CONTROLLING PROCESSES FOR THE ELECTROLYTIC RECOVERY OF METALS

Abstract: A method for controlling a process for the electrolytic recovery of metals from solutions containing dissolved metals is disclosed in which the conditions of electrolysis are adjusted such that light reflected by the electrodeposit corresponds to that reflected by a preferred electrodeposit structure. The source of light may be a laser or quartz-halogen lamp and the conditions of electrolysis which may be adjusted include the concentrations of additives.

Description: CONTROLLING PROCESSES FOR THE ELECTROLYTIC RECOVERY OF METALS This invention relates to a method for controlling the electrolytic recovery of a metal from solutions containing said metal. The present inventors have found that the manner in which light is reflected by electrodeposits is very sensitive to variations in the structure of the deposited metal. By using the light reflecting characteristics of the electrodeposit as a control parameter, the conditions of electrolytic recovery are adjusted in order to electrodeposit the metal in a preferred form. SUBSTITUTE SHEET Electrolytic recovery of metals is defined herein as including processes for the electrowinning and electrorefining of metals as well as the electroplating of a metal onto a substrate. In such procedures the metal is recovered from a solution containing the dissolved metal, known as an electrolyte, by passing an electric current through the electrolyte. The composition of an electrolyte is particularly important in electrolytic metal recovery processes. When electrolytes are prepared, non-essential and sometimes unwanted components may inadvertently enter the electrolyte (e.g. by corrosion of the electrodes used in the process or by inadequate prior purification of the electrolyte) . Some components of electrolytes are electroinactive and can be tolerated at relatively high concentrations in the electrolyte. Other components, termed impurities, are electroactive and may be deleterious to the electrolytic metal recovery process, even when present at trace concentrations. There are major difficulties associated with the determination of impurities in electrolytes, and in general, on-line analytical procedures do not exist. Consequently, the operator of an electrolytic metal recovery process is often unaware of the types and concentrations of impurities in the electrolyte. Under such circumstances, inadequate process performance is often the first indication of a deterioration of electrolyte purity. This situation is particularly prevalent for electrowinning and electrorefining operations, where the intake of the metal involves complex raw materials of variable composition drawn from a number of sources, each source having substantially different compositions. The problem of impurities is sometimes compounded by the interaction of two or more components of the electrolyte by a synergistic effect, such that the performance of the electrolytic metal recovery process is affected more deleteriously than would be predicted by summing the effects of the individual impurities. When this occurs the concentration limits at which an impurity can be tolerated in an electrolyte become uncertain. It may be essential that the electrodeposited metal should exhibit special physical characteristics. For instance in the recovery of metals by electrowinning and electrorefining processes, the metal is often electrodeposited onto a reusable electrode. Obviously, at the end of a deposition cycle the electrodeposit must be stripped from the cathode. The presence of impurities in the electrolyte can lead to inadequate stripping, either because the electrodeposit is tenaciously bound to the substrate or because the electrodeposit is brittle and prone to fracture. The ability to strip electrodeposits effectively is a prime factor underlying the achievement of production targets in processes involving the electrolytic recovery of metals. Conversely, in electroplating operations it may be essential to ensure that the electrodeposit is homogeneous and adheres tenaciously to the substrate surface, as well as possessing specified visual qualities, such as a high lustre. It is also important that processes for the electrolytic recovery of metals are energy efficient. For instance, when a metal is recovered from an acidic electrolyte the codeposition of hydrogen at the cathode competes with the electrodeposition of the metal. Any factor which increases the relative rate of hydrogen deposition results in less efficient deposition of the metal. Such factors include the increasing acidity of the electrolyte and the presence of any impurity or impurities which electrocatalyse the deposition of hydrogen. The efficiency with which the electric current is utilised for the electrodeposition of a metal is characterised by the faradaic efficiency, sometimes termed the current efficiency, of the process. The energy efficiency of an electrodeposition process also decreases as the cell voltage increases. Processes for the electrolytic recovery of metals are interfacial phenomena and the resultant electrochemical reactions are a function of the interaction of the electrolyte and the electrodeposit. There is a tendency for the unevenness or