I. Leachate composition and purification method thereof

Zinc calcine or other zinc-containing materials (such as zinc oxide soot, zinc oxide ore, etc.) are leached to produce a neutral leachate, although the Fe ^{3 + is} completely hydrolyzed and precipitated by controlling the acidity of the end point during the leaching process. Some impurities such as arsenic and antimony , but many remaining impurities (such as Cu, Cd, CO, Ni, AS, Sb, Ge, etc.) have great harm to the zinc electrolysis process, which will reduce the efficiency of electrolysis current and increase the power consumption. , affecting the quality of cathode zinc, corroding the cathode and causing difficulty in stripping.

Therefore, all the impurities that damage the zinc electrowinning must be removed by solution purification, and the qualified purification liquid can be sent to the zinc electrolysis tank.

Table 1 Composition range and average content of neutral leachate (g/L)

The purpose of purification is to remove impurities such as copper , cadmium , cobalt , nickel , arsenic and antimony in the neutral leachate to the allowable content of the electrowinning process, to ensure the normal progress of the electrowinning process and to produce a higher grade of zinc. sheet. Meanwhile, by the enrichment of the purification process, so that valuable accompanying elements, such as copper, cadmium, cobalt, indium, and thallium in the starting material is enriched, to facilitate further recovery of valuable metals from the slag purification metal component.

The purification method can be divided into two categories according to the purification principle: 1 adding zinc powder to replace copper and cadmium, or adding zinc powder to remove copper and cadmium while removing nickel and cobalt in the presence of other additives. According to the different additive components, the method can be further divided into zinc powder-arsenic salt method, zinc powder-antimony salt method, alloy zinc powder method and the like; 2 adding organic reagent to form insoluble compound to remove cobalt, such as xanthate purification method And nitroso β-naphthol purification method. A summary of the process for various purification methods is given in Table 2.

Table 2 Several typical processes for various zinc sulfate solution purification methods

It can be seen from Table 2 that since the impurity components of the neutral leachate of each plant are different from the control standards of the new liquid component, the purification methods of the respective plants are also different, and the setting of the purification section is also different. According to the setting of the purification section, the purification process has two sections, three sections and four sections. There are two kinds of intermittent and continuous operations according to the different cleaning methods. Intermittent operations are relatively easy to operate and control, and can be adjusted in time according to changes in solution composition, which is widely used in medium and small wet zinc smelters. Continuous operation has high productivity, small floor space, and easy to realize large-scale and automatic equipment. Therefore, it has developed rapidly in recent years, but the operation and control requirements of this method are relatively high.

Since the potentials of copper and cadmium are relatively positive, it is relatively easy to purify and remove impurities, so each factory firstly removes copper and cadmium first in the first stage. When copper and cadmium are replaced by zinc powder, since the potential of copper is more positive than that of cadmium, it is easier to precipitate preferentially, and it is relatively difficult to replace cadmium with zinc powder. It is necessary to add excessive zinc powder to meet the purification requirements.

Since cobalt and nickel are the most difficult impurities to remove in the leachate, the difference in the purification process of each factory (Table 2) is essentially the difference in the cobalt removal method. In addition to the addition of additives, cobalt and nickel are replaced by a replacement method, and at a higher temperature, an excessive amount of zinc powder is added to achieve the purification requirement. Or use expensive organic reagents, and reasonable choice of cobalt removal process can reduce purification costs.

Second, zinc powder replacement in addition to copper, cadmium

(1) Basic reaction of copper and cadmium by replacement method

Since the standard potential of zinc is relatively negative, that is, the metal activity of zinc is strong, it can remove most of the more positively-charged metal impurities from the zinc sulfate solution, and the Zn ^{2 +} product of the displacement reaction does not cause the solution. Secondary pollution, so all wet zinc smelting plants choose zinc powder as a displacer. When the metal zinc powder is added to the zinc sulfate solution, it undergoes a displacement reaction with a relatively positive metal ion such as Cu ^{2 +} , Cd ^{2 +} or the like.

The standard electrode potentials of the four metals Cu, Cd, Co, and Ni are positive compared with zinc. However, since the potential of copper is much higher than that of zinc, Cu ^{2 +} can be compared with Cd ^{2 +} , Co ^{2 +} , and Ni ^{2 . + It} is easier to be replaced. In production practice, if other impurity components in the purification liquid can meet the electromagnetism requirement, then Cu ^{2 +} can fully meet the new liquid quality standard.

