The impurity ions discharged on the cathode are in the cathode region, and the influence of impurities on the electrolysis process mainly depends on their precipitation potential and the overvoltage of hydrogen thereon. All the ions that can be discharged on the cathode have a common point, that is, their precipitation potential is always positive than zinc , and the reduction potential of some impurities is positive, and some are negative as well as zinc, but the absolute value is relatively small. Although these impurities can precipitate at the cathode and adversely affect the electrolysis production, the effects of different impurities are not exactly the same. The main reason for this difference is different from the hydrogen bonding force of the metal there are significant differences, therefore, according to which the hydrogen overvoltage and degree of the magnitude of the stable hydride is divided into the following three groups such impurities .
A lead , cadmium , tin , antimony and other metal ions Lead and cadmium ions are often present in industrial zinc electrolytes, while tin and antimony are rare, only in some special cases will enter the solution. Impurity lead is mainly from the anode plate, while cadmium comes from the concentrate. When the concentration of these impurity ions in the solution is high, there is a tendency to increase the dissolution of zinc due to the formation of microbatteries by zinc and these impurity metals.
B. Metal ions such as cobalt , nickel , copper, etc. The commonality of these metals is that the supervoltage at which hydrogen is deposited is different. Since they are also impurities of the precipitation potential which are positive compared with zinc, according to thermodynamic regulations, they will precipitate on the cathode earlier than zinc, which is the deposition of the first group of metals such as lead, cadmium, tin and antimony. similar. However, after they are deposited on the surface of the cathode, the metallic zinc does not precipitate thereon and covers them. Only hydrogen discharge occurs where such impurities are deposited. This is because the super-voltage deposited by hydrogen here is low, and the precipitation potential is higher than the discharge potential of zinc (the absolute value of the negative number is small). If there is a certain concentration of such impurities in the electrolyte, it will cause great interference to the electrolysis process. At this time, various holes will appear on the cathode sheet to cause the burning phenomenon.
C germanium, arsenic, antimony and other impurities which an impurity element has a unique set of acts that are not the first two groups of elements, after they are discharged at the cathode can generate hydride, and the hydride gas is easily decomposed and volatilized. Niobium is a positively charged metal, so it is easy to discharge on the cathode, and because the super-voltage on which hydrogen is deposited is not high, it will be followed by discharge of hydrogen ions and simultaneous generation of active hydrogen atoms. The atom is further combined to form a hydrogen halide gas. If hydrogen telluride can escape immediately like hydrogen and oxygen after formation, it will not cause much harm to the electrolysis operation, because the content of antimony in industrial electrolytes is generally very small. However, it turns out that the hydrogen halide formed does not escape from the electrolyte bath, but also decomposes and circulates continuously during the electrolysis process.
The circulation mechanism of ruthenium in the electrolysis process is likely to be: after the cesium ions in the solution are discharged on the cathode, the hydrogen halide gas formed is adsorbed on the surface of the cathode, and this gaseous insulator will prevent the Zn from continuing there. Deposition; thereafter, hydrogen halide is oxidized by ions having strong oxidizing power which are often present in industrial electrolytes, such as Fe 3+ , MnO 4 - , Cu 2+ , etc., taking Fe 3+ as an example. The corresponding oxidation reaction is:
GeH 4 + 4Fe 3+ — → Ge 4+ + 4Fe 2+ +2H 2 ↑
Next, the Ge 4+ ions are again discharged near the bubble adsorption site and regenerate the hydrogen halide gas. Such repeated cycles and cycles will cause great harm to the zinc electrolysis process, which not only greatly reduces the current efficiency but also reduces the quality of the cathode zinc. When the content of cerium in the electrolyte is large, a large area of ​​acicular pores will appear on the cathode zinc, and when the pores are dense, they will be connected into one piece, so that the deposit is in the form of a mesh.
