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2009:056
M A S T E R ' S T H E S I S
Optimisation of Influential Factorsin Electrowinning of Tellurium by
Means of PLS Modelling
Seyed Mohammad Khosh Khoo Sany
Luleå University of Technology
Master Thesis, Continuation Courses Minerals and Metallurgical Engineering
Department of Chemical Engineering and GeosciencesDivision of Process Metallurgy
2009:056 - ISSN: 1653-0187 - ISRN: LTU-PB-EX--09/056--SE
Optimisation of Influential Factors in Electrowinning of Tellurium by Means of
PLS Modelling
Master Thesis By:
Seyed Mohammad Khoshkhoo Sany [email protected]
Supervisors: Prof. Åke Sandström (Luleå University of Technology)
Dr. Nils Johan Bolin (Boliden Mineral AB)
Abstract Electrowinning of tellurium is a relatively simple process that can be carried out effectively by employing the existing technology. In order to obtain tellurium with the least amount of impurities, the effects of three main factors were studied, namely current density, free concentration of caustic soda and initial concentration of tellurium. Five elements were chosen as the main impurities: Ag, Bi, Cd, Ni and Pb. A series of 17 experiments arranged in a CCF (central composite faced-centred) design has been carried out and the results were fitted using PLS (partial least squares) method. The effect of free caustic concentration was found to be the most important of the three parameters studied. Current density was also important, yielding the least amount of impurities at high current densities. Tellurium concentration had the smallest effect of the three parameters studied. From the model, current densities of 330-350 A/m2, tellurium concentrations of 90-120 g/l and free sodium hydroxide concentration of around 120 g/l are suggested as optimal conditions for obtaining the purest tellurium.
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Table of Contents Abstract..................................................................................................................... 1 Acknowledgment ...................................................................................................... 3 Aim of the project ..................................................................................................... 4 Chapter 1 – Tellurium ............................................................................................... 5
1.1 Introduction ..................................................................................................... 5 1.2 Geochemistry .................................................................................................. 5 1.3 Resources ........................................................................................................ 6 1.4 Production ....................................................................................................... 6 1.5 Purification...................................................................................................... 9
1.5.1 Vacuum Distillation .................................................................................. 9 1.5.2 Zone Refining ........................................................................................... 9
1.6 Applications .................................................................................................. 10 1.7 Economy ....................................................................................................... 10
Chapter 2- Electrowinning........................................................................................12 2.1 Introduction to Electrowinning ...................................................................... 12
2.1.1 Introduction to electrometallurgy ............................................................ 12 2.1.2 Basic Facts.............................................................................................. 12 2.1.3 Faraday Law ........................................................................................... 14 2.1.4 Current Efficiency................................................................................... 14 2.1.5 Electrode Potential and Nernst’s Equation............................................... 14 2.1.6 Cell Voltage and energy consumption ..................................................... 15 2.1.7 Metal Deposition..................................................................................... 15
2.2 Electrochemistry of Tellurium ....................................................................... 16 2.2.1 Cathodic Reduction of Tellurite .............................................................. 16
2.3 Electrowinning of Tellurium.......................................................................... 18 Chapter 3 - Experimental Part ..................................................................................21
3.1 Design of Experiments................................................................................... 21 3.1.1 Introduction ............................................................................................ 21 3.1.2 Definition of Factors ............................................................................... 21 3.1.3 Definition of Responses .......................................................................... 22 3.1.3 Design of Model ..................................................................................... 23
3.2 Experiments................................................................................................... 25 3.2.1 Investigating the Effect of Periodic Short-Circuited Current.................... 25 3.2.2 Running the Designed Experiments......................................................... 26
3.3 Analysing ...................................................................................................... 27 Chapter 4- Results and Discussion............................................................................29
4.1 Results........................................................................................................... 29 4.2 Fitting the Model and Evaluation of Fit.......................................................... 31 4.3 Using the Model (Optimisation)..................................................................... 35 4.4 Conclusion..................................................................................................... 40 4.5 Future Work .................................................................................................. 41
References................................................................................................................43
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Acknowledgment I am grateful to the financial support given by Boliden Mineral AB which enabled me to carry out this project. Also, I wish to thank Prof. Åke Sandström for his guidance throughout my master studies at Luleå University of Technology and for giving me the opportunity to perform this thesis. Dr. Nils-Johan Bolin is greatly acknowledged for his help with the theoretical and experimental parts of the project. I am very thankful to Paul Kruger who always had time to answer my questions. In addition, I appreciate all the assistance I received from Rolf Danielsson, Amang Saleh, Mikael Widman, Mikael Eriksson and Carina Andersson during the laboratory work. Lars Eric Carlsson is sincerely thanked for analysing my samples and teaching me how to play badminton (you have been very patient). Boliden’s musical band, Anders Nyström, Conny Jacobsson, Gösta Pettersson and his lovely wife, Per Ivar Marklund, Mehran Mousavipour, Ulrika Holmgren and Daniel Holmgren, is appreciated for inviting me to their group and for their open mindedness attitude towards my music. There is no enough space to name all other nice friends that I have found in Boliden. They made a nice and joyful stay for me during the time I was working on the thesis. I am grateful to all of you. If it wasn’t for my friend Sepehr, I would have never been able to come so far in my education. He was the one who gave me his invaluable consults helping me finding the right path. A considerable part of my success is because of him. Thank you Sepehr. Above all, I can’t find suitable words to express thanks to my family for all they have done for me. I am indebted to my dad for his never-ending care and support, and to my mum for her unconditional love. My lovely sisters have been emotionally supporting me during all these times. I love you all!
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Aim of the project Tellurium is a rare metal with a very promising market. Demand for highly pure tellurium to be used in strategic applications is increasing day by day. Boliden AB possesses considerable resources of tellurium both in its gold bearing minerals and in the copper anode slimes. Recovery and purification of this tellurium indeed is a promising business in a near future. This thesis work focuses on a single stage of every tellurium processing line; electrowinning. Electrowinning is an important stage to produce commercial grade tellurium of higher value. Interaction between important factors and their influence on the purity of electrowon tellurium have been investigated in this project. In order to obtain tellurium with a minimum content of impurities, optimum values are suggested by modelling the process, using the partial least square (PLS) method.
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Chapter 1 – Tellurium
1.1 Introduction Tellurium with atomic number of 52 is one of the rarest elements in the earth crust with an abundancy of only 10 ppb. It was first discovered by a Hungarian mineralogist Franz-Joseph Müller von Reichenstein in 1782. The route of the word comes from Tellus which means earth in Latin. Tellurium is categorized amongst mettalloids and is in the 16th group (chalcogens) in the peridoc table of elements. In its crystalline form, i.e hexagonal, tellurium is bright silver-white in colour and extremely brittle. It has 8 natural stable isotops and 21 artificial unstable ones. Tellurium is a p-type semiconductor which shows piezoelectricity behaviour. It boils and melts at relatively low temperatures, 449.5 and 1053.9 °C, respectively. Tellurium is insoluble in water however, it dissolves in sulphuric and nitric acid as well as in dilute hydrochloric acid. There are many properties of tellurium similar to that of sulphur and selenium. However, tellurium is less reactive, more basic, more metallic and shows strongly amphoteric behaviour [1].
1.2 Geochemistry
The chalcogens, including tellurium, are mainly components of intrusive and extrusive magmas and volcanic gases. Hence, it is expected that they belong to volcanic sulphur deposits. Nevertheless, tellurium in fact is not essentially a constituent of the igneous rock-forming minerals. From magmatic and pegmatitic to hydrothermal, especially where these deposits are associated with epithermal gold and silver, tellurium can be found widely in many different types of deposits [2]. Tellurium occurs in small concentration but large quantities in porphyritic as well as pyritic sulphide deposits of copper, nickel and frequently lead. Pentlandite ((FeNi)9S8), chalcopyrite (CuFeS2), pyrite (FeS2), sphalerite (ZnS), and pyrrhotite (FeS) are the most common pyritic deposits that tellurium minerals coexist with them. Copper-molybdenum, lead-zinc, gold, tungsten-bismuth, uranium and mercury-antimony are other common tellurium-bearing deposits [2]. Usually, tellurium occurs as binary minerals in microscopic sizes. Predominantly it can be found as tellurides, although occurrences in other forms such as tellurates, native tellurium and in alloy with selenium has been reported. Tellurium occurs with gold and silver in ten minerals, ten other minerals with bismuth and six with iron. The following list of tellurium minerals includes better-known tellurium minerals: hessite (Ag2Te), petzite (Ag3AuTe2), calaverite (AuTe2), sylvanite (AuAgTe4), altaite (PbTe), tetradymite (Bi2Te2S), rickardite (Cu4Te3), nagyagite (Au(Pb,Sb,Fe)8(Te,S)11) and tellurite (TeO2) [2].