The zinc in the leachate of the wet zinc smelting plant is generally around 150g/L, and the equilibrium potential of the zinc electrode is -0.752V. Then, the above replacement reaction can be carried out until the equilibrium potential of impurity ions such as Cu, Cd, Co, Ni, etc. reaches -0.752 V, that is, these impurity metal ions can be completely replaced in theory. However, this is only a result obtained from the thermodynamics calculation, which is greatly deviated from the actual situation. For example, from the comparison of thermodynamic data, the equilibrium potential of cobalt is relatively more positive than the equilibrium potential of cadmium, and should be preferentially replaced by cadmium. However, due to the higher overvoltage of Co ^{2 +} reduction, it is actually difficult for Co to be zinc powder. Displacement removal, even hundreds of times the theoretical amount of zinc powder, is difficult to remove Co to the requirements of zinc electrowinning. The result is just the opposite, so it is necessary to take other measures in production to precipitate the cobalt out of solution.

(2) Influencing factors of the replacement process

Since copper and cadmium are relatively easy to remove, most factories choose to remove both copper and cadmium in the same stage. The replacement process is affected by the following aspects:

1, zinc powder quality

In addition to Cu, Cd should use relatively pure zinc powder, in addition to avoiding the introduction of new impurities, while reducing the amount of zinc powder. Since the displacement reaction is a reaction between a liquid phase and a solid phase, the reaction rate mainly depends on the specific surface area of ​​the zinc powder. Therefore, the larger the surface area of ​​the zinc powder, the more the chance of the impurity component in the solution contacting the metal zinc powder. The faster the reaction speed.

However, the excessively fine zinc powder is likely to float on the surface of the solution, which is also disadvantageous for the displacement reaction. Since the zinc powder for purification inevitably has partial surface oxidation during preparation, storage, etc., the displacement capacity of the zinc powder is greatly reduced. Therefore, some factories first acidify the pre-purification liquid with waste liquid to purify the zinc powder. The surface of ZnO reacts with sulfuric acid to make the zinc powder appear on the surface of the fresh metal to improve the displacement reaction ability of the zinc powder. It should be noted that the acidification of the solution must be appropriate. If the acidity is too low, it is difficult to achieve the purpose. If the acidity is too high, the zinc powder consumption will increase. Generally, the acidification pH of the factory is 3.5 to 4.0.

If zinc and powder are added at the same time to remove Cu and Cd, the particle size of the zinc powder is generally required to be -0.149 to -0.125 mm. However, some factories use copper and cadmium in two sections because of the high copper content in the leachate. For example, in the Belgian Barron Electric Zinc Plant, when the solution contains more than 400 mg/L of copper, the zinc powder is first thickened to sink copper. Feilong Industrial Co., Ltd. When the solution contains more than 500mg/L of copper, the crude zinc powder is added to deposit the copper first, and the sponge copper is produced, and then the solution is sent to the cadmium removal section. In the single cadmium removal process, zinc powder having a relatively coarse particle size can be selected.

2, stirring speed

Since the displacement reaction is a reaction between the liquid phase and the solid phase, increasing the stirring speed is advantageous for increasing the chance of Cu ^{2 +} and Cd ^{2 + in} contact with the zinc powder in the solution. In addition, the stirring can also promote the deposition on the surface of the zinc powder. The sediments fall off and expose the fresh surface of the zinc powder, which is beneficial to the reaction. At the same time, the enhanced agitation is more conducive to the diffusion of the displaced ions to the surface of the zinc powder, thereby achieving the purpose of reducing the unit consumption of the zinc powder. However, if the stirring strength is too high, the improvement of the reaction rate is not significantly improved, but the energy consumption is increased, and the purification cost is increased. Therefore, it is important to select an appropriate stirring strength. In order to strengthen production, some factories use fluidized clean liquid tanks for purifying copper and cadmium.

When the zinc powder is replaced by copper and cadmium, the stirring method should be mechanical agitation. If air agitation is used, the surface of the zinc powder will be oxidized and passivated. In addition, the oxygen in the air will cause the copper and cadmium which have been replaced to precipitate. Dissolved.

3, temperature

Increasing the temperature can increase the reaction rate of the replacement process and the completeness of the reaction, but increasing the temperature also increases the dissolution of the zinc powder and the reconstitution of the precipitated cadmium. Therefore, in addition to Cu, Cd should be replaced with zinc, and Cd should be controlled to a suitable reaction temperature, generally about 60 °C. Studies have shown that cadmium has a transformation point of allotropes between 40 and 45 ° C. Excessive temperature will promote cadmium re-dissolution.