The physicochemical properties of arsenic and antimony and their behavior during electrolysis are very similar to those of strontium. Arsenic and antimony are positively charged metals. When electrolyzed, these two elements are easily discharged on the cathode, and the overvoltage of hydrogen deposited thereon is not high. They, like hydrazine, can also combine with hydrogen to form the corresponding hydrides AsH 3 and SbH 3 . The mechanism in which they are formed and the decomposition in the electrolysis process are also similar to GeH 4 . [next]
The impurity ions discharged on the anode are moved by the external electric field, and the cations move toward the cathode, and only the anions tend to the anode. However, due to the circulating flow of the electrolyte, certain cations can still reach the anode surface by mechanical displacement and diffusion. Whether these cations can be discharged and further lose one or several electrons depends on the size of the corresponding ionization energy, which in turn is related to the structure of the extra-ion electron layer. For constant-value ions or ions that are already at their highest state, they will not discharge on the anode. There are many kinds of cations belonging to this class, K + , Na + , Ca 2+ , Mg 2+ , A1 3+ , Zn 2+ , Cd 2+ , Fe 3+ , Ni 3+ , C0 3+ , Cu 2+ . , Ag + , Ge 4+ plasma are all of this type. Whether cations of variable valence state and in a low-priced state will be further oxidized on the anode depends mainly on their oxidation-reduction potential and industrial electrolysis conditions. There are many such cations, such as Cu + , Fe 2+ , Co 2+ , Ni 2+ , Mn 2+ , Pb 2+ , Sn 2+ , etc., which are known to oxidize low-priced tin to high valence. The standard potentials of tin (Sn 2+ /Sn 4+ ) and low-valent copper oxidized to high-priced copper (Cu + /Cu 2 + ) are -0.15V and -0.153V, respectively, so these two metal ions can be before oxygen evolution. It is oxidized to a high price state.
Manganese is a very unique metal. It has five different valences such as 2, 3, 4, 6, and 7. Four of them can exist in solution in an ionic state, while tetravalent manganese can only be in solid state. Mn hatred exists. The oxides of divalent and trivalent manganese are basic, tetravalent manganese is neutral, and the compounds of hexavalent and heptavalent manganese are acidic and strongly acidic.
Under industrial electrolysis conditions, divalent manganese is easily oxidized to a tetravalent state, that is, manganese dioxide powder is formed, and at the same time, it can be partially oxidized to a heptavalent state, that is, permanganate Mn04 is formed. It should be noted that if oxygen is not deposited on lead, the overvoltage is higher, then MnO 4 - cannot be formed, just as the overvoltage without hydrogen does not give zinc at the cathode.
Now, let's discuss the behavior of iron in the electrolysis process. It is known that the standard potential of the conversion reaction between metallic iron and divalent iron ions and ferric ions is;
Fe 2+ + 2e — → Fe E ө = -0.440V
Fe 3+ +3e — → Fe E ө =0.771 V
It can be seen that the oxidation process of ferrous ions at the anode is easy because its standard reduction potential is -0.440 V, which is higher than the precipitation of oxygen and the conversion of divalent manganese to the standard potential of tetravalent manganese (both -1.23 V). To be positive, it is much more positive than the standard oxidation potential (1.52 V) of oxidation of divalent manganese to potassium permanganate, so that ferrous ions are preferentially oxidized as long as they enter the anode region. In addition, atomic oxygen and oxygen precipitated from the anode and potassium permanganate ions in the solution can also oxidize low-valent iron ions into a high-priced state.
The high iron ions produced on the anode and in the solution will tend to the cathode and are easily re-reduced to a low temperature on the cathode because the standard reduction potential of this electrochemical reaction is much more positive than zinc, with a difference of more than 1.5V. Therefore, even at a low concentration, high iron ions are preferentially discharged compared to zinc ions.
However, ferrous ions are not easily reduced to metallic iron, although the standard reduction potential of metallic iron precipitation is more positive than that of zinc. The reason for this is that the standard reduction potential of metal iron and zinc is not far from the numerical value. The super-voltage of metal iron precipitation is higher, and the activity of ferrous ions in the electrolyte is much lower than that of zinc.