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1.3 Resources There is not any formally stated deposit for tellurium extraction around the world. Like selenium, almost all the tellurium produced worldwide is a by-product of other metals processing lines. Specifically, tellurium is chiefly produced from the anode copper slime that is formed in copper electro-winning plants [3, 4, 5, 6]. However, tellurium can be accumulated during refining of other primary metals such as lead and zinc [1]. Presence of tellurium with gold has been reported mostly as a problem rather than as a resource. Refractoriness of gold and silver tellurides (also known as invisible gold) in cyanide leaching processes arises considerations in order to process gold tellurides separately. There are a number of approaches [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17] - including flotation, hot cyanide leaching, oxidising, (chemically or roasting) and direct chloride leaching - in order to recover the gold from tellurides. Whenever it is economically possible, tellurium or its compounds are recovered from tellurium-bearing gold minerals [10, 13]. For example, wet chlorination has been used in Soviet Union to recover tellurium from tetradymite, BiTe2S, in a gold ore [18]. The recovery of tellurium from secondary resources stands for 10 to 20 percent of world tellurium production [1]. Tellurium recovery from CdTe photovoltaic modules has been the subject of an investigation [19]. Furthermore, recycling of tellurium from the flue dust, which is produced during addition of tellurium into molten steel, has been patented [20]. Since copper ores are the main resources for tellurium production, estimation of reserves for this metal is based on copper reserves. By applying a fixed recovery factor of 0.065 kg tellurium per tonne of copper, the world resource for tellurium is quoted as 34 000 tonnes [1]. The European share of tellurium resources by using this method is about 5 000 tonnes. It should be noted that this quotation stands only for the tellurium in copper minerals and does not include other tellurium-bearing minerals.
1.4 Production Tellurium, as mentioned above, is usually produced from the slimes that are accumulated in the bottom of copper electro-refining tanks. Due to the presence of precious metals, treatment of the slime is of utmost importance. However, in order to treat the slimes for their precious metals, they should be freed of inferior metals such as copper, selenium and tellurium. Tellurium consists 0.5-10% of the slimes and generally is in the intermetallic compounds with copper, silver and occasionally gold. A complete review of different methods for anode slime treatment can be found in literature [21]. These methods are to be used individually or in combination with each other, depending on the content of selenium and tellurium. Here, short summaries of most important methods, regarding tellurium recovery, are given. The first step of slime treatment is usually leaching in aerated dilute sulphuric acid or, alternatively, by oxidative pressure leaching with dilute sulphuric acid under 250-350 kPa of oxygen pressure and at 80-160 ºC. Running the leaching under suitable
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conditions of oxygen pressure and temperature will favour in all copper and tellurium dissolution. Tellurium can thereafter be cemented out as copper telluride by copper shots above 80 ºC. Copper telluride is dissolved by sodium hydroxide to give sodium tellurate in the solution. Then, by acidifying the solution, highly pure tellurium dioxide can be precipitated. Obtained tellurium dioxide is dissolved again by sodium hydroxide in suitable concentration and metallic tellurium with purity of >99.5% is produced from this solution by electrolytic methods (see Fig 1.1) [1, 22]. Soda ash roasting is another alternative in order to treat anode slimes. In this method, all selenides and tellurides are converted to their respective hexavalent state. Hexavalent telluride (sodium tellurate, Na2TeO4), which is insoluble, can be separated sharply from soluble sodium selenate by water leaching. The residue contains tellurium (as sodium tellurate), lead and precious metals and can be treated by sulphuric acid at 90 ºC to convert sodium tellurate to telluric acid and then by means of hydrochloric acid and sulphur dioxide reduce the telluric acid to tellurium. It is also possible to send the residues to a doré furnace. By adding lime and silica, an alkaline tellurium slag is separated from an alloy phase containing precious metals. The slag is washed with water to give sodium telluride in the solution. Analogously to the first method, tellurium dioxide is precipitated from the solution by neutralising. Tellurium dioxide can be re-dissolved again by sodium hydroxide to obtain the electrolyte for electrowinning process [1]. There is also another alternative method namely sulphation mentioned in the literature [18]. In this method, anode slimes are firstly roasted below 350 ºC to obtain metal sulphates. Then, in a second stage of roasting, above 400 ºC, tellurium and selenium sulphates convert to their respective dioxide compounds. Selenium dioxide is expelled at this temperature and tellurium remains as tellurium oxysulphate (TeO2.SO3), which is partly leached with water and is treated further by copper chippings to precipitate copper telluride. The leach residue is sent to a doré furnace and is processed analogously as mentioned before. This method has been very popular in the United States, Soviet Union, Canada, Finland, Hungary, China and Japan. However, emission of sulphide dioxide makes this method undesirable from environmental point of view.
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Fig 1.1. Tellurium production flow sheet from copper anode slimes
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Regarding recovery of tellurium from gold ores, wet chlorination process has been developed [18]. Tellurium, which exists as Bi2Te2S, is firstly collected in a flotation process followed by suspending in a sodium chloride (or ferric chloride) solution. Tellurium is then precipitated by sulphur dioxide.
1.5 Purification Electrolytic methods are usually able to produce metallic tellurium with purity of 99.5%. This purity is satisfactory for most usages of tellurium. However, application of tellurium as a semiconductor in electrical and optoelectrical fields, has been raising the demand for tellurium with ultra high purity i.e. >99.999% (5N). Hence, an extensive volume of researches has been devoted to purification of tellurium [18]. Amongst them, two recent developments and a short summary of each are presented here:
1.5.1 Vacuum Distillation By virtue of difference in vapour pressure between tellurium and various impurities, it is possible to remove them by vacuum distillation. This method is generally capable of reducing the major impurities. However, those impurities, such as Sb, Mg, Zn, Na, As, Se, Cd, K, P, Cs, S, and Hg with comparatively higher vapour pressure (at the working temperature) than that of tellurium are expected to remove with lower efficiencies. In order to remove selenium, some scientific works suggest using an environment of hydrogen gas. A large number of reports have been issued regarding producing of 5N or even 6N tellurium, via single or multiple vacuum distillation [23, 24,
24]. The important factors in vacuum distillation has been found to be soaking time, distillation temperature, vapour pressure of impurities at working temperature, vacuum level and initial concentration of impurities [25].
1.5.2 Zone Refining Since most admixtures have a Keff (effective segregation coefficient) less than unity, zone refining is a relatively easy and effective purification method for tellurium [23, 26]. This method is suggested to be used after vacuum distillation in order to remove those impurities that are difficult to be treated in vacuum distillation. The product purity can reach to 7N tellurium. During the zone refining, a quartz tube is filled with initial tellurium and then a narrow region, starting from one head, is melted and moved along the whole tube to the other end. Impurities with Keff<1 are accumulated at the tail of the ingot while other impurities at the beginning. Several attempts have been made to understand the behaviour of different impurities during zone refining of tellurium. The results show that the value of Keff decreases with increasing atomic number [23, 26]. For the best possible result, multiple cycles of zone refining has also been tried with a good success. It is suggested that besides Keff, zone length and number of passes plays critical roles in regard to the purity [27].
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1.6 Applications Despite small abundance of tellurium in the crust, it has found many industrial usages, especially over recent decades. A short summary of tellurium applications in different industries is given here. For a detailed description, readers are encouraged to see reference number [2]. More than half of the world production of tellurium is used in steel industries. Tellurium addition to ferritic steels improves machinability considerably. In iron castings tellurium has been in use in order to control the chill depth. In this regard, tellurium is 100-150 times as efficient as chromium. Because of its high surface activity, tellurium is a superior absorbent of nitrogen in steel making processes. Minor additions of tellurium to liquid steel act as a mild deoxidiser, enhance the soundness of casting, refine the grain size, counteract the effect of sulphur in ductility, distribute the inclusions evenly, eliminate sulphide clustering on grain boundaries and improves the low temperature impact strength. Tellurium, furthermore, results in an increase of fatigue strength in bearing steels. Besides steel industry, tellurium has been deployed in copper alloying to advance machinability without reducing conductivity. Alloying lead with tellurium will result in higher re-crystallization temperature and better corrosion resistance. Moreover, tin-base bearing alloys take the advantage of tellurium to improve the work-hardening properties, tensile strength and creep resistance. Tellurium compounds has found applications in metal coatings of silverware, aluminium and brass. Pigments and glass, catalysts, self-lubricating composites, explosives as well as medical and biological industries are other areas that tellurium compounds have received attentions to be exploited. Finally, one of the most rapid growing areas of tellurium applications is electronic and optoelectronic industries. Ultra high pure tellurium (+5N) has nowadays a high demand for strategic semiconductor compounds to be deployed in optical devices, nuclear detection, photo refractivity, IR optoelectronics and phase change memories. This growing application implies the need for development of methods to produce the tellurium with impurities in the order of ppb.
