GB2539878A - Process lines and their use for the production of metals through electrolysis - Google Patents

Abstract

A process line for electrorefining (ER) or electrowinning (EW) of metals comprises an electrolytic cell 321 fitted with at least one anode-cathode pair. Rectified mains power (can be three-phase) is supplied via a transformer 324 and a rectifier, such as a thyristor rectifier 322. In order to reduce the effect of ripple, the frequency of operation of the power supply is raised by incorporating a frequency converter 326, such as a motor-generator set, so that the input to the thyristor rectifier is at a frequency f2 substantially higher than 100 Hz, e.g. 400 Hz. As a result the output of the rectifier contains no ripple components below 360 Hz. Since the impedance of the electrolyte increases at higher frequencies, the deleterious effect of the ripple components is reduced. The frequency converter 326 can be before the transformer, as shown, or after it.

Description

Process lines and their use for the production of metals through electrolysis.

The invention relates to the use of elements of production lines for the electrolytic production of metals. The invention has applications in the metallurgical industry.

Metals can be obtained by electrolysis through, for example, electrorefining (from impure anode metal) and electrowinning (from solution). With both of these methods, an electric current flows between two electrodes, an anode and a cathode respectively, submersed in an electrolyte consisting of a solution containing the metal in a dissolved, ionised, impure form. The flow of the electric current causes the metal to be deposited on the surface of the cathode. Electrorefining is an electrolytic process for the purification of metal in which the impure metal plays the role of an anode. As a result of the flow of the current the anode is dissolved in the electrolyte and the metal ions travel to the cathodes where pure metal is deposited. In the electrorefining of copper, for example, the anode is made of impure copper obtained through pyrometallurgical process (fire-refining) and the electrolyte, based on sulphuric acid, contains dissolved copper ions. The impurities from the copper of the anode may be separated in the form of solid particles which fall to the bottom of the container with the electrolyte, may dissolve in the solution and stay there, or may dissolve in the solution and precipitate on the surface of the cathode, thereby contaminating the copper produced by this method. Typical impurities include arsenic, nickel, bismuth and antimony, whose concentration grows in proportion to the copper produced, and which are removed from the electrolyte by means of so-called decopperisation (electrowinning) and the further chemical processing of the electrolyte.

In order to improve the quality of the surface where the copper (as well as other metals) is deposited and to avoid the formation of dendrite which causes short circuits and the precipitation of impurities (occlusions) on the surface of the metal produced, a small quantity of thiourea or animal glue is added to the electrolyte.

Electrowinning is an electrolytic process for obtaining metal from the electrolyte in which it is contained, where the anode is made of a metal different from the metal being precipitated and fundamentally not soluble in the electrolyte .

Many different metals are produced by electrolysis, for example: copper, nickel, gold, silver, cobalt, zinc, tin, chromium, manganese or aluminium. In the industrial process of electrorefining of copper, a current is typically applied in an individual cell containing several alternately arranged anodes and cathodes with an applied voltage of the order of 0.25V to 0.4V, while in the industrial electrowinning process a voltage of the order of 1.5V to 2V is applied and strongly depends on electrolyte composition and production cycle called crop (i.e. changing anode-cathode distance). In both of the processes described above, the current density usually applied is of the order of 200 to 400 Amperes per square metre of cathode surface, while the surface area of each anode and each cathode is typically around Im^ each side. The typical flow rate of the electrolyte through the cell is approximately 20 litres per minute, while the typical temperature of the electrolyte is approximately 40°C to 60°C.

An increase in the current density, and the corresponding absolute increase in the current flowing in the electrolyte, leads to an increase in the output of the metal produced. There is, however, a certain limiting value to the current density for the electrolyte and the entire system for the electrolytic production of metal which, when exceeded, leads to an unfavourable precipitation of impurities on the cathode surface (i.e. the deposit of metal may be spongy or powdery and contain occluded salts and other impurities) and/or to the passivation of the anode, making it impossible to obtain further metal. For this reason, it has been the ambition of the metallurgical industry to achieve conditions for the electrolytic production of metal whereby the value of the current density, and thereby the productivity, are as high as possible while maintaining the expected quality of the metal produced.

To this end, various conditions are selected, including the concentration of the acid in the electrolyte, the temperature of the electrolyte, the rate at which the electrolyte flows through the cell, the concentration of additives such as thiourea or animal glue, the distance between anode and cathode, etc. US 6511591 (Virtanen et al.) describes the use of reverse-polarisation current flows (Pulse Reverse) in the electrolytic production of metal in order to limit the passivation of anodes and to increase current density in electrolytic production; the document discusses the limits encountered when high current densities are used. US 2007/200725659 by Christian Hecker sets out the use of a superimposed AC signal on a cathodic reduction process for copper production; this needs considerable additional equipment.