Since iron and manganese cannot be precipitated in the form of metals on the cathode, they are only reduced to Fe 2+ and Mn 4 - remain in the solution, which gives them a chance to re-enter the anode region and be oxidized again to Fe 3+ And Mn 4 - . Obviously, the preferential discharge of high-iron ions and permanganate ions generated in the electrolysis process in the cathode region leads to a decrease in cathode current efficiency. [next]
Cationic impurities such as potassium, sodium, aluminum , magnesium , calcium, etc., which are not deposited on the cathode and anode, are more electronegative than zinc, and the absolute value of the negative of the standard reduction potential is much larger than that of zinc, so according to the laws of thermodynamics They do not discharge on the cathode during electrolysis. At the same time, these metals have only one valence type, unlike iron and manganese, which have several different valence ions, and thus do not undergo oxidation reaction on the anode. Such impurities will gradually accumulate in the wet zinc smelting industrial solution to a certain concentration. Among them, calcium and magnesium impurities are harmful to the production process because their sulfate solubility is relatively small, which will cause fouling of the electrolyte vacuum cooler and related pipelines. The crystal of this calcium magnesium sulfate adheres strongly to the tube wall and eliminates the difficulty.
Anions such as fluorine and chlorine tend to the anode during the electrolysis process. Among them, Br - and I - have little practical significance in production because they are rarely present in the electrolyte. Here, we mainly study two anions of C1 - and F - . The standard oxidation potential values ​​for these two anions are as follows:
Cl - F -
â–³FÓ©298/(å¡/克离å) -31350 -66080
E Ó© /V 1.36 2.87
As can be seen, due to the large negative free energy of formation, and would require a high voltage discharge to make them deposited on the cathode, for F - it is especially true. The negative value of the standard oxidation potential of F - far exceeds the oxygen evolution potential and the formation potential of lead peroxide, so that it will never discharge at the anode during electrolysis.
Obviously, F - ions will not discharge at the cathode because there are no more expensive fluoride ions.
Although F - ion does not discharge at the cathode or at the anode, it is still a very harmful impurity for the electrolysis process because it corrodes the plates, especially the cathode aluminum plate.
Aluminum is a more active metal than zinc, which can be used as a plate in an acidic electrolyte, mainly due to the presence of a corrosion-resistant oxide film on its surface. The chemical composition of this oxide film is A1 2 0 3 and A1(OH) 3 . It is known that there are similar oxide films on many metal surfaces.
The industrial electrolyte is a dilute sulfuric acid solution. The reason why the aluminum plate is resistant to corrosion and the zinc sheet deposited thereon is easily peeled off is due to the presence of an oxide film or an adsorption layer on the aluminum plate. If the passivation layer is partially or completely destroyed, then the above two functions will of course be weakened or disappeared.
Some research data show that when the electrolyte contains halogen ions such as F - , Cl - , Br - , I - , etc., the passivation layer formed on the metal aluminum will be partially damaged, and the corrosion of F - is the largest.
Chlorine easily penetrates into the anode from the pores of the anode and acts with lead, and the following reaction occurs:
Pb + 2Cl - â†- → PbCl 2
E Ó© = 0.268V
The solubility of PbC1 2 produced is much higher than that of PbSO 4 . The concentration of SO 4 2- ion in industrial zinc electrolyte is much higher than the concentration of chloride ion. Therefore, when the concentration of PbC1 2 in solution reaches saturated solubility, the corresponding Pb 2+ The concentration is already supersaturated for PbS0 4 , so that solid phase PbS0 4 will precipitate. [next]
After C1 - ions are regenerated, they can again penetrate into the interior of the anode and lead. This repetitive action of chloride ions in zinc electrolysis will cause constant corrosion of the anode plates. Due to the corrosion of the aluminum plate by the chlorine, as the surface of the anode and the PbSO 4 suspended in the electrolyte are also significantly increased, the possibility of being entrained to the cathode region for various reasons is greatly increased. Solid PbS0 4 particles will enter the cathode deposit in the form of mechanical inclusions and, like Pb 2+ ion discharge deposition, will also degrade the quality of the electrolytic zinc product.