roughness of an electrodeposit to increase as the amount of metal electrodeposited per unit area of cathode increases. Unfortunately, there is also a tendency for the electrochemical reactions which compete with metal recovery to increase relative to metal deposition as the electrodeposit becomes more massive. Thus the faradaic efficiency of metal recovery decreases. The use of inappropriate operation conditions including electrolyte compositions aggravates this tendency and can, particularly at long deposition times, result in considerable increases in electrical power consumption per unit mass of recovered metal. To some extent this difficulty can be circumvented by the use of additives in the electrolyte. Although the exact function of these additives is unclear, it is often apparent that they modify the structure of the electrodeposited metal. When the modified electrodeposit is more level and less rough this is usually beneficial because even at long deposition times the electrochemical processes which compete with metal recovery are suppressed and increased efficiency of electrical energy consumption is achieved. Additives, which can be either organic, or inorganic compounds, are often active at very low concentrations, sometimes at trace levels. In some cases, the distinction between an additive and an impurity becomes very fine. For instance, the presence of a compound may result in a detrimental modification of the electrodeposit in one concentration range, thus acting as an impurity, while the same compound may cause a beneficial modification of the electrodeposit in another concentration range, in this case acting as an additive. Clearly processes involving the electrolytic recovery of metals are technically complicated procedures in which small variation in electrolyte composition, particularly impurity and additive concentrations, can substantially affect the performance of the process. Indeed, the variation of additive concentrations is the main control option available to the practitioner of these processes. In the absence of suitable on-line analytical procedures for either additives or impurities it is difficult to achieve the optimum control of the process, which is consequently often operated inefficiently. U.S. Patent 4,146,437 described the use of an electrochemical procedure in a test electrochemical cell to provide a method of optimising the control of a process for the electrolytic recovery of metals. In this method the electrochemical measurement corresponds to the electrode potential at which the metal commences to deposit onto a substrate electrode at the beginning of a deposition cycle. The concentrations of additives can then be adjusted until the electrode potential in the test electrochemical cell is the same as an empirically determined value, which corresponds to optimum performance of the process. While this approach represents a substantial advance in the control of electrolytic metal recovery processes, it suffers from significant deficiencies. Firstly, the electrode potential which is used to monitor and control the process is an interfacial measurement which reflects both the characteristics of the electrolyte and the electrodeposit. Thus if the composition of the electrolyte varies, as occurs in many large scale electrodeposition processes, the optimum electrode potential at which the process is controlled changes. Consequently, the control point must be continuously corrected for changes in the composition of the electrolyte. Furthermore, for any given metal there are often substantial differences in the composition range of the electrolyte from which the metal is electrodeposited. These differences in electrolyte often result from differences in the raw materials and the techniques of electrolyte preparation which are used, particularly in metal recovery operations. Consequently, the control potential for each electrolytic recovery process tends to be unique and must be determined empirically for each operation, and possibly for each change in feedstock. Secondly, the electrode potential is measured at an electrode on which the metal is commencing to deposit, rather than measuring the deposition of the metal onto a substantial metal deposit, corresponding to the condition which exists throughout the majority of an electrolytic process for the recovery of metals . A paper by Mackinnon, Brannen and Kerby, titled "The effect of cadmium on zinc deposit structures obtained from high purity industrial acid sulphate electrolyte" as published in the Journal of Applied Electrochemistry Volume 9, pages 71-79 (1979) describes the characterisation of electrodeposits using X-ray diffraction techniques. This article and other papers indicate that X-ray diffraction methods provide valuable information concerning the structure of electrodeposits and demonstrate that it is essential to achieve preferred electrodeposit structures in viable commercial processes. However, X-ray diffraction techniques only provide semi-quantitative information and are not suitable for applications involving the on-line control of electrolytic metal recovery processes . An objective of the present invention is to provide a method for the control of processes involving the electrolytic