Numerous works have been written on optimising the composition of the electrolyte in order to increase productivity and the quality of the metal deposit produced. The literature also includes works relating to an increase in the output of electrolytic copper by changing the geometry of the cathodes in order to allow larger quantities of metal to precipitate, as published in international patent application WO 2005/080640 (Outokumpu), confirming the possible limitations on the use of current densities and the limitations arising from the passivation of anodes; they do not suggest any solution involving changing the power supply systems and modifying power signal quality supplied to production cells. A method is therefore sought for appropriately supplying power to the electrolyte cells in order to increase the current density and the output of electrolysis. This problem was unexpectedly solved by the Invention presented here .

One aspect of the invention relates to a process line for obtaining metal through electrolysis by electrorefining or electrowinning on an Industrial scale. Including an electrolytic cell comprising at least one anode-cathode pair connected to a rectified power supply having on the AC input side no significant components at frequencies less than 100 Hz, preferably none less than 400 Hz. The rectified power supply can include a rectifier to which is connected the mains or a transformer supplying current from the mains, the rectifier being for instance a diode, thyristor or IGBT (generally SCR) rectifier, and the process line can additionally include a frequency converter connected either between the transformer and the input of the SCR or before the transformer, so as to supply an AC voltage to the rectifier at a frequency f2 substantially higher than 60Hz.

For a three-phase, six-pulse thyristor rectifier the first harmonic, which represents the lowest frequency component f3 of the output of the power supply, will be at six times the basic frequency, so at least 300 Hz for European systems and 360 Hz for American systems with an input at mains frequency. With the invention the frequency f2 of the output of the frequency converter is preferably at least twice the mains frequency fl, i.e. at least 100 Hz, preferably at least 1 kHz. Thus for the standard setup the output frequency f3 will be at the very least 720 Hz, preferably more than 1 kHz, Ideally more than 2 kHz.

The significance of the frequency of the ripple component of the voltage applied to the cell is thought to be as follows. The voltage applied is ideally DC. However, in industrial processes power has to be supplied from the mains, which is AC at a frequency fl, and so it has to be rectified to provide the DC voltage. The resulting voltage typically has a slight ripple, e.g. at frequency 2f for full-wave rectification or 6f for a three-phase setup, as would be usual. While this ripple, as applied to the large electrolytic capacitance of the cell, appears nearly to disappear, and is therefore often ignored, it does have an effect within the electrolyte, particularly near the cathode where the metal is being deposited.

The invention is based on the recognition that the impedance of the cell increases considerably at frequencies above a minimum value, which for typical copper-plating cells is at about 70 Hz, which is not very far from the typical applied frequency of 300 Hz. A typical impedance curve against frequency for a copper bath is shown in Figure 5. If the ripple component frequency in the voltage applied is therefore substantially higher than this minimum, it is effectively filtered out and the deposition of the metal of interest becomes considerably more efficient and can be conducted with higher average current density.

There are various ways of ensuring that the ripple component has such a higher frequency as set out above.

For instance, the invention may make use of an alternating-current motor-generator set in order to increase the frequency of the current supplied to the SCR (e.g. thyristor) rectifier in the process of obtaining metals through electrolysis. Such sets are readily available, typically being used for altering output voltage or frequency (e.g. 50 Hz to 60 Hz, 50 Hz to 400 Hz for aircraft systems) or suppressing transients, for instance. Alternatively, semiconductor frequency converters (AC/AC) may be used or inverters (DC/AC) instead of m-g sets with a secondary frequency above double the mains frequency fl, preferably above IkHz.

Embodiments of the invention exhibit a number of advantages in methods of obtaining metals by electrolysis: metal can be produced at a higher current density, i.e. with a greater productivity expressed in tonnes per day, whilst maintaining a comparable quality of the metal produced, the increased current density does not lead to passivation of the anodes or the release of impurities on the surface of the produced metal, which was not possible with the previous use of higher current densities, the quality of the metal produced can be improved: where production is carried out using the conventional method at the same current density as the method according to the invention, production according to the invention results in the reduced release of impurities on the surface of the metal, because the actual voltage In the electrolyte is a purer DC voltage; the selectabllity of cathodes with highest quality (e.g. equal or better than CATH-01, so called Grade A on LME) increases within a population of cathodes produced in the tankhouse, the electricity necessary to produce a given quantity of metal can be reduced, the current efficiency of the cathode can be increased by approximately 1 percentage point, e.g. from 95% to 96%; the consumption of additives such as thiourea or animal glue can be reduced.

For a better understanding of the invention, embodiments will now be described with reference to the attached drawings, in which:

Figures 1 and 2 show conventional systems for electrolytic deposition of metals such as copper;

Figure 3 shows an arrangement for the large-scale industrial production of copper in accordance with a first embodiment of the invention;

Figure 4 shows a second embodiment exchanging the places of frequency converter and transformer; and

Figure 5 is a trace of electrolyte impedance against the frequency of an applied AC voltage.