Electro-Zinc Quality Control With the development of science and technology, the consumption of zero-zinc in the industrial sectors such as die-casting parts, electroplating and hot-dip plating, medicine, and chemical reagents has increased year by year. The zinc ingot standards specified in the national standards are listed in Table 17-17. In terms of chemical composition, China's zinc electrowinning existing technology and equipment can already produce a large amount of zero zinc, and some large wet zinc smelting plants, such as Zhuzhou Smelter, the zero zinc output rate is close to 100%. Some of the advanced zinc (the Zn content of 399.885%) produced by the electric zinc plant has been registered with the London Metal Exchange and can be exempted from entering the international market. Production practices have proved that the impurities that affect the quality of electric zinc are mainly lead, tin and iron. In order to meet the requirements for the production of zero zinc, the production conditions must be strictly controlled, and the key problem is to strive to reduce the content of impurities in the effusion.
Chemical composition of zinc ingots specified in GB/T470-1997 | ||||||||||
Brand | Chemical composition /% | |||||||||
Zn is not less than | Impurity content, not greater than | |||||||||
Pb | Cd | Fe | Cu | Sn | Al | As | Sb | sum | ||
Zn99.995 | 9.995 | 0.003 | 0.002 | 0.001 | 0.001 | 0.001 | -- | -- | -- | 0.005 |
Zn99.99 | 99.99 | 0.005 | 0.003 | 0.003 | 0.002 | 0.001 | -- | -- | -- | 0.01 |
Zn9.95 | 99.95 | 0.02 | 0.02 | 0.01 | 0.002 | 0.001 | -- | -- | -- | 0.05 |
Zn99.5 | 99.5 | 0.3 | 0.02 | 0.04 | 0.002 | 0.002 | 0.01 | 0.005 | 0.01 | 0.5 |
Zn98.7 | 98.7 | 1 | 0.2 | 0.05 | 0.005 | 0.002 | 0.01 | 0.01 | 0.02 | 1.3 |
Note: Zinc is 99.99% zinc ingot for the production of die-cast alloys with a maximum lead content of 0.003%.
In the process of zinc electrowinning, impurities not only affect the electrochemical and crystallization processes of zinc electrowinning, but most of the impurities will have a greater reduction rate than Zn 2+ under similar conditions.
However, the rate at which any ion is precipitated cannot be greater than its limiting current density.
nFD ′ αi
Di = —————
δ(1-kt)
Where Di - the actual precipitation rate, cm 2 / s;
D ' - ion diffusion coefficient;
δ - effective diffusion layer thickness, cm;
Αi - the activity of ions in solution, mol / L;
T——the number of ion migrations;
K——the conductivity of the ions S/m;
F - Faraday constant. [next]
For impurities, the concentration is small and does not actually carry the charge transfer, therefore,
nFD 'C i
D i = —————
δ
Where C i is the impurity ion concentration.
The calculation of Cu 2+ in ZnS0 4 electrolyte is taken as an example. After two stages of purification, when the concentration of Cu 2+ in the obtained electrolyte is C i = 3 × 10 -6 mo1/L, D ′ = 8.0 × 10 -6 cm 2 /s, δ = 3 × 10 -2 cm When the limit current density is:
J Cu = 2 × 96500 × 8 × 10 -6 × 10 -4 × 3 × 10 -6 × 10 3 × 3 × 10 -2 × 10 -2
= 1.544 × 10 -3 A/m 2
The cathode current density of electrolytic zinc sulphate is Jx=500A/expanded, and the current efficiency is usually 90%, so the copper content in the cathode deposit is
2 × 63.5 × 1.54 × 10 -3
——————————— = 0.00033%
2 × 65.4 × 500 × 0.9
Below the first grade electro-zinc copper standard (<0.001%). Therefore, in order to improve the quality of electro-zinc, the key is to reduce the content of harmful impurities in the solution and increase the current density.