1.7 Economy Ever since tellurium unique properties as a semiconductor has been deployed in electronics, especially in photovoltaic cells, the world price has experienced a considerable upsurge. Consequently, consumption of tellurium in other applications decreased and steel industries as well as chemical industries which were the major consumers of tellurium started to use other substitutes for tellurium which was not anymore as cheap as it used to be. Demand for pure tellurium in electronics has been growing and as a result, the price had a pick in 2005. However, the price of tellurium fell due to a decreased demand from Chinese manganese producers to almost 80
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USD/kg (Fig. 1.2). A very recent quotation from metalprices.com states the average price for May 2009 as high as 178 USD/kg.
0
20 000
40 000
60 000
80 000
100 000
120 000
1975 1980 1985 1990 1995 2000 2005 2010
Year
Pric
e [U
SD/M
T]
Fig. 1.2. Tellurium price from 1976 to 2007 [28]. Unfortunately, trends for production and consumption of tellurium worldwide are not easy to determine. One reason is that not all companies intend to report the production and another reason is that some part of the trade is in scrap and semi-refined products. Estimations by USGS, issued in 2007, suggest an annual production of 450 to 500 MT/yr. However, the capacity of tellurium production on the base of copper anode slimes is 1200 MT/yr. This report also predicts that the demand for tellurium in electronics and solar cell application will still be going up while the price will also increase. At the same time, the consumption of metallurgical alloying and chemical is expected to decline due to the high price of tellurium [28].
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Chapter 2- Electrowinning This chapter is about electrowinning in general and then some electrochemical aspects of tellurium is given and finally electrowinning of tellurium will be discussed and important factors in such a process are introduced.
2.1 Introduction to Electrowinning
2.1.1 Introduction to electrometallurgy In contrast to pyro- and hydrometallurgy, which have a long history, electrometallurgy is a very young technology that was born just after the discovery of electric current in the nineteenth century. In 1800, Alessandro Volta made the first electric pile and in the same year, Carllisle and Nicholson used Volta’s pile to decompose water into hydrogen and oxygen. Humphry Davy in 1807 officially was the first one who used the knowledge in electrochemistry for metallurgical aims. He decomposed sodium and potassium from caustic soda and caustic potash in a large battery and, for the first time, identified these two elements as metals. His assistant, Michael Faraday, in 1830 found relationships between the current and amount of deposited material. Since then, considerable developments in purification of metals via electrometallurgical methods have been achieved. Nowadays, it is difficult to imagine production of aluminium from bauxite using methods other than electrometallurgy. More than 50 percent of copper and zinc are similarly produced and purified by electrometallurgical processes. There is also a huge interest within different metal industries to shift already well-established pyrometallurgical routes to electrometallurgical ones [29]. Generally, there are four main categories under electrometallurgy namely electrowinning, electrorefining, electroplating and electroforming [30]. Extraction of metals from aqueous solutions or their salts is called electrowinning while, electrorefining is purification of metals by anodically dissolving the impure metals followed by catholically depositing the pure metals. Electroplating is used to modify surface of metals (and in some cases non-metals) in order to improve appearance or corrosion and abrasion resistivity. A special branch of electroplating, in which the electroplated metal can be removed from the cathode as an entity, is called electroforming. In this chapter a short introduction about principles and concepts of electrowinning, in general, will be given. Then, different issues of tellurium electrowinning, from theoretical aspects to practical difficulties, will be presented.
2.1.2 Basic Facts In metallic conductors, free electrons are responsible for transportation of electric charge while, in electrolytic conductors the charge is transferred by ions. To be able to charge the current into an electrolyte, two electrodes are needed namely anode and
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cathode. These two electrodes are connected to a DC power supply. A simple schematic of an electrowinning circuit, consisting of an electrowinning cell, a switch, a resistance, a voltmeter, and an ampere-meter, is shown in Fig. 2.1. This is the simplest approach to build an electrowinning circuit.
Fig. 2.1. Schematic of the simplest possible electrowinning circuit To maintain a steady current in the circuit showed in Fig 2.1, electrochemical reactions should take place at the interfaces of electrolyte and electrodes. These reactions are heterogeneous reduction or oxidation reactions, which consequently result to reduction or oxidation of compounds. Depending on the type of the electrolyte and the electrodes these reactions can be varied considerably. However, since inert electrodes are always used in electrowinning [29], the reactions can be summarized as below: At cathode, metal cations are always reduced by the following reaction:
Men+ + ne- � Me (2.1) and at the anode, the following reactions take place:
H2O � 2H+ + ½O2 + 2e- (in acidic media) (2.2) 4OH- � 2H2O + O2 + 4e- (in basic media) (2.3)
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2.1.3 Faraday Law In 1830, Faraday formulated the relationship between the amount of deposited material, the quantity of electricity and the chemical equivalent weight of the metal as:
FIt
nM
m ��
���
�= (2.4)
where m is the amount of deposited metal, M is the atomic weights of the metal, n is the valance of the metal, I is the amperage of the electricical current and F is the Faraday constant which is equal to 96485 [A.s].
2.1.4 Current Efficiency The ratio between the weight of the deposited substance obtained by electrowinning and that calculated by faraday law is known as the current efficiency. Current efficiencies are not always 100 percent and this does not mean that the application has failed. It only indicates that there are also some other electrochemical reactions that the current takes part in. These reactions, depending on the type of the electrolytes and electrodes, are evolution of hydrogen, interaction of anode and cathode products, electrolytic reversal of electrode processes, interaction of the product with the electrolyte and interaction of anode components with the electrolyte [29].
2.1.5 Electrode Potential and Nernst’s Equation From a thermodynamic point of view, the change in free energy of reaction 2.1 must be negative in order to forward the reaction towards production of metal on the cathode.
+−=∆ nMeMe GGG (2.5) However, �G is positive and metal always tends to dissolve in the electrolyte. Therefore, a driven force in the form of an electrical current should be applied to make the reaction 2.1 happen.
nEFG −=∆ (2.6) E is called electrode potential and in practice, it is the voltage applied to the cell. Nernst’s equation suggest measurement of electrode potential at any concentration of dissolved metal, +nMe :
++= nMe
nFRT
EE ln0 (2.7)
where 0E is the standard electrode potential and is defined as the voltage of the reduction reaction of one molar solutions at 25 ºC against the hydrogen standard electrode.
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2.1.6 Cell Voltage and energy consumption Also known as cell potential, cell voltage is the sum of the electrode potential difference between anode and cathode, absolute values of anode and cathode overvoltages ( Aη and Cη ) and total ohmic electric resistances:
�+++−= RIEEV CACAc ηη (2.8) Cell voltage has no theoretical significance and it is only important for calculation of energy consumption:
ItVW C= (2.9) By applying Faraday’s law (equation 2.4) and considering current efficiency (eff), specific energy consumption, w, is calculated by equation 2.10[30]:
effMnFV
mW
w C
.== (2.10)
2.1.7 Metal Deposition According to Faraday’s law, the only important factor in determination of metal deposition rate is the quantity of the electricity and other factors like temperature, concentration, flow rate, etc. do not play any role in this regard. Nevertheless, these factors along with some others are responsible for the character of the electrowon metal. Nucleation and crystal growth are two phenomena that take place during metal deposition on the surface of the cathode. If the nucleation rate is larger than the rate of growth of crystals, the deposited metal will be powdery. Conversely, if the crystal growth rate is larger than the nucleation the metal will be coarse-grained. When the process is diffusion controlled, i.e. the concentration of metal ion at the interface is close to zero, nucleation dominates the metal deposition and the product will be fine powder. On the other hand, if the electrochemical reactions are chemically controlled, then the rate of crystal growth will be higher and, again, the coarse-grained metal will be formed on the cathode. There are a number of factors in electrowinning processes, which affect the concentration of metal ions at the interface: Current Density: At low current densities, the electrochemical reactions are occurring slowly and the process is controlled chemically. Consequently, a coarse-grained crystallization occurs on the cathode. The opposite is also true. Concentration of electrolyte: When the concentration of metal ions in the electrolyte is low, the process is, inevitably, diffusion controlled. The product, therefore, is in powder form. Analogously, the opposite is true.
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Temperature: Temperature has the effect similar to the concentration. With increasing the temperature, diffusion increases and the rate of crystal growth will be greater than nucleation rate. As a result, coarse-grained deposition is favoured. Stirring of bath: By stirring the electrolyte, the thickness of the double layer is decreased and diffusion rate will rise. This will result in production of coarser deposited material. The presence of an indifferent electrolyte: The presence of another electrolyte, which does not react at the cathode, can lower the rate of diffusion and, consequently, makes it the rate controlling phenomenon. In this case, a powdery product is expected. Presence of colloidal substances: Addition of colloidal reagents such as agar, glue, gums, sugars, etc., in small amounts (0.05 g/l) has a positive effect to obtain a smooth powdery deposition. The reason is that the presence of these particles on the cathode hinders the crystal growth. Hence, nucleation will be the dominant process, resulting in a fine-grained deposit. More concentration of colloidal reagents causes production of very loose metal deposition on the cathode.