recovery of metals from solutions of dissolved metals whereby the deposition process is characterised quantitatively according to a physical property of the electrodeposit. The prior art control procedures for such processes involve electrochemical measurements, which are sensitive to variations in both the electrolyte composition and the electrodeposit structure. The present invention consequently has the advantage of being more direct than the procedures of the prior art and is not confounded by the variability of other process properties not related to the structure of the electrodeposit. A further objective of the present method is to relate variations in the structure of the electrodeposit, as characterised by a physical property of the electrodeposit to the composition of the electrolyte from which the deposit was formed. If the physical property of the electrodeposit indicates an unsatisfactory electrolyte the process can be controlled by adjusting the procedures for the purification of the electrolyte until a satisfactory electrodeposit is obtained. Alternatively it may be advantageous to make adjustments to the amount and type of additives present in the electrolyte thus producing electrodeposits possessing the preferred physical properties. The applicants have now found that the electrolytic recovery of metals from solutions of dissolved metals can be advantageously controlled by characterising the structure of the electrodeposited metal. Surprisingly it has been found that the manner in which light is reflected by electrodeposits is very sensitive to variations in the structure of the deposited metal. Consequently, it is possible to determine the manner in which light is reflected by an industrially preferred form of electrodeposit e.g. one which is formed with minimum consumption of energy and posssesses any desirable properties, such as mechanical strength, ductility or surface homogeneity and lustre. If light reflectance measurements indicate that the process product, the electrodeposit, has deviated from the preferred form as characterised by the manner in which it reflects light, the procedure of the process is adjusted until the preferred deposit structure is again formed. A particularly advantageous form of controlling the process is to adjust the concentrations of additives. However, it may also be necessary to improve the purity of the electrolyte by making adjustments to the electrolyte purification procedures. One embodiment of characterising the light reflecting properties of electrodeposits involves comparing the intensities of the incident and reflected light beams. Spectrometers are suitable instruments for conducting such measurements, which can be made using either polychromatic or monochromatic radiation. A wide range of wavelengths have been found to be useful, spanning the ultra-violet, visible and infra-red frequency ranges. In another embodiment of the invention the polarisation of the reflected light is measured. It will be obvious to those skilled in the art that there are a number of ways of comparing the polarisation of light reflected by an electrodeposit. By using a polarising filter the reflected light beam can be characterised into two components which are polarised in orthogonal planes. When the light source is plane polarised, or has a strongly preferred direction of polarisation, it will be found to be advantageous to define one of the components of the reflected light as being polarised parallel to the preferred direction of polarisation of the incident light. This component of the polarised reflected light is defined as having an intensity of V. The component of reflected light which is polarised orthogonal to the component V, is defined as having an intensity H. An advantage of this method of measurement is that samples may be compared by variations in the relative magnitudes of H and V, irrespective of minor variations in the intensity of the incident light beam. However, assuming that the reflection of light by electrodeposits is compared using a function such as the ratio H/V, it may be found that in some instances two different electrodeposit structures result in the same ratio H/V. In such instances it may be advantageous to compare between electrodeposits the individual magnitudes of H or V normalised to a constant incident light intensity, as well as a function of the reflected components such as H/V or ^H 2 + V 2 . The latter function * H 2 + V 2 , relates to the peak total reflected intensity which is equivalent to the total reflected, or unpolarised light. In general it is found that for a given wavelength the ratio H/V is a more sensitive measure of surface structure than v /H2 + V2. However • /H2 + V2 is simpler to measure because both the incident and reflected light can be unpolarised. In a preferred embodiment of the invention it has been found to be advantageous to measure the reflected light at a wavelength which is of the same order of magnitude as the dimensions of the electrodeposit structure. In one embodiment of the invention it has been found to be advantageous to use a laser as the source of incident radiation. Lasers may be adjusted to generate light which is almost completely plane polarised. For the characterisation of zinc