electrode lytic process.

2. A system according to Claim 1, sub stantially as herein described with reference to, and as shown in, Figure 2 or 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. star, there being at the output of each section a thyristor 13, 14 and 15, each assembly of thyristors 13, 14 and 15 being controlled by a common programming device 16 which establishes the conduction periods thereof, within a determined angle, which conduction angles can be increased. Thus, the voltage applied to each electrolytic chain is the result of three partial voltages out of phase with each other and, therefore, partially superimposed, the resultant wave 17 being clearly shown in figure 5 wherein the reference numbers 18, 19 and 20 represent the three partial voltages which combine in the formation of the wave 17. It can likewise be appreciated from figure 5 that the conduction time T5 is smaller than the zero conduction time T,, and in this case too there are provided after a number of semi periods some further pauses, wherein the thyristors 13, 14 and 1S do not conduct with the same end purpose as the further pauses T4 of the preceding embodiment. In any case, and as previously mentioned, a certain correspondence is obtained between the necessary depolarization voltage and the voltage in fact applied, as can be seen from a mere observation of figures 3 and 5, comparing them with the theoretical graph of figure 1. The mentioned saving in energy, constituting the main aim of the present invention, is likewise reflected in the practical embodiments described above. The invention will be further described with reference to the following illustrative Examples. Example 1. The following results were obtained from a copper deposition starting from a solution of copper sulphate and sulphuric acid: Using CuSO4.5H2O with a concentration of 100 g/l and H,SO, with a concentration of 25 g/l and using a stainless steel anode and a copper cathode, employing a direct current density of about 1.2 A/dm2 and a voltage of 2 volts, a consumption in energy of approximately 2.2 Wh/g of deposited copper was obtained. With the pulsing current obtained by the system according to the invention and on the basis of this same data, a consumption of about 1.6 Wh/g is achieved. Comparing the values obtained, it can be seen that the saving in energy achieved is calculated as more than 25%. Example 2. The following results were obtained from the deposition of zinc starting from a solution of zinc sulphate and sulphuric acid: Using a solution of ZnSO4 with a concentration of 250 g/l and H2SO4 with a concen tration of 125 g/l, and using a lead anode and an aluminium cathode with a current density of about 3.5 A/dm2 and a voltage of 3.7 volts, a consumption of energy, according to conventional methods, of about 3 Wh/g of deposited zinc was obtained, while with the system of the present invention the consumption was reduced to 2.5 Wh/g. Comparing the values obtained in this case, it can be seen that a saving in energy achieved is calculated as more than 17%. WHAT WE CLAIM IS:

1. A system for producing direct current for an electrolytic process wherein to each of the phases of a three-phase alternating current network there is connected a thyristor rectifier the thyristors of which are controlled by a programmed control unit to start the conduction of the thyristors at an angle close to zero degress and greater than this value, the thyristors remaining conducting until an angle close to 90 degrees and smaller than this value is reached, and are then non-conducting for the remainder of the half period, thus obtaining within each half period a voltage having an increasing value followed by a zero value for the remainder of the half period, the thyristors further being controlled so that after a predetermined number of half periods a pause lasting another predetermined number of half periods is produced, which number will depend on the electrolyte used in the electrode lytic process.

2. A system according to Claim 1, sub stantially as herein described with reference to, and as shown in, Figure 2 or 4 of the accompanying drawings.