2.2 Electrochemistry of Tellurium In order to optimise the electrowinning process, it is necessary to understand the electrochemical behaviour of tellurium. Besides the thermodynamic calculations, that Pourbaix has published in his Atlas of Electrochemical Elequilibria in Aqueous Solutions, electrochemistry of tellurium and its ions have been studied under a handful studies via polarographic, coulometric, stationary and rotating electrode methods as well as cyclic voltammetry [31, 32, 33, 34]. The results of these studies were in a close agreement with thermodynamic calculations done by Pourbaix [35]. Apart from tellurium, behaviour of selenium that is a probable impurity co-depositing with tellurium has been the subject of studies as well [31, 32, 22].
2.2.1 Cathodic Reduction of Tellurite As an amphoteric element, tellurium passes into solution both in the form of cations and anions. In reduced state, it can form telluride (Te2-) and ditelluride (Te2
2-) and in oxidized state, it can exist in tetra- or hexa-valent compounds. Since in the electrowinning of tellurium in alkali media it is the reduction of +4 state tellurium (tellurite), which is of utmost interest, a short summary of the studies in this regard is presented in this section. For more discussions on other ionic species of tellurium over entire pH range, readers are referred to the introduced references.
Tellurite undergoes a stepwise reduction form Te4+ to Te(0) and finally Te2-, according to below reactions:
TeO3
2- + 3 H2O + 4e- � Te + 6OH- (2.11) Te + 2 e- � Te2- (2.12)
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Lingane and Niedrach [218] showed that reduction of Te4+ occurs at a half wave potential of about –1.2 V, which is very close to that of tellurium reduction to telluride (reaction 2.12). They concluded that Te4+ reduction proceeds all the way to telluride and reaction (2.12) is potential determining step. Therefore, the overall reaction describing the reduction of Te4+ to telluride is as follow:
TeO3
2- + 3 H2O + 6e- � Te2- + 6OH- (2.13) Following the reaction (2.13), there is a complex of other reactions starting with the reaction between partially formed telluride and Te4+ in the double layer resulting in a colloidal dispersion of Te(0) (Reaction 2.14). As the concentration of Te4+ in the double layer falls, telluride starts to react with colloidal Te to form ditelluride and other polytellurides (Reaction 2.15). As a consequence, the electrode surface is freed and a density gradient is formed which will result in a stirring action on the surface of the electrode. This stirring action causes tellurite ions concentration to increase again in the double layer. Polytellurides reduces back to telluride (Reaction 2.16) and finally telluride takes part in reaction with tellurite to produce elemental tellurium on the surface of the electrode (Rea. 2.17). This process cycles repeatedly in the sense that the rate of elemental tellurium deposition is higher than reduction of tellurite to telluride and consequently elemental tellurium deposition becomes possible:
Te(4+) + 2 Te2- � 3 Te (2.14) (x-1) Te + Te2- � Tex
2- (2.15) Tex
2- + 2(x-1) e- � x Te2- (2.16) 2 Te2- + 3 H2O + TeO3
2- � 3 Te + 6 OH- (2.17) In more recent works by Ha et al. [35] and Handle [22], it is postulated that reduction procedure of tellurite ends up in ditelluride, and then ditelluride is diproportionated to telluride by
Te22- � Te + Te2- (2.18)
and again via the reaction (2.17), ditelluride reacts with incoming tellurite ion and precipitates colloidal Te.
As it is discussed above, reduction of tellurite, without considering the route of the reduction, will end up in depositing of tellurium. Nevertheless, it is only the direct reduction of tellurite to tellurium (Rea. 2.11), which gives a dense metallic deposition of tellurium. The tellurium that is produced through the intermediate reactions (Rea. 2.14 to 2.18) is in colloidal form and results to a powdery deposit. Ha et al. [35] carried out a thorough polarization and cyclic voltammetry experiments on tellurite in order to find out the range of potential in which only a dense metallic tellurium can be produced. This range found to be very narrow and it is almost between –0.8 to –0.95
18
V (vs Hg/HgO electrode). However, in industrial scale where a stable dense deposit is not necessary, more negative potentials can be allowed. There is still another issue to be discussed in this regard and that is the presence of impurities in the solution and probability of them to co-deposit with tellurium. Copper has a more positive reduction potential in highly alkaline solution comparing to tellurium and, as a result, deposits during the electrowinning. However, it is possible to precipitate copper to the order of a few ppm by treating the leach liquor with Na2S [2]. Selenium, studied extensively by Handle [22], has similar thermodynamic and electrochemical behaviour as tellurium, consequently, is completely electrowon with tellurium. Other impurities have more negative electrode potential than that of tellurium and are not considered as major problems in the electrowinning process. Nevertheless, nature of resource materials and the method that leach liquors are treated differs in different plants. Therefore, type and amount of major impurities that are reported to the cathode might be different from a producer to the other.
2.3 Electrowinning of Tellurium
Tellurium electrowinning has been tried from both alkaline and acidic medias. There are several Russian investigators [34], who worked on acidic tellurium solutions. Different medias such as hydrofluoric acid, sulphuric acid, hydrochloric acid and acetic acid have been tested. Besides the corrosion problem, control of impurities seems to be a severe challenge in acidic electrowinning. Nevertheless, electrowinning or electrorefining of tellurium in acidic medias can be an interesting subject for further investigations. Production of tellurium from alkali media proceeds according to following general reactions: Cathodic reaction TeO3
2- + 3H2O + 4e- � Te +6OH- (2.1)
Anodic reaction 4OH- � O2 + 2H2O + 4e- (2.20) Overall TeO3
2- + H2O � Te + O2 + 2OH- (2.21) The process has been patented in the United States [36]. Presently, treatment of copper refinery slimes followed by electrowinning of tellurium from an alkali media in industrial scale is carried out in several copper plants such as CCR Division of Noranda Metallurgy Inc. (Canada), Cerro Corporation (Peru), Saganoseki (Japan) [18] as well as Pacific Rare Metals Industries (Philippines) [2]. Parameters for electrowinning technique are generally categorized into electrochemical and physical parameters. Some electrochemical parameters include composition of electrolyte, temperature, current density, concentration of soluble impurities, type and amount of additives, presence of suspended solids and electrical conductivities. On the other hand, physical parameters are cell arrangements, anode-cathode distance, current distribution, electrode cleanliness, deposition time and
19
electrolyte flow [22]. Finally at the end of the process, performance of electrowinning can be judged by evaluation of different indicators such as purity of the electrowon deposit, current efficiency, morphology of the deposit and so on. Table 2.1. Working parameters for tellurium electrowinning in two industrial plants [2]
Parameter CCR Refinery PRM Refinery Te [g/L] in fresh electrolyte 150-200 250 in spent electrolyte 90-140 80 NaOH [g/L] in fresh electrolyte 40 85 in spent electrolyte 80 200 Electrolyte Temperature [ºC] 40-45 45 Flow rate [L/min] 60 40 Number of anodes 15 9 Number of cathodes 14 8 Cathodic current density [A/m2] 160 160 Cell Voltage [V] 2.0-2.5 2.5 Deposition time [days] 3 8 Current efficiency [%] ca 90 90
For electrowinning of tellurium in alkaline solutions, main parameters are initial concentration of tellurium in the solution, concentration of free sodium hydroxide, batch temperature of the electrolyte, current density, as well as agitation (or flow rate). In Table 2.1, a summary of operating conditions in CCR Division of Noranda Metallurgy Inc. and Pacific Rare Metals Industries is given. In some older studies, other parameters like material of cathode and anode, and distance between them have been discussed [37]. However, it is presently known that the electrode distance has no effect on purity of deposited tellurium. The best material for cathodes and anodes are also found to be stainless steel due to the ease of stripping. Table 2.2 summarizes the optimal parameters suggested by different investigators. Table 2.2 declares that the values of parameters are not similar in all investigations. One reason, indeed, is the difference between the source material as well as the method of obtaining the electrolyte, which will raise the difference in type and concentration of impurities. Another reason is the different objective of the studies. For example Handle [22] were focused on decreasing co-deposition of lead and selenium while, Ha et al. tried to get a stable deposit. It is important to find out the relation between each single parameter and a performance indicator. However, owing to the complexity of inter-relationships between the parameters and performance indicators listed above, it would be very difficult to achieve this goal. This complexity is pointed out by Handle [22] under two main categories: ��A specific setting for a parameter may improve one performance indicator at the
expense of another. For instance, an increase in temperature raises the quality of deposit, but it also lowers the current efficiency and purity.
��There is an unknown synergetic cause-effect between the parameters. This makes
it difficult to identify the main parameter that is out of balance.