metal electrodeposits prepared from sulphate solutions a CO- laser tuned to a wavelength of 10.59 jam has proved to be a suitable light source. When light is reflected by a surface without a change in polarisation the reflection is predominantly specular (i.e. a single mirror reflection) and hence V represents the intensity of specularly reflected light. However, some of the light will be subjected to multiple reflections. In this case the light is scattered and a change in polarisation occurs. Consequently, the magnitude of the component H of the reflected light indicates the extent to which light scattering occurs. Those familiar with the processes of specular reflection and light scattering will appreciate that the relative intensities of specularly reflected and scattered light vary with the geometry of the measurement system. When the angle of incident light to the sample equals the angle at which reflected light is sampled, the collection of the specularly reflected component V will be optimised. For many applications the geometry of the measurement system will not be critical, however it will be obvious to those skilled in the art that for certain applications there will be an advantage in selecting a preferred geometry. For instance if a detector is sequentially measuring H and V it may be desirable to select a geometry such that the magnitudes of the components H and V differ by less than a factor of 1,000. The preferred geometry meeting this requirement will of course vary with the structure of the surface of the electrodeposit. Thus in an embodiment of this invention the reflected light is sampled by a single detector, in which the geometry of the measurement system can measure accurately both the components H and V. A paper by Depew and Weir entitled "Surface Roughness Determination by the Measurement of Reflectance" as published in Applied Optics, Volume 10, Number 4, Pages 969-970 (1971) describes the specular component of the relative reflectance of a metal surface by the following equation. R/R = exp[-(4τrσ /λ ) 2 .cos 2 θ] where σ is the root-mean-square roughness, λ is the wavelength of the incident light, 0 is the angle of incidence and R is the specular reflectance of a perfectly smooth surface of the same material. It is evident from this equation that the specular component is large for all θ if σ/λ « 1 and is large for all σ/λ if θ approaches 90°. In another embodiment of the invention it has been found advantageous to use a quartz-halogen lamp as the source of incident radiation. The optimum measurement wavelengths vary with the structure of the surface of the electrodeposit and with the geometry of the measurement system. When the angle of incidence is near-normal the 8-12 _um wavelength range has been found to be suitable for the characterisation of zinc metal deposits prepared from sulphate solutions. Light emitted by the envelope of a quartz-halogen lamp is collimated with apertures and directed at the zinc surface at a near-normal angle of incidence. This embodiment corresponds to Depew & Weir's case of σ/λ <<1 and smoother surfaces have higher specular reflection than rougher surfaces. A detector adapted to measure light in the 8-12 jum wavelength range is positioned to preferentially intercept and measure the specular reflection. In an embodiment of this invention the specularly reflected light is sampled by a single detector which accurately measures the intensity of that specular reflection. In a further preferred embodiment of the invention it has been found advantageous to use the visible emission of a laser as the source of incident radiation. In a geometry corresponding .to Depew & Weir's case where θ approaches 90 when the angle of incidence is near to grazing (> 86°) the 633 nm wavelength has been found to be suitable for the characterisation of zinc metal deposits prepared from sulphate solutions. Light from a 633 nm HeNe laser is directed at the zinc surface at > 86 angle of incidence. The light is partially reflected specularly and partly scattered. As described above smoother surfaces have higher specular reflection than rougher surfaces. A detector adapted to measure 633 nm light is positioned to intercept and measure preferentially the specular reflection. In an embodiment of this invention the specularly reflected light is sampled by a single detector which accurately measures the intensity of that specular reflection. It will be evident to those skilled in the art that various lasers and various incoherent light sources may be used at various angles of incidence to realise this embodiment. In particular it will be evident that the use of longer wavelengths will allow angles of incidence nearer to normal and that the use of shorter wavelengths will require angles closer to 90 . A particular advantage of the further preferred embodiment of the invention is that it permits the use of wavelengths at which the electrolyte is transparent, thus allowing measurements to be made on deposits immersed in electrolyte. In another embodiment of this invention a plurality of detectors, each having a different geometry, are used to characterise the reflected light. Conversely, the possibility also exists of using a single detector and a plurality of incident