4, the composition of the leachate

The zinc concentration, acidity and impurity content of the leachate and solid suspended matter all affect the progress of the displacement reaction. The lower zinc concentration in the leachate is beneficial to the outward diffusion of Zn ^{2 + on the} surface of the zinc powder during the replacement process, but the concentration is too low to facilitate the precipitation of hydrogen, thereby increasing the consumption of zinc powder. Therefore, the production practice generally controls the zinc content of the leachate to be 150-180 g/L.

The higher the acidity of the solution, the more favorable the precipitation of hydrogen, resulting in unhelpful loss of zinc powder and promoting re-dissolution of cadmium. In the production practice, in order to make the residual Cu and Cd of the purification solution reach the purification requirement, the pH of the solution must be maintained above 3.5.

5. The occurrence of side reactions

Although most of the AS and Sb have been removed by co-precipitation during the leaching process, there is still a certain amount of AS, and Sb is present in the leachate. In the replacement process, especially in the case of high acidity, the pH of the actual solution. Under the conditions, highly toxic AsH ₃ and SbH ₃ gases are inevitably produced (the latter is very unstable, and SbH _{3 is} easily decomposed under the condition of zinc electrowinning), and therefore, arsenic and antimony should be completely removed as much as possible in the leaching section. In addition, ventilation in the workplace should be strengthened during production to ensure production safety.

(3) Measures for reconstitution of cadmium and avoiding cadmium reconstitution

As mentioned above, the re-dissolution of cadmium has a great relationship with temperature, so it is necessary to control the appropriate operating temperature. In addition, production practices have shown that reconstitution of cadmium is also related to factors such as time, slag amount and solution composition. The contact time between copper and cadmium slag and solution has a great influence on the re-dissolution of cadmium.

Since the copper and cadmium slag which are separated by the replacement are in contact with the solution for a longer period of time, the cadmium content of the liquid after the replacement is higher, so the solid-liquid separation should be quickly performed after the purification operation is completed. Production practice shows that the slag content of copper and cadmium slag in solution also has a great influence on cadmium re-dissolution. The more slag amount, the more cadmium re-dissolving, so the tank should be cleaned regularly during the production process, and the fluidized purification should be adopted. The slag discharge cycle should be shortened as much as possible.

The presence of impurities AS, Sb in the solution not only increases the unit consumption of zinc powder, but also promotes re-dissolution of cadmium. Therefore, these impurities should be completely removed as much as possible during neutral leaching. In addition, it is also necessary to control the concentration of Cu ^{2 +} in the neutral leachate, and the concentration of copper ions is preferably controlled at 0.2 to 0.3 g/L.

In order to avoid the re-dissolution of cadmium in the process of copper and cadmium purification, in addition to controlling the operating technical conditions, it is necessary to control the appropriate excess of zinc powder. Some factories divide zinc powder in copper and cadmium. Batch injection, and a small amount of zinc powder pressure tank before purification and pressure filtration, and reduce the re-dissolution of cadmium by increasing the amount of metal zinc powder in copper and cadmium slag.

(4) Control of main technical conditions for copper and cadmium by replacement method

Due to the difference in raw materials, some hydrometallurgical plants have high copper content in the factory leachate. Copper and cadmium are deposited separately by two-stage purification. However, most factories remove copper and cadmium in the same purification section. The main technical conditions are listed in Table 3. Since the composition of the solution varies from plant to plant, the composition of the copper and cadmium after replacement is also different, and the chemical composition of the produced copper-cadmium slag is different. Generally, the copper-cadmium slag contains 38% to 42% of zinc, and contains copper. % to 6%, containing 8% to 16% of cadmium. The copper cadmium slag produced is used to recover copper, cadmium and other valuable associated metals.

Table 3 Main technical conditions for replacement of copper and cadmium

Third, the organic reagent method to remove cobalt, nickel

The organic reagent precipitation method removes cobalt by removing a compound which is insoluble in impurities such as cobalt or nickel in the solution. The organic reagents currently used in the production of cobalt removal methods include xanthate cobalt removal and alpha nitroso-beta naphthol removal cobalt.

Fourth, the removal of fluorine and other impurities purification method

If the content of fluorine, chlorine, potassium, sodium, calcium and magnesium in the neutral leachate exceeds the allowable range, it will also adversely affect the electrolysis process, and different purification methods can be used to reduce their content.