20
Table 2.2. Results of laboratory- scale studies on electrowinning of tellurium Parameter Napolitano [37] Soshinkova [34] Handle [22] Ha [35] Te [g/L] min 75 100-200 120 50 NaOH [g/l] ND 60-80 ND 100 Cathodic current
density [A/m2] 200 60 250 250 Temperature [ºC] 46 40-45 35 40 Flow rate [L/min] ND ND 5 ND Notes ��No agitation
��Cathodes protection by plastic membrane
In order to understand this complexity, one can take the advantage of using the techniques of “design of experiments” (DOE) to outline a series of goal-oriented experiments in order to use statistical models in analysing the data extracted from the experiments. This will be discussed in the next chapter where some theoretical aspects about the statistical method used in this project to design and analyse the experiments are given.
21
Chapter 3 - Experimental Part This chapter is devoted to describing the practical part of the thesis i.e. designing of the experiments and practical details of performing them.
3.1 Design of Experiments
3.1.1 Introduction Design of experiments (DOE) nowadays has found a wide application in different industries such as, chemical, polymers, car manufacturing, pharmaceutical, foods, pulp and paper, marketing etc. The main idea at the back of the DOE is to get maximum possible information from as few experiments as possible. DOE can be used to develop a new process, enhance an existing process, optimise performance of a process, screen main factors in a process, or to minimize the cost of a process and pollution [38]. For detailed technical information about DOE, readers are referred to literature [38, 39]. In this investigation, a composite design was chosen in order to model the relation between factors and responses and finally optimise the electrowinning process of tellurium to get minimum amount of impurities co-deposited with tellurium. For this purpose, the software designed and developed by UMETRICS AB called MODDE 8.0 has been employed. A complete description of important factors and results indicators (responses) in tellurium electrowinning has been given previously in section 2.3. Here, definition of factor and responses are reviewed from modelling point of view.
3.1.2 Definition of Factors As it was mentioned earlier, the main factors in tellurium electrowinning are generally addressed as current density, flow rate, tellurium concentration in electrolyte, concentration of free sodium hydroxide and temperature. In most investigations, optimum temperature has been found to be 35-45 ºC. Thus, in this project, temperature has been constant at 40 ºC. On the other hand, high agitation (flow rate) decreases thickness of diffusion double layer and favours a coarse-grained crystal deposition, which inhibits dendrite growth of the metal and finally reduces the entrapment of impurities on the surface. Then, this variable also has been kept constant by keeping the stirring rate on a medium-to-high stirring action equivalent to 280 rpm. By reducing variables form five to three, it is possible to design a CCF model with 17 experiments instead of at least 29 experiments in the case of having five factors. The positive effect of using a periodic short-circuiting current in electrowinning processes has been understood for a long time [40]. This will result in disturbing the diffusion layer and will help the deposition to grow more uniformly and with coarser
22
crystals. The effect of periodic short-circuiting current has been tried in two preliminary tests, while other parameters have been kept constant. It was observed that the concentration of most impurities (Ag, Bi, Cd, Cu, Fe, Ni, Pb and Sb) decreased in a significant order. The details are presented in section 3.2.1. Table 3.1 summarizes the parameters and the range they have been investigated. The range of the parameters has been concluded from a collection of references [1, 2, 18, 22,
34, 35, 37]. Table 3.1. Parameter definition and values Parameter Abbreviation Min. Max. Current Density [A/m2] i 150 350
Tellurium Concentration [g/l]
Te 20 200
Free caustic concentration [M]
NaOH 1 3
Temperature [ºC] - (constant) 40
Agitation [rpm] - (constant) 280
Periodic short-circuiting [sec/sec]
- (constant) on 0.5/10
Obviously, the initial concentration of impurities is also important in their abundance in the deposited product. However, in this study, since the same solution has been used in all experiments and because of the small area of the cathodes and consequently very little amount of deposition of impurities on the cathode, the change in impurities concentration from one test to another has been neglected.
3.1.3 Definition of Responses In this project, the amount of impurities in the deposited tellurium has been investigated as the responses. Produced tellurium obtained from each experiment was sealed and sent to laboratory for analysis. More details regarding analysing will be given in section 3.3. As it is shown in Table 3.2 five impurities including silver, bismuth, cadmium, nickel and lead have been nominated as the responses. These impurities have been selected amongst a number of 47 elements considering that they can be a potential danger to tellurium purity. Table 3.2. List and settings of responses Elements Unit Transform PLS Scale Type
Ag ppm Power Unit Variance Regular Bi ppm Power Unit Variance Regular Cd ppm Log Unit Variance Regular Ni ppm None Unit Variance Regular Pb ppm Log Unit Variance Regular
23
The necessity to transform each response and the best type of transformation has been checked by using Box Cox method, before fitting the model [38].
3.1.3 Design of Model To be able to perform RSM (response surface modelling) study on three varying parameters, a CCF (central composite faced-centred) model has been used. Although a CCC (central composite circumscribed) design seems to be more accurate, because of investigating each parameter in five levels instead of three in CCF, lack of raw tellurium dioxide and other practical problems encouraged using a CCF design. It should be noted that CCF design still has good abilities for investigating quadratic relations and curvatures in order to produce a reliable RSM study. Design matrix is shown in Table 3.3 and Fig. 1 shows the schematic design region of experiments. Table 3.3. Design matrix
Exp. No.
Current Density Te Free
NaOH1 -1 -1 -12 1 -1 -13 -1 1 -14 1 1 -15 -1 -1 16 1 -1 17 -1 1 18 1 1 19 -1 0 0
10 1 0 011 0 -1 012 0 1 013 0 0 -114 0 0 115 0 0 016 0 0 017 0 0 0
The region of design, as Fig. 3.1 depicts, is a cube and is completely symmetrical. Each dimension of the cube stands for one parameter. The experiments are shown with orange points on the cube. There are eight experiments on the corners, six on the faces, and three replicated experiments in the centre, which totally make 17 experiments. The equivalent experimental conditions are finally presented in Table 3.4.
24
Figure 3.1. Region of experiments in a CCF model with three parameters and three replicated centre experiments. Table 3.4. Designed series of experiments
Exp. No. Current Density [A/m2]
Te [g/l]
Free NaOH [g/l]
1 150 20 402 350 20 403 150 200 404 350 200 405 150 20 1206 350 20 1207 150 200 1208 350 200 1209 150 110 8010 350 110 8011 250 20 8012 250 200 8013 250 110 4014 250 110 12015 250 110 8016 250 110 8017 250 110 80
25
3.2 Experiments
3.2.1 Investigating the Effect of Periodic Short-Circuited Current In order to understand if a periodic short-circuited current has a positive effect on impurities or not, two preliminary tests, one using short-circuiting and one without it, was carried out. Other parameters including current density, concentration of free caustic soda, temperature and tellurium concentration has been kept constant on values suggested by Y.C. Ha et al [35]. Tellurium dioxide used to prepare the electrolytes had purity of 99.3% and it was provided by Sigma-Aldrich. Firstly, required amount of sodium hydroxide to suffice participation in reaction with tellurium dioxide (Reaction 3.1) as well as providing free sodium hydroxide was dissolved in deionised water.
TeO2(s) + 2NaOH � Na2TeO3(aq) + H2O (3.1) Afterwards, tellurium dioxide was added to the caustic solution and mixed till a clear solution was obtained. In order to have impurities in the electrolyte, similar to a real electrolyte that can be obtained from an industrial process, 50 ppm of certain elements such as Cu, Ag, Se, As, Sb, Bi, Pb, Ni, Co, Fe, S, P, F, Mn, Mo, Cd, Zn, Sn, and Si were added in the form of oxides, or other compounds. The solution was heated up to 80 ºC and stirred for 2 hours. Finally, the solution was filtered and filtrate has been considered as the electrolyte. The set-up of electrowinning experiments consisted of a two litre glass beaker as the cell, two stainless steel plates of 10cm x 4cm as the anodes, a cylindrical stainless steel of D4.4cm x L1.2cm with total surface area of 39 cm2. The electrode distance was 5 cm. 1.5 litre of electrolyte was used for test 0-1. In order to prepare the electrolyte for the second test, the equivalent amount of deposited tellurium from test 0-1 was added to the spent electrolyte. A summary of experimental conditions and results are given in Tables 3.5. Table 3.5. Preliminary experiments to investigate effect of periodic short-circuiting
Parameters Deposit analysis
Exp. Code
i [A/m2]
Temp. [ºC]
Stirring [rpm]
Short-Circuit
Te [g/l]
Free NaOH [g/l]
Ag [ppm]
Bi [ppm]
Cd [ppm]
Ni [ppm]
Pb [ppm]
0-1 250 40 280 No 50 95 12.7 2420 676 10.1 8580 0-2 250 40 280 Yes 50 95 5.48 1820 202 3.8 6720
The positive effect of using a periodic short-circuited current is apparent from the table above. The length of short-circuiting was kept on half a second for each 10 sec of direct current passing.