beams. Systems of these types are capable of measuring differences between samples in the dependence of the intensity of the scattered and specularly reflected light on the geometry of the measurement system. For instance, the ratio H/V will be a function of the geometry of the detection system. Hence while two structurally different samples may exhibit the same ratio H/V at a single detector, they may exhibit dissimilar ratios H/V at other detectors with different geometries. Hence, electrodeposits which cannot be discriminated by a single detector system may each exhibit unique reflected light characteristics when a multiple detector system is used, in which each detector has a different geometry with respect to the incident and detected reflected light beams . In another preferred embodiment of this invention the electrodeposit is characterised by measuring the light reflected by the actual electrodeposit formed in the process which is being controlled. For this mode of measurement it will usually be necessary to remove the electrodeposit from the electrolyte because the electrolyte may absorb the incident radiation. For example when zinc electrodeposits are characterised at wavelengths in the infra-red range it is essential that the sample is dry because water absorbs infra-red radiation strongly. By careful selection of the angles of incidence and reflection it is possible to obtain substantial reflection at the shorter wavelengths and thus characterise the electrodeposit while it is still immersed in the electrolyte or when the electrolyte is displaced by water or some other fluid. In commercial electrolytic metal recovery processes electrodeposits are often formed over prolonged deposition periods. For example in the electrowinning of zinc from acidic sulphate solutions deposition times of up to 72 hours, or even longer, are employed. In some instances, particularly when the process is inadequately controlled, the electrodeposit becomes uneven, and gross surface irregularities such as nodules and pinholes may occur. These irregularities, which tend to have a random distribution, can affect the manner in which an electrodeposit reflects light and it may be necessary to adopt measurement procedures which minimise their effect. In one embodiment of this invention the light reflecting properties of the electrodeposits are measured after the deposition of a constant amount of material (i.e. at constant deposition time when faradaic efficiency is constant). More preferably this measurement is made under conditions in which the amount of material which is electrodeposited is sufficiently small to eliminate the occurrence of uneven electrodeposits and to avoid the formation of nodules and pinholes. For the electrowinning of zinc from sulphate solutions, this preferred deposition period prior to the measurement is in the range 0.5 to 8 hours. However, by expeditious selection of wavelength and measurement geometry it is possible to extend this range, which may be particularly advantageous for in-situ measurement in electrolytic cells. There is often an advantage in measuring short duration deposits in that the control of the electrodeposition process can be particularly critical at the start of the deposition period. For instance, if the structure of the zinc electrodeposit is non-ideal when deposition commences, the deposit will often redissolve towards the end of the deposition period and overall the electrodeposition process will be energy inefficient. Fortunately, when large scale industrial processes for the electrolytic recovery of metals are operated continuously, a range of electrodeposits of different ages are usually available. In another embodiment of the invention the electrodeposit for measurement is formed in a pilot cell, i.e. a small scale cell separate from the commercial process. Although the light reflection measurements may be made in-situ in the pilot cell, it may for the aforementioned reasons be necessary to remove the electrolyte from the deposit. There may be an advantage in using a moving wire or foil as the cathode in the pilot cell. When this method is used the conditions in the pilot cell are adjusted until the resultant electrodeposit exhibits light reflecting characteristics which correspond to those of a preferred electrodeposit. For example the concentrations of additives may be changed. When satisfactory conditions have been established the main commercial process is adjusted so that it operates under identical conditions. In a further embodiment of the invention the characteristics of light reflected by an unsatisfactory electrodeposit may be used to diagnose the reasons for inadequate process operation. For instance, if the electrolyte contains an unacceptably high concentration of impurities, the resultant electrodeposit may exhibit unique light reflectance features which can be attributed to a specific impurity or impurities . In such circumstances it may not be possible to control the process by adjusting the concentration of additives and it may be necessary to improve the purity of the electrolyte, for instance by improving the purification of the electrolyte in other process steps which precede the electrodeposition step. In this embodiment of the invention