First, known systems for electrorefining will be described. Figure 1 shows a typical view of an electrolyte cell 10 with alternating anodes 11 and cathodes 12 immersed in electrolyte 13 within the cell. The anodes have a common electrical connection 14 while the cathodes have a common electrical connection 15 in the form of copper bus bars, being the anode and cathode bus bars. The anodes are insulated from the cathode bus bar by means of insulating support elements. The cathodes are similarly insulated from the anode bus bar.

Figure 2 shows a typical process line for obtaining copper and other metals by electrolysis, comprising several electrolyte cells 10, typically numbering more than ten, containing anodes and cathodes, connected in series. For simplicity fifteen cells W1 to W15 are shown, located in a tankhouse 21. The cells are connected in series to a direct-current (DC) power supply, including a SCR (e.g. thyristor) rectifier 22. This rectifier 22 is powered by a low-voltage three-phase alternating current (AC) line 23 whose voltage is adjusted to the number of cells and oscillates at mains frequency fl, i.e. 50Hz or 60Hz. The voltage in the low-voltage AC power supply line 23 is typically adjusted by means of a transformer 24 which reduces the voltage of the mains power supply line 25 from, for example, 6kV or 400V as supplied by the energy operator to the required voltage of, e.g., lOOV or 30V, depending on the number of cells used. The mains AC power supply line 25 supplies power with a sinusoidal course, most often three-phase, at a frequency of 50Hz or 60Hz, depending on the relevant standards of the country in question.

In view of the fact that in North America and the majority of countries of South America a frequency fl of 60Hz is used in electricity supply lines, while in Europe and the other countries of the world a frequency of 50Hz is used, the output of the SCR (e.g. thyristor) rectifier 22 contains a considerable component at a frequency of 360 or 300 Hz (6f). Unfortunately this is not far from the minimum impedance of the cell's electrolyte, so there is a substantial unwanted ripple in the deposition current, which is inefficient (when the ripple takes the voltage below the ideal current density) and deleterious (when the ripple takes the voltage above the level at which smooth deposition takes place and only the desired metal, e.g. copper, is plated).

This embodiment of the invention involves the use of so-called frequency converters or generators to supply current to the SCR (e.g. thyristor) rectifier at a frequency f2 exceeding 50Hz/60Hz, preferably by at least a factor of two. Converters of this type can be in the form of semiconductor units or mechanical AC motor-generator sets. In practical embodiments of the invention, available AC generators for the aeronautical industry with a frequency of 400Hz and motor-generator sets manufactured by Emerson Industrial Automation and Kato Engineering Inc. were used, which allowed the conversion of 50Hz/60Hz power lines to 1200Hz at an output and voltage typically used in industrial electrorefining and electrowinning applications. Use was also made of programmable sources of AC current (inverters). These sets give rise to a power line frequency of 400 Hz and above .

Figure 3 illustrates a first system for obtaining copper and other metals by electrolysis in accordance with the invention. For this purpose, the pre-existing or newly-installed electrolyte cells 10 installed in the tankhouse 21, as in Figure 2, are connected to an SCR (e.g. thyristor) rectifier 322 adjusted, if necessary, to operate at the frequency selected above (f2). The rectifier, depending on the manufacturer, may require an upgrade or complete replacement. However, there is no need to upgrade the cells, and therefore the metal manufacturer's existing infrastructure in his tankhouse may be used.

The SCR rectifier 322 is connected at 303 to a higher-frequency source of electricity (f2), in the form of a frequency converter 326 connected to a transformer 324, the assembly being powered by conventional mains. Depending on the scale of the electrolytic installation, including the number of cells 10 and the voltage and power rating of the rectifier 322 used, it is possible to place the frequency converter 326 before the transformer 324, resulting in the arrangement shown in Figure 3, or after the transformer 324, resulting in the alternative embodiment depicted in Figure 4.

Figure 3 illustrates a typical method of arranging a larger industrial system for the production e.g. of copper, in which the main alternating-current power supply line 307 can be three-phase at a voltage of e.g. 6kV and a frequency fl of 50Hz/60Hz. The mains power supply line 307 in this case supplies the frequency converter 326, which takes the form of an AC motor-generator set, and provides an output current at the required AC voltage, but at a higher frequency f2 of e.g. 400 Hz or 1200Hz.

This output 305 is fed to the transformer 324, which supplies a low-voltage branch used to connect the SCR (e.g. thyristor) rectifier 322. The output voltage might typically be in the region of 5-250V, depending among other things on the number of cells in series and on the process in question. The lowest frequency of the ripple in the DC output of the rectifier would in the example be 2400 Hz or 7200 Hz.