26
3.2.2 Running the Designed Experiments This part of experiments has been carried out in a similar manner to previous part. However, in order to run 17 experiments with consuming minimum amount of tellurium dioxide, three synthetic electrolytes with 20, 110 and 200 g/l of tellurium were prepared and the run order was set as shown in Table 3.5. Following this order, after completing each experiment at a specific tellurium level, the amount of caustic soda required for the following experiment was adjusted to the required level. After completing each test, the amount of deposited tellurium was added to the spent electrolyte. Finally, the electrolyte was filtered and the volume was adjusted to one litre, to obtain a fresh electrolyte for the next experiment. Table 3.5. The order and the conditions of experiments
Exp No. Run
Order Current
Density [g/l] Te [g/l] Free
NaOH [g/l] Temperature
[ºC] Stiring Per. Short-Cir.
(0.5s/10s) 2 1 350 20 40 40 M-H* Yes 13 2 250 110 40 40 M-H Yes 4 3 350 200 40 40 M-H Yes 1 4 150 20 40 40 M-H Yes 3 5 150 200 40 40 M-H Yes 17 6 250 110 80 40 M-H Yes 11 7 250 20 80 40 M-H Yes 9 8 150 110 80 40 M-H Yes 12 9 250 200 80 40 M-H Yes 10 10 350 110 80 40 M-H Yes 16 11 250 110 80 40 M-H Yes 15 12 250 110 80 40 M-H Yes 5 13 150 20 120 40 M-H Yes 7 14 150 200 120 40 M-H Yes 6 15 350 20 120 40 M-H Yes 8 16 350 200 120 40 M-H Yes 14 17 250 110 120 40 M-H Yes
* M-H abbreviates for medium to high, which was set to 280 rpm. During these experiments the cathodes were stainless steel plates with the size 4cm x 7cm which was taped in a way to have 32 cm2 of total cathodic area. By using the Faraday’s law (equation 2.4), duration of each experiment was calculated in the way that almost three grams of tellurium would be deposited after each experiment. Figure 3.2 shows the set-up of the experiments as well as the apparatus to provide the current control.
27
Figure 3.2. (Left) The set-up for laboratory scale of electrowinning of tellurium. (Right) Electrical apparatus providing direct current and programming for the periodic short-circuiting. Before the experiments, cathodes were carefully washed and rinsed with acetone and weighted. After the tests, cathodes were immersed and kept in stirred hot water for one hour in order to wash out any remaining electrolyte. Then, the cathodes were secured and kept for 24 hours to be completely dried. Consequently, the cathodes were weighted and tellurium was stripped off the cathodes, sealed, coded and sent to the laboratory for analysing. All other required information such as duration of the experiment, amperage, voltage and their variation over the time was accurately recorded by the computer software LabView and were used in the final calculations of actual current density and current efficiency.
3.3 Analysing The methods for tracing impurities in materials such as Te, Cd, In, Ga, As and Se which form the basis of modern electronics, have been under extensive investigation recently. According to Swenters [41] the analytical technique to study these materials should have five main features:
��Panoramic elemental capability (non metals like S, Cl, Br, I, P included) ��Good detection limits (<10 ppb) ��Uniform sensitivity and maximum selectivity ��Short analysis time ��Good precision and accuracy
28
Presently, two techniques namely spark source mass spectrometry (SSMS) and glow discharge mass spectroscopy (GDMS) are economically available for highly pure materials and can satisfy all the five features to a good extent. Application of other methods such as radio frequency glow discharge optical emission spectrometry (RF-GDOES) as well as inductively coupled plasma mass spectrometry (ICP-MS) have also been used by other researchers analysing trace concentrations of impurities in high purity tellurium [42]. In this project, however, the methods mentioned were not available. In addition, since the purity of tellurium that is obtained by electrowinning varies between 3N and 4N, other methods with higher detection limits (around at least 10 ppm for each impurity) could satisfy the goals of the project. Therefore, the samples were sent to ALS Laboratory Group, which deploys ICP-MS and ICP-AES techniques*. The detection limits offered by ASL were not sufficiently low for a number of impurities such as Al, Ba, Ca, Fe, K, Mg, Na, P, S, Zn and Ti. Nevertheless, it was the best available option for this study. As it will be shown in the next chapter, apparently the method used to analyse the samples was not reliable enough to make a robust model out of the results. One reason for this can be bad detection limit for some important elements such as Fe and Ca. Another reason can be the wide spectrum produced by tellurium, which causes various interferences with other elements and, as a result, the inaccuracy in the measurements. This implies that for future investigations, SSMS or GDMS should be used. The results of the experiments and details of modelling procedure will be presented during the next chapter.
* The company’s internal code for the service package is ME-MS61c. The description of the method is available at http://www.alsglobal.com/Mineral/ALSContent.aspx?key=75
29
Chapter 4- Results and Discussion In this chapter, the results of the experiments are presented and the PLS model which has been constructed on the base of these results will be introduced. Finally, the model will be analysed in order to suggest the optimised values of each parameter in electrowinning of tellurium.
4.1 Results The samples were analysed for 47 elements covering all the impurities that have been found to have an impact on the process. When considering the final analyses, the impurities can be categorized as follow: A) Elements with very low concentrations: This group are those impurities that have a good detection limit in the analysing method and their concentrations were low enough (<5 ppm) in all 17 samples: As, Be, Ce, Co, Cs, Ga, Ge, Hf, In, La, Li, Mo, Nb, Rb, Re, Sc, Sn, Sr, Ta, Th, Tl, U, V, W, Y, and Zr. B) Elements with bad detection limit: This group includes impurities that can be a possible threat. However, because of the low detection limit of the analytical method, their analysis in all or a part of samples were not available: Al, Ba, Ca, Fe, K, Mg, Mn, P, S, Sb, Se, Ti and Zn. Amongst these impurities Se is the most important one. Selenium has been reported as one of the most problematic impurities for tellurium. However, the amount of Se in all the experiments was below detection limit (10 ppm). This arises again some cautious and questions about the reliability of the analytical procedure. C) Elements with no importance: Copper and sodium can be categorized in this group. Copper has a concentration of 30 to 150 ppm in the samples. However, since copper can be removed from electrolyte by cementation, it was not considered as a problem. Sodium had a higher concentration of 200 to 800 ppm. It is suggested that this high amount of sodium comes from the remaining electrolyte on the cathode and can be eliminated to a great extent by washing the deposited material in a more effective way after the tests. D) Elements with unreliable results Only Cr falls in this group. Results for Cr in four experiments were reported to be exactly 10 ppm. The rest of the experiments had 20 ppm of Cr. As a result, chromium has also treated like other elements in group B. E) Impurities with significant importance: The last group contains five elements that have high concentration to be considered as a threat and, in addition, their concentration is in the range of the detection limit in all samples. These are Ag, Bi, Cd, Ni and Pb, which have been introduced previously in section 3.1.3 and the model has been built based on the result obtained for these elements.
30
The ultimate worksheet of the experiments, including the responses, is shown in Table 4.1. Table 4.1. Ultimate worksheet including actual parameters and responses
Experiment Parameters* Deposit analysis
Exp. No.