it may be advantageous to conduct the electrodeposition step in a pilot cell, preferably in the absence of additives, which tend to ameliorate and hence mask the effects of any impurities which may be present in the electrolyte. The applicants wish to emphasise that there are many ways in which the light reflecting characteristics of electrodeposits may be measured and used to detect differences in the structure of electrodeposits. The essential features of this invention are the comparison of the characteristics, for example intensity and polarisation, of incident and reflected light beams. The invention contemplates measurement of all possible variations of those features, including, but without prejudice to the generality of the foregoing, changes in the polarisation characteristics of the incident light (e.g. from plane to circularly or elliptically polarised light); in the geometry of the measurement system (i.e. the angle at which the incident beam of light impinges on the electrodeposit and the angle(s) at which reflected light is sampled relative to the electrodeposit); the wavelength or the polarisation of the incident light beam. The following examples indicate ways in which the procedure of the present patent application has been applied, but in no way is the application restricted to the scope of these examples. Example 1 (a) Zinc electrodeposits were formed from an acidic sulphate electrolyte in a pilot cell using a deposition time of 1 hour under a variety of conditions (Table 1) including the presence of beneficial additives (e.g. glue, 2 naphthol and antimony) and impurities which are detrimental to electrolysis. In each experiment the initial zinc, sulphuric acid and cobalt concentrations were 54, 100, and 0.006 g/L respectively, the temperature was 35 C and the current density was 424 A/m 2 . Table 1 Electrodeposition Conditions in Pilot Electrolysis Experiments From Table 1 it can be seen that electrodeposit samples 1 and 2 were formed at a high faradaic efficiency (i.e. current efficiency). Important features of the electrolytes from which these electrodeposits were formed included a satisfactory antimony concentration, which acts at low concentrations as a beneficial additive, and very low concentrations of the detrimental impurities, copper and nickel. Electrodeposits 3 to 6 were formed at lower faradaic efficiencies due either to unacceptably low antimony concentrations or exceptionally high concentrations of the detrimental impurities copper and nickel. The light reflecting characteristics of the electrodeposits were measured at 10.59 jam wavelength using a C0_ laser as a light source. The incident light beam was highly polarised. A detection system was used in which the electrodeposit was scanned with the incident beam at an angle which was almost perpendicular to the electrodeposit. The reflected light beam was detected at a similar angle. The reflected light was sampled as two components. (a) Light polarised parallel to the polarisation of the incident light. (V). (b) Light polarised perpendicular to the polarisation of the incident light. (H) . The results of the reflection measurements are shown in Table 2. Table 2 Reflection of Polarised 10.59 jam Wavelength Laser Light by Electrodeposits Table 2 shows that the electrodeposits which were formed at high faradaic efficiencies (1 and 2) reflect less light polarised parallel to the incident beam (V) , but more light polarised perpendicular to the incident beam (H) . This occurs because the preferred electrodeposits have structures which scatter 10.59 um laser light more efficiently than the electrodeposits formed at low faradaic efficiencies. Reflected light which is polarised perpendicular to the incident beam (H) is predominantly a consequence of light scattering processes, whereas the light polarised parallel to the incident beam (V) mainly occurs by specular reflection. A convenient method of comparing the polarisation of reflected light between samples is the ratio H/V, which is >5 times higher for deposits formed at high faradaic efficiencies than for the samples formed at low faradaic efficiencies. The y H2 + V2 relates to the peak total reflected intensity which, for the purposes of this embodiment, is equivalent to the total reflected light, also provides adequate discrimination between the two sets of deposits. The results in Table 2 shows that it is possible to adjust the conditions for the electrolytic recovery of zinc such that the electrodeposit has either a high ratio (H/V) or I 2 2 ~ a low value of the function - H + V . For instance, when the polarisation ratio (H/V) had a value of the order of 0.2 the electrolysis was conducted at high faradaic efficiency, which is an essential feature of an energy efficient electrolytic process for the recovery of metals. The intensities, V and H, of the reflected light, geometry of the measurement system. Although there is usually considerable flexibility in the selection of the measurement system for detecting reflected light, certain geometries will often be either preferred or even optimum. Obviously for any given measurement system the characteristics of the light reflected by a preferred type of electrodeposit must be determined initially in order to