Figure 4 depicts a typical solution for smaller systems for the electrolytic production of metals requiring a lower power supply; here, the voltage-reducing transformer 424 supplies power at a frequency fl by a connection 405 to a frequency converter 426, which in this case is typically a semiconductor converter, though it could also be motor-generator set. The frequency converter outputs (403) at a frequency f2 at least twice as great as f1. A motor-generator set typically has an upper frequency limit of about 1200Hz, so for higher frequencies, e.g. up to 50 kHz or so, a semiconductor converter would be used.

The particular embodiment of the system for the electrolytic production of copper or other metals depends on the type of mains power line 407, the electric power required to obtain the metal by electrolysis, the need to share the mains power line 407 and/or the transformers with other equipment used for other purposes than electrolytic production, the possibility of using and adapting existing transformers 424 and rectifiers 422 and on economic considerations, as weighed up by a specialist in power engineering.

It should be taken into account that the positive effect consisting of an increase in productivity as a result of supplying a SCR (e.g. thyristor) rectifier is achieved as a result of rectifying a current having a frequency substantially above 50Hz/60Hz. The use of a power line supplying electricity at the same voltage but a higher frequency increases electrolytic productivity without loss of quality and purity of the metal produced. It was confirmed that supplying the SCR rectifier with power at a higher frequency allows the process of electrorefining or electrowinning to be carried out at a current density 20% to 40% higher than that using a conventional current at 50Hz/60Hz for the same quality.

Tests were carried out using an installation following Figure 4. At a frequency of 400Hz, a current density increase of about 18%, at 1200Hz of about 32% and at a frequency of about 8300Hz an increase of 40% were obtained. It was also confirmed that there was no longer any necessity to use additives such as thiourea or animal glue. The electrolyte temperature was 60°C, the electrolyte flow rate in the cell was 20 litres per minute, the electrolyte composition 46g Cu/litre, 160g H2S04/litre, 20g As/litre, 0.7g Sb/litre, 25g Ni/litre, 2g Fe/litre, 0.6g Bi/litre, 0.03g Cl/litre. The cathodic current efficiency in each instance reached a value in excess of 97% and the total consumption of energy required to produce 1 tonne of copper was reduced by 4% compared with a traditional thyristor rectifier arrangement.

Claims (14)

Claims

1. A process line for the electrolytic production of metals by electrorefining or electrowinning, comprising an electrolyte cell (10), fitted with at least one anode-cathode pair, and a power supply, to be connected to AC mains and including a rectifier (322, 422), the power supply additionally including a frequency converter (326, 426) arranged to increase the frequency of the voltage applied to the rectifier to a frequency f2 of at least 100 Hz.

2. A process line according to claim 1, wherein f2 is at least 400 Hz.

3. A process line according to claim 1 or 2, wherein f2 is chosen so that the output of the rectifier includes no ripple component at a frequency below a frequency f3, where f3 is at least 500 Hz, preferably at least 1 kHz, further preferably greater than 2 kHz, still further preferably greater than 10 kHz.

4. A process line according to any preceding claim, further including a transformer (324, 424) for reducing the voltage applied to the cells (10).

5. A process line according to claim 4, wherein the transformer (324) is connected upstream of the frequency converter (326).

6. A process line according to claim 4, wherein the transformer (324) is connected downstream of the frequency converter (326).

7. A process line according to any preceding claim, wherein the frequency converter (326) is a motor-generator set producing an output at 400 Hz.

8. A process line according to any of claims 1 to 6, wherein the frequency converter (426) is a semiconductor converter .

9. A process line according to any preceding claim, wherein the rectifier is an SCR rectifier, e.g. a thyristor rectifier.

10. A process line according to any preceding claim and configured for 3-phase power input.

11. A method of electrorefining or electrowinning, in which power is supplied from AC mains at a frequency fl and is rectified, and this rectified output is used to generate a voltage applied to the electrolyte in which no ripple component has a frequency below f3, where f3 is at least 500 Hz.

12. A method according to claim 11, in which an alternating-current motor-generator set or semiconductor frequency converter is used to provide an input to the rectifier at a frequency f2 higher than fl, preferably at least 100 Hz, so that the rectifier's output is produced at the higher frequency f3.

13. A method according to claim 8 or 9, wherein f3 is at least 1 kHz, preferably at least 2 kHz, further preferably at least 10 kHz.

14. A method of conversion of a process line for the electrolytic production of metals by electrorefining or electrowinning, the process line comprising an electrolyte cell (10), fitted with at least one anode-cathode pair, and a power supply, to be connected to AC mains and including a rectifier (322, 422), in which a frequency converter is inserted before the rectifier so as to increase the frequency of the input of the rectifier so that it supplies the rectifier at a frequency f2 of at least 100 Hz, preferably at least 400 Hz.