Current Density [A/m2]
Te [g/l]
Free NaOH [g/l]
Ag [ppm]
Bi [ppm]
Cd [ppm]
Ni [ppm]
Pb [ppm]
Weight of deposit
[g]
Current Efficiency
[%]
1 156 20 40 49.4 82.4 20.2 46 450 2.7238 91.61 2 315 20 40 47.9 47.1 10.9 12 192 3.5541 93.62 3 153 200 40 18.2 38.8 84.4 48 740 2.8071 97.16 4 353 200 40 18.5 9.7 53.6 5 643 2.8078 97.88 5 150 20 120 18.4 99.6 31.2 104 236 2.6917 93.35 6 353 20 120 1.5 188 39.5 100 150 2.7185 95.01 7 153 200 120 13.2 327.6 82 88 472 2.7455 98.61 8 353 200 120 0.5 150 2.5 65 120 2.8222 98.52 9 159 110 80 22.8 107.7 136.2 63 774 2.9216 95.15
10 353 110 80 12.8 72.4 75.6 72 308 2.8543 95.87 11 262 20 80 15.9 57.3 14.7 69 216 2.8781 99.58 12 262 200 80 20.1 177.6 115.8 51 543 3.0008 99.07 13 266 110 40 20.5 26.1 107 8 1320 2.8052 96.98 14 253 110 120 2.1 48.3 14 133 119 2.8506 97.91 15 266 110 80 3 47.4 16.8 54 138 2.9342 98.18 16 259 110 80 16 92 91.5 95 350 2.9105 97.74 17 262 110 80 16.8 51.2 99.2 50 822 2.2972 97.53
* Constant parameters including temperature, stirring rate and periodic short-circuiting are not mentioned in this table. Table 4.1 provides that actual current densities applied to experiments are not exactly the same as they were designed (Table 3.3). The small cathodic area in the laboratory scale of tellurium electrowinning implies that even a small variation in adjusting the amperage of the cell will result in considerable changes in the current density. Furthermore, there have been some small changes in the amperage of the cell over the time. Therefore, an average of the amperage over the time has been considered as the final cell amperage and current densities have been calculated according to that average measure. Nevertheless, this does not affect the functionality of the model. Composite designs have a good ability to tolerate these small changes in the parameters. Besides the chemical analysis of the electrowon tellurium, the morphology of the cathodes was also differing from one experiment to the other. As it was expected from the literature [22], a colloidal black surface layer (BSL) was observed in all cathodes (see Reaction 2.14). BSL is a powdery but yet stable layer, which is formed on the base metallic tellurium layer and its thickness, according to Handle [22], has a direct relation to agitation rate and potential. Higher potential with higher agitation rate increases the thickness of BSL. BSL can potentially provide a suitable place for impurities to accumulate. However, it has been shown in this study that in the same agitation rate BSL thickness can vary considerably. Furthermore, periodic short-
31
circuiting has certainly positive effect on avoiding BSL growing. The reason can be attributed to the disturbing the diffusion double layer, which favours crystal growth rather than powder formation. This can also be the reason that higher current densities with lower impurities and thinner BSL was achieved in this study. Unfortunately, in this project the thickness of BSL has only been recorded as qualitative response in two levels; thin and thick. Attempts to model BSL thickness on the base of available data was not successful. It seems that it is necessary to have agitation rate as a varying parameter along with current and periodic short-circuiting parameters, and then to include the variation of the thickness of the BSL as a response in the model.
4.2 Fitting the Model and Evaluation of Fit Since there are five responses, the model has been fitted using PLS (Partial Least Squares) method. Results of the first fit showed that exclusion of term i*i (second order effect of current density) which was insignificant in all responses improves the model. Furthermore, experiment No. 15 was found to be a weak outlier for Ag, Cd and Pb. After excluding term i*i as well as experiment No. 15, the model was refitted and the improvement in the fit was considerable. PLS total summary with four components and summary of fit are presented in Fig. 4.1 and 4.2.
0.0
0.2
0.4
0.6
0.8
1.0
Comp1 Comp2 Comp3 Comp4
R2
& Q
2
PLS Total Summary (cum) R2Q2
Fig. 4.1. PLS total summary plot
32
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Ag~ Bi~ Cd~ Ni Pb~
Summary of Fit
N=16 Cond. no.=3.763DF=7 Y-miss=0
R2Q2Model ValidityReproducibility
Fig. 4.2. Summary of fit Fig. 4.1 provides that total measure of fit (R2) after the fourth component is 0.79, which is excellent for a model with five responses. However, the prediction measure (Q2) is not acceptable at all. By fractioning the overall performance statistics to individual R2s and Q2s, Fig.4.2, it can be seen that R2 for silver is 0.93 and for bismuth, cadmium and lead it is 0.76, 0.73 and 0.72, respectively. Finally, nickel is fitted with R2 equal to 0.82. Model validity for all responses is higher than 0.25, which means that there is no any significant lack of fit. Nevertheless, Q2 values are not acceptable in all responses. Residuals lie all between normal lines (Fig. 4.3). Therefore, the residuals are distributed normally and there are no distinct outliers. Yet, some gaps between observations in all responses states that non-linear relationships between some factors and responses are probable. This is also confirmed by coefficient list (Table 4.2) in which the values of quadratic coefficients are relatively considerable (|Bij|>0.2) especially for Bi, Cd and Pb.
33
Fig 4.3. Normalprobability graphs for all responses By plotting the total variable importance graph (VIP), it is possible to evaluate the total importance of each term on the model. This is shown in Fig. 4.4. All terms have the importance coefficient higher than 0.7 except for the last three terms. However, as it is shown in Table 4.2, these terms (underlined) have influence on some individual responses and excluding any of these terms does not help improving the model, but conversely reduces the goodness of fit. Table 4.2. List of normalized coefficients Term Ag~ Bi~ Cd~ Ni Pb~ i -0.40 -0.24 -0.32 -0.13 -0.27 Te -0.25 0.14 0.27 -0.20 0.30 NaOH -0.74 0.66 -0.25 0.83 -0.62 Te*Te 0.12 0.35 -0.46 -0.15 -0.22 NaOH*NaOH -0.12 -0.05 -0.44 -0.12 -0.30 i*Te 0.07 -0.25 -0.20 -0.14 -0.12 i*NaOH -0.23 0.06 -0.15 0.06 -0.17 Te*NaOH 0.20 0.27 -0.26 -0.08 -0.10
34
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
NaO
H i
Te*T
e
Te*N
aOH Te
NaO
H*N
aOH
i*Te
i*NaO
H
VIP
Variable Importance Plot
Fig. 4.4. Variable importance plot
-1.0
-0.5
0.0
0.5
1.0
2 4 6 8 10 12 14 16
Sta
nda
rdiz
ed R
esid
ual
s
Run Order
Ag~
1
2
3
4
5
6
7
8
9
10
11
12
13 14
1617
-1
0
1
2 4 6 8 10 12 14 16
Sta
nda
rdiz
ed R
esid
ual
s
Run Order
Bi~
1
2
3
4 5
6
7
8
9 10
11
12
13
14
16
17
-1.0
-0.5
0.0
0.5
1.0
2 4 6 8 10 12 14 16
Sta
nda
rdiz
ed R
esid
ual
s
Run Order
Cd~
1
2 3
4 5
6
7
8
9
10
11
1213
14
1617
-1.0
-0.5
0.0
0.5
1.0
2 4 6 8 10 12 14 16
Sta
nda
rdiz
ed R
esid
ual
s
Run Order
Ni
1
2
3
45
6
78
9
10
11 12
13
14
16
17 -1.0
-0.5
0.0
0.5
1.0
2 4 6 8 10 12 14 16
Sta
nda
rdiz
ed R
esid
ual
s
Run Order
Pb~
1
2
3
4
5
67
89
1011
12
13
14
16
17
MODDE 8 - 2009-06-22 15:32:30 Fig. 4.5. Residuals versus run order for all responses
35
1
2
3
4
5
6
7
1 2 3 4 5 6 7
Obs
erve
d
Predicted
Ag~
12
345
6
7
8
9
1011
12 13
14
1617
5
10
15
4 6 8 10 12 14 16 18
Ob
serv
ed
Predicted
Bi~
1
2 3
4
5
6
7
8
9
1011
12
13
14
16
17
0.5
1.0
1.5
2.0
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Obs
erve
d
Predicted
Cd~
1
2
3
4
56
7
8
9
10
11
1213
14
1617
0
20
40
60
80
100
120
0 20 40 60 80 100 120
Ob
serv
ed
Predicted
Ni
1
2
3
4
5 67
8 91011
12
13
14
16
17
2.0
2.2
2.4
2.6
2.8
3.0
2.0 2.12.2 2.32.4 2.5 2.62.72.8 2.9 3.03.1
Ob
serv
ed
Predicted
Pb~
1
2
34
5
6
7
8
9
10
11
12
13
14
16
17
MODDE 8 - 2009-06-22 15:37:53 Fig. 4.6. Observed versus predicted values for all responses Fig. 4.5 shows the residuals plotted against run order and it states that there is no any systematic relation between residuals and run order. As the final step in order to evaluate the fit, it is worthy to look at observed versus predicted values for all responses. In these graphs, Fig. 4.6, it is visually shown that Q2s are low. The reason for low Q2s can be traced back to the analysis problems and lack of accurate control on the other constant parameters like concentration of different impurities in the electrolyte. To conclude, the present statues of the model is the best possible fit, and can be effectively used for explaining the relationship between the parameters and responses with a good reliability. However, the model is not a trustable tool in order to predict the amount of impurities for new series of data. Hence, in this report it will be tried to optimise around the experienced sub- regions and not move very far from them.
4.3 Using the Model (Optimisation)
The model, as it is mentioned, can be used with a good reliability to evaluate the main effects of parameters on the responses as well as inter-relation of both parameters and responses. In this regard, scattering plot of loadings, as it is a common tool in evaluating PLS models, cannot be of interest because of having four PLS components [38]. Instead, normalized coefficients (Table 4.2) can be used to interpret the importance of parameters and their interactions on each response. This is shown graphically in Fig. 4.7. .
