Study on Optimization of Deep Purification Process Design of Zinc Oxygen Pressure Acid Leaching Solution

According to the characteristics of high impurity content and complex composition of oxygen pressure leaching (OPL) solution of zinc sulfide concentrate, combined with the quality requirements of