provide a reference for control purposes. (b) In part (a) of this example it was suggested that the structure of an electrodeposit and hence the manner in which it reflects light could be varied by changing the concentration of an additive. The results in Table 3 substantiate this suggestion in the case of the electrodeposition of zinc from an acidic sulphate electrolyte under conditions of high faradaic efficiency using glue as an additive. The ratio of the reflected light intensities in directions perpendicular and parallel to the incident beam (H/V) decreases as the glue concentration increases. The electrodeposits characterised in Table 3 were formed under similar conditions to those described in part (a) of this example, except that the antimony concentration was constant at 0.05 mg/L and 2 naphthol was not used as an additive. The electrolyte was "pure" in that copper and nickel were nominally absent. Cobalt was present at a level of 3.5 mg/L. Table 3 Variation of Infra-red (H/V) , V and Visible Reflectance with Glue Concentration During Pilot Cell One Hour Duration Electrodeposition of Zinc Grazing Angle Glue H/V H 2 + V 2 V ' H 2 + V 2 Relative Concentration 10.6 jum 10.6 jam 8-12 urn Reflectance mg/L Laser Laser Filtered* 0.633 urn 15 0.11 0.14 1.47 1.8 20 0.046 0.29 2.26 2.03 25 0.034 0.3 2.85 2.91 30 0.013 1.65 9 5.9 40 0.0065 3.6 25.5 15.3 *Incoherent light source filtered through a 8-12 am filter. In the preferred embodiments of the invention either the incident light or the reflected light is filtered. However, both the incident light and reflected light can be filtered prior to detection of the reflected light. The geometry of the measurement system used for the results reported in the second and third columns of Table 3 was different from that used to obtain the Table 2 results. The reflected light beam was detected under conditions which favoured the sampling of V compared to H. Hence all of the ratios H/V in Table 3 are lower than those reported in Table 2 for preferred electrodeposits. The 10.6 _um infra-red measurements were made with laser light at near-normal incidence. The 8-12 _um infra-red measurements, reported in the fourth column, were made with filtered unpolarised incoherent light at near-normal incidence. The visible measurements were made at a grazing angle of incidence using a 633 nm HeNe laser. From Table 3 it is apparent that H/V has a wide range, the laser /H2 + V2 has a wider range than the filter ■ f /H2 + V2 " which in turn has a larger range than the visible parameter. The laser and filter methods use a convenient angle of incidence but need a dry deposit surface. The filter method does not offer the range of the laser method but is simpler and cheaper to implement in part because the light is unpolarised. The visible method uses a less convenient angle of incidence and provides the smallest range but could be used on immersed samples. The results in Table 3 substantiate the point that a wide range of measurement techniques are all capable of discriminating changes in deposit structure resulting from a variation in glue concentration. (c) The ratio H/V of eight electrodeposits formed at one hour deposition time was measured at 10.59 am and 0.633 _um using a CO- and a HeNe laser respectively. In both cases a method was used which measures the reflection at near normal incidence to the sample. The results are compared in Table 4. Table 4 Comparison of the Ratio H/V at Two Wavelengths From Table 4 it is apparent that in the instance of near normal incidence the range of the ratio H/V values is much broader at 10.59 μm than at 0.633 μm. For instance, the ratio (H/V) max: (H/V) min is 119 at 10.59 μm compared with 5.2 at 0.633 μm. Furthermore, the ratio H/V values measured at 10.59 and 0.633 μm are very poorly correlated. These results demonstrate the advantage of measuring the reflectance of the electrodeposits at a preferred wavelength, 10.59 μm in the present near normal instance. However, the data of Table 3 demonstrates that there are advantages in making measurements at a different wavelength with a different geometry, particularly if one wishes to perform the measurement whilst the deposit is immersed. Example 2 Samples formed at 3 hours deposition time were taken from a block of 180 cells in an electrolytic zinc plant. Each cell had a nominal capacity of c. 0.5 tonne zinc/day. The reflectances were measured using filtered unpolarised incoherent light (8-12 μm) at near normal incidences as described for the results in the fourth column of Table 3. For each electrolytic zinc plant condition four deposit samples were measured. The results are reported in Table 5 as reflectance values standardised against the reflectance of a standard (a sand-blasted aluminium surface) . Table 5 Reflectance of 3 Hour Deposits Sampled from Electrolytic Zinc Plant Cells and Measured with 8-12 μm Filtered Infra-red The results show that the reflectance of the 3 hour deposits increases as the 48 hour faradaic efficiency decreases. In order to improve electrolytic performance corresponding to condition 3 in Table 5, the additives used to modify the deposit structure should be adjusted until the reflectance of 3 hour deposits is of the magnitude given for condition 1 in Table 5.