Lead is the most important impurity affecting the quality of electro-zinc. The content of lead in the cathode zinc increases with the increase of the temperature of the effluent, and decreases with the increase of the cathode current density. If the temperature rises by 10 °C , the solubility of lead increases, and the concentration of lead ions in the solution can be increased by 15%-20%. The cathode current density is increased from 200 A/m 2 to 500 A/m 2 , and the lead content in the cathode zinc is reduced by three-quarters. This is because when the current efficiency is constant, the cathode current density is increased. Although the amount of zinc precipitated per unit time increases, since the lead ions are precipitated at the limiting current density, the amount of lead precipitated at the same time does not change, so the lead content in the cathode It decreases as the cathode current density increases.
However, the lead entering the electro-zinc is not completely derived from the discharge of lead ions in the solution, and may also be due to mechanical attachment of Pb0 2 suspended in the liquid to the cathode, or first to reduction to lead ions, and then precipitation. Therefore, the above concept of limiting current density may not be applicable at some time.
According to the lead balance in the zinc electrowinning process, 76.4% of the lead ions in the solution enter the cathode, and about 70% of them come from the lead- silver anode, thus reducing the dissolution of the anode lead and preventing the lead ions in the electrolyzed liquid from entering the cathode. It has become the main direction of improving the quality of electric zinc.
Since the Pb0 2 film is directly formed and indirectly formed dense, many factories use an anodic coating method to reduce the amount of lead from the anode into the human effusion. The treatment of lead-silver anodes in a potassium fluoride-containing aqueous solution by the Terrell smelter in Canada can also reduce lead-to-lead zinc contamination.
In order to reduce the precipitation of lead ions in the electrolyzed liquid at the cathode, each factory adds strontium or barium carbonate to the electrofluid of the electrolyzer. Since SrS0 4 or BaS0 4 has a lower solubility than PbS0 4 , the lead ions can be partially substituted to form a homomorphic coprecipitation. Since Sr 2+ or Ba 2+ in the crystal is not completely replaced by lead ions, the degree of lead removal ions is determined by the amount of such carbonate added. Usually 1t of zinc is added to 2kg of SrC0 3 , but it is also added to 6kg. The Kokkola Electric Zinc Plant produces It Zinc with 9.86 kg BaC0 3 .
The content of fluorine, chlorine and manganese in the electrolyzed liquid is appropriately controlled, and a cathode zinc having a low lead content can be obtained. Generally, the lead content in the zinc cathode increases as the content of fluorine and chlorine in the electrowinning liquid increases. Therefore, it is required to control the fluorine content in the electrowinning liquid to be below 80 mg/L and the chlorine content to be below 100 mg/L. However, when the concentration ratio of Mn to Cl in the effusion is greater than or equal to 3-3.5, even if the chlorine in the effusion reaches 350-1000 mg/L, the lead content in the cathode zinc can be less than 0.005%. Generally, 1-3 g/L of Mn 2+ is present in the electrolyzed liquid, and Mn0 2 with good adhesion is formed on the anode, so that the pores of the Pb0 2 film are reduced, thereby hindering corrosion of the anode. For example, at a current density of 450 A/m 2 , the temperature of the electrowinning solution is controlled to 37 ° C, the acidity is 100 g / L H 2 SO 4 , and the anode of the lead plate containing 1% of silver is electrowinned. Experiments show that with the anode Pb0 2 film When the content of MnO 2 is increased, there is no change in the lead content in the electrowinning solution.
In order to ensure the quality of electro-zinc to improve electrolysis conditions, various additives are commonly used in industrial electrolytic zinc production. Commonly used additives are mainly the following, such as plant or animal glue, surface active substances and salt additives, etc., please refer to the relevant literature for related content.
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