(1) Dechlorination

In general, the main sources of chlorine are chlorides in zinc soot and chloride ions in tap water. The presence of chloride ions in the solution will corrode the anode of the zinc electrolysis process, increasing the lead content in the electrolyte and lowering the grade of precipitated zinc. When the solution contains more than 100 mg/L of chloride ions, the chlorine should be purified. Commonly used chlorine removal methods include silver sulfate precipitation, copper slag removal, and ion exchange.

1. Silver sulfate precipitation Dechlorination is the addition of silver sulfate and chloride ions to the solution to form a poorly soluble silver chloride precipitate. The method is simple in operation and has good chlorine removal effect, but the silver salt is expensive and the silver regeneration recovery rate is low.

2. Dechlorination of copper slag is based on the interaction of copper and copper ions with chloride ions in the solution to form a poorly soluble cuprous chloride precipitate. The sponge copper slag (25%-30% Cu, 17% Zn, 0.5% Cd) produced during the process of producing cadmium by treating copper cadmium slag is used as a chlorine leaching agent. The process temperature is 45-60 ° C, the acidity is 5-10 g/L, and after stirring for 5-6 h, the chloride ion in the solution can be reduced from 500-1000 mg/L to less than 100 mg/L.

3. Ion exchange method Dechlorination is the exchange reaction between the exchangeable ions of the ion exchange resin and the ions to be removed in the electrolyte, so that the ions to be removed in the solution are adsorbed on the resin, and the corresponding exchangeable ions on the resin enter the solution. . A domestic factory uses 717 strong alkaline anion resin, and the dechlorination efficiency is 50%.

(2) Fluorine removal

Fluoride is derived from fluoride in zinc soot and enters the solution upon leaching. Corrosion of zinc fluoride ion cell cathode would be aluminum plate, sheet is difficult to peel off the zinc. When the fluoride ion in the solution is higher than 80 mg/L, the fluorine must be purified. Generally, a small amount of lime milk can be added during the leaching process, so that calcium hydroxide and fluoride ions form insoluble calcium fluoride (CaF) and then polymerize with silicic acid, and adsorb on silica gel, and the silica gel is regenerated by water elution of fluorine. The method has a fluorine removal rate of 26% to 54%.

Because the effect of removing fluorine and chlorine from the solution is not good, some factories use pre-fire method (such as multi-hearth furnace) to remove fluorine and chlorine from zinc dust, and simultaneously remove arsenic and antimony, so that fluorine and chlorine are not Enter the wet system.

(3) Removal of calcium and magnesium

The total amount of alkaline earth metal ions such as K ⁺ , Na ⁺ and Mg ^{2 + in the} electrolyte can reach 20-25 g/L. If the content is too high, the density, viscosity and electrical resistance of the zinc sulfate solution will increase, causing difficulty in sedimentation and filtration. The cell voltage rises.

K ⁺ , Na ⁺ ions in the solution, if the iron removal process uses the jarosite method, they participate in the formation of jarosite and accompany the slag discharge system. For example, after the iron and potassium iron in the Anzhong zinc smelting plant in Japan, the potassium and sodium ions in the solution were reduced from the original 16g/L to 3g/L.

When zinc is electrowinned, magnesium should be controlled below 10~12g/L. The magnesium concentration is too large, and the magnesium sulfate crystallizes out and blocks the pipeline and the trough. Most plants extract some of the electrolyte from magnesium, and replace it with a new liquid with low impurities.

1. Ammonia method uses 25% ammonia water and neutral electrolyte for magnesium removal. Its composition is (g/L): Zn130～140, Mg5～7, Mn2～3, K13, Na2～4, CL0.2～0 .4, control temperature 50 ° C, PH = 7.0 ~ 7.2, after 1h reaction, zinc is

The basic zinc sulfate [ZnSO ₄ ·3Zn(OH) ₂ ·4H ₂ O] was precipitated, and the precipitation rate was 95% to 98%. 98% to 99% of Mg ^{2 +} , 85% to 95% of Mn ^{2 +} and almost all of K ⁺ , Na ⁺ , CL ^- ions in the impurity element remain in the solution.

2, lime milk neutralization and removal of magnesium India Debari zinc plant draws 4.3m ^{3 of} waste electrolyte every hour with lime milk at room temperature, precipitates zinc hydroxide, discards the filtrate containing most of the magnesium, can prevent magnesium in the system The accumulation in the middle. Or adding lime milk to the waste electrolyte or neutral zinc sulfate solution at a temperature of 70-80 ° C and a pH of 6.3-6.7 to precipitate basic zinc sulfate, the result is 70% magnesium and 60% of the fluoride can be removed.

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