36
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Ag~ Bi~ Cd~ Ni Pb~
Normalized Coefficients (Regular) iTeNaOHTe*TeNaOH*NaOHi*Tei*NaOHTe*NaOH
Fig. 4.7. Normalized coefficients for all responses The strong effect of concentration of free caustic soda is again confirmed with this plot. It should be noted that this parameter has positive effect on the amount of Bi and Ni, and negative effects on other impurities. This will complicate the process of optimisation. The case is the same for tellurium concentration (Te), which has positive effect only on Bi, Cd, and Pb. Current density (i) has negative effect on all responses1. The effects of other quadratic terms vary considerably from one response to another. Optimizer tool in MODDE cannot be used here because of low predictability of the model. Nevertheless, Fig. 4.8 to 4.10 provide contour plots for impurities in relation to current density and tellurium concentration in different levels of caustic concentrations. These plots can be used to find out the experiment region in which the amounts of impurities are as low as possible.
1 It is worthy to state that “positive” and “negative” terms here are mathematical terms. For instance, when it is said that current density has “negative” effect on all impurities, it means that for higher current densities the amount of impurities will be smaller. In fact, higher current density has “positive” effect on the process.
37
Fig. 4.8. Variation of impurities (ppm) versus tellurium concentration (g/l) and current density (Amp/m2). Free NaOH concentration is kept constant at its lower level (40 g/lit)
Fig. 4.9. Variation of impurities (ppm) versus tellurium concentration (g/l) and current density (Amp/m2). Free NaOH concentration is kept constant at its medium level (80 g/lit)
38
Fig. 4.10. Variation of impurities (ppm) versus tellurium concentration (g/l) and current density (Amp/m2). Free NaOH concentration is kept constant at its higher level (120 g/lit) As it was expected from the coefficient plot, amounts of Ag, Pb and Cd were reduced with increasing caustic concentration, while Bi and Ni behave vice versa. It is also visible that how increasing current density generally lowers the amount of impurities. Since lead is the strongest impurity, minimization of this element would be the first priority in order to find the optimum region. It is needless to state that if other impurities, for any reason, would become of more importance to be eliminated this approach can be changed. From these figures, it is possible to assign several experimental regions in order to obtain low amounts of impurities. These areas can be found in all three figures and are summarized below. a) Free caustic concentration to be kept at its low level (Fig. 4.8): The corner in the right bottom part of the graph represents a minimum amount for all impurities, except for Ag. However, the amount of silver is not considerable and varies between 30 and 40 ppm in this region. Current density and tellurium concentration vary between 300-350 amp/m2 and 20-60 g/lit, respectively. A rough estimation of the amount of impurities is presented in Table 4.3. b) Free caustic concentration to be kept at its medium level (Fig. 4.9): At right bottom corner again, where current density is 320-350 amp/m2 and tellurium concentration is 20-50 g/lit, an optimum region can be located. Nickel amount in this area is relatively high and if higher amount of lead can be tolerated in the deposit, the
39
corner at right top can be suggested in order to have less nickel and similarly all other impurities. Different amounts of impurities in regard to these two regions can be found and compared in Table 4.3. c) Free caustic concentration to be kept at its high level (Fig. 4.10): In this case, which is the best case for lead, the area in the right middle part of the graph can be suggested as a possible safe region. The region is limited to current density of 330-350 amp/m2 and tellurium concentration of 90-120 g/l. It should not be forgotten that this case is the worst case in regard to bismuth and nickel. However, it can be a good solution to minimize the amount of lead considerably. Table 4.3. Rough estimation of the amount of impurities with different composition of parameters with regard to Fig. 4.8 - 4.10
No.
Free NaOH
[g/l]
Current Density
[amp/m2]
Tellurium Conc. [g/l]
Ag [ppm]
Bi [ppm]
Pb [ppm]
Cd [ppm]
Ni [ppm]
1 40 300-350 20-60 30-40 30-60 370-500 20-40 20-25 2 80 320-350 20-50 ~15 70-100 280-350 30-50 ~75 3 80 330-350 170-200 ~10 55-80 380-440 35-55 45-55 4 120 330-350 90-120 <5 95-110 140-155 ~20 ~100
Fig. 4.11. Variation of impurities versus free NaOH conc. (g/l) and tellurium conc. (g/l) at constant current density of 350 amp/m2
40
From Table 4.3 it is understood that the last composition (No.4) is the best in regard to total amount of impurities. However, it is worthy to look at another contour plot to be able to conclude the optimum conditions. Since the coefficient of current density for all responses is negative, this factor should be at its highest level to minimize the amount of impurities. Fig. 4.11 shows how impurities vary versus variation of free concentration of caustic soda and tellurium concentration while current density is fixed at 350 amp/m2. In Fig. 12 three possible optimum regions can be found: right top corner, right bottom corner and left bottom corner. Parameters in these regions and estimation on the amounts of impurities in each area are given in Table 4.4. Table 4.4 Estimation of the amount of impurities with different composition of parameters with regard to Fig. 4.11
No.
Free NaOH
[g/l]
Current Density
[amp/m2]
Tellurium Conc. [g/l]
Ag [ppm]
Bi [ppm]
Pb [ppm]
Cd [ppm]
Ni [ppm]
5 100-120 350 150-200 <5 85-165 120-260 5-35 60-120 6 100-120 350 20-60 <5 90-130 110-230 15-40 95-110 7 40-60 350 20-60 20-40 30-75 360-500 20-55 25-50
With comparing Table 4.3 and 4.4, it can be concluded that regions number 4 and 6 are the best experimental areas in regards to minimize the total amount co-deposited impurities in the process of electrowinning of tellurium. In most studies it is mentioned that a low current density is favourable to keep the amount of impurities as low as possible. However, in our study higher current densities are suggested. This can be attributed to employing of periodic short-circuiting technique, which enabled us to perform electrowinning under higher current densities without the risk of obtaining a high amount of impurities. The thickness of BSL in elevated current densities is not considerable too. The reason may be the fact that by using periodic short-circuiting the diffusion double layer is vanished and deposition is controlled by chemical reaction. As a result, coarse-grain deposit is produced and BSL formation decreases.
4.4 Conclusion It has been shown that electrowinning of tellurium is a relatively simple process. However, using a proper analytical method to accurately evaluate the outcome of the process is a much more challenging step. The usefulness of using a mathematical modelling in optimisation of the process has also been proved. The concentration of free caustic soda was found to have the most influential effect on the amount of co-deposited impurities. Free caustic soda had a positive effect on Bi and Ni and a negative effect on Ag, Cd and Pb. Current density was less influential than free NaOH but effected negatively for all responses. Periodic short-circuiting technique was shown to have a good effect on the process and in fact enabled the process to run under higher current densities. Tellurium concentration was not that
41
much influential, however, second order effects of all parameters had reasonably significant effect on the amount of impurities. It was possible to locate several safe experimental regions comprising of completely different arrangement of parameters. Amongst them, following two regions are suggested as the optimised areas (Table 4.5). Table 4.5. Best experimental areas
Priority
Free NaOH
[g/l]
Current Density
[amp/m2]
Tellurium Conc. [g/l]
First suggestion 120 330-350 90-120 Second suggestion 100-120 350 20-60
More investigation should be carried out in order to understand the mechanism of co-deposition of different impurities. Furthermore, the mechanism of formation of BSL and its contribution to accumulation of impurities is yet to be studied.
4.5 Future Work Since there are no previous studies on electrowinning of tellurium in order to minimize concentration of a group of impurities at the same time, this work plays a key role as a starting point in optimisation of electrowinning process not only for tellurium, but also for other metals. The most important achievement of this work is to reveal the main problems that can happen during the investigations. On the base of what has been experienced and investigated in this work, following actions can be recommended for further studies in the future:
1) The most important problem to be solved is to find or develop a reliable analytical method, not only for the solid samples, but also for liquid samples containing high concentrations of tellurium. This will ensure that the out-come of the research can be evaluated properly and finally the results can be trusted and used effectively.
2) It would be of a great interest if the effect of other parameters such as agitation
rate (flow rate) and initial concentration of impurities (as uncontrolled parameters) can be investigated in a new series of experimental design. This should essentially be accompanied by a trustable analytical method.
3) Conditions that favours in BSL formation, amount of different impurities that
are accumulated in BSL and underlying layer and distribution of them should be investigated in future. Using SEM and EDS analysis to study morphology and impurities entrapment can assist in this regard. If there is a huge difference between amount of impurities in BSL and the metallic phase, a practical method to separate these two phases should be suggested.
42
4) The effect of periodic short-circuiting can also be investigated as a varying parameter along with other parameters. It is believed that longer periods of short-circuiting will lead to better surface morphology, less BSL and fewer impurities.
5) The mechanism of co-deposition of different impurities with tellurium and
with each other should be studied in separate electrochemical experiments. Polarographic, voltametric, cyclometric and other usual electrochemical studies can be used for this purpose.
6) The process should be scaled up to a bigger laboratory experiments or a pilot
process in order to face practical problems that exist in industrial scale, now that the initial questions are answered.
43
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