Claims: CLAIMS : 1. A method for controlling a process for the electrolytic recovery of metals from solutions containing dissolved metals characterised by adjusting the conditions of electrolysis such that light reflected by the electrodeposit corresponds to that reflected by a preferred electrodeposit structure. 2. A method according to Claim 1, in which the light source used to measure the light reflected by the electrodeposit is polarised. 3. A method according to Claim 1, in which the electrodeposit is characterised by the intensity and/or angular spread of the reflected light. 4. A method according to either of Claims 1 and 2, in which the electrodeposit is characterised by the polarisation of the reflected light, which is resolved as two components which are polarised in orthogonal planes. 5. A method according to any of Claims 1 to 4, in which the incident and/or reflected light is filtered. 6. A method according to any of Claims 1 to 4, in which the light source is a laser. 7. A method according to any of Claims 1 to 5, in which the light reflected by an electrodeposit is measured at a single wavelength. 8. A method according to any of Claims 1 to 5, in which the light reflected by an electrodeposit is measured at a multiplicity of discrete wavelengths. 9. A method according to any one of Claims 1 to 8, in which the electrodeposit is characterised by differences in the intensity of incident and reflected light. 10. A method according to any one of Claims 1 to 8, in which the electrodeposit is characterised by the ratio of the intensities of two polarised components of the reflected light beam which are polarised in orthogonal planes. 11. A method according to any one of Claims 1 to 8, in which the light reflected by the electrodeposit is characterized by differences in the intensities of two components of the incident and reflected light beams, said components being polarised in orthogonal planes. 12. A method according to any one of Claims 1 to 8, in which the light reflected by the electrodeposit is simultaneously characterised by differences in the intensities of two components of the incident and reflected light beams as well as the ratio of the intensities of said components of the reflected light beam, said components being polarised in orthogonal planes. 13. A method according to any one of Claims 1 to 12, in which the reflected light is simultaneously characterised by a multiplicity of detectors, each said detector sampling light reflected at different angles by the electrodeposit. 14. A method according to any one of Claims 1 to 13, in which the conditions of electrolysis are adjusted by varying the concentration of compounds added to the electrolytic cells in order to modify the deposit structure. 15. A method according to Claim 14, in which glue and an antimony compound are added. 16. A method according to any one of Claims 1 to 15, in which the light reflected by the electrodeposit is compared with that reflected by an electrodeposit previously prepared under electrolysis conditions involving high faradaic efficiency, said electrodeposit having a preferred structure. 17. A method according to any one of Claims 1 to 16, in which the electrodeposit is formed in a small scale electrolysis cell separate from the commercial process for the electrolytic recovery of metals. 18. A method according to any one of Claims 1 to 17, in which the characteristics of light reflected by the electrodeposit are measured at a constant deposition time. 19. A method according to Claim 18, in which the deposition time is in the range 0 to 80 hours. 20. A method according to Claim 18, in which the deposition time is in the range 0.5 to 8 hours.

Inventors: ADCOCK PETER ANTHONY, AULT ANTONY ROBERT, EBERHARDT JOHN EDWARD, HAUB JOHN GEORGE, NEWMAN OLIVER MICHAEL GRIFFITH, PRYOR ARTHUR WILLIAM, ADCOCK, PETER, ANTHONY, AULT, ANTONY, ROBERT, EBERHARDT, JOHN, EDWARD, HAUB, JOHN, GEORGE, NEWMAN, OLIVER, MICHAEL, GRIFFITHS, PRYOR, ARTHUR, WILLIAM
Applicants: COMMW SCIENT IND RES ORG, ELECTROLYT ZINC AUSTRALASIA, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR, ELECTROLYTIC ZINC COMPANY OF AUSTRALASIA LIMITED
Application year: 1988
Main classification: C25C1
IPC codes: C25C1, C25C7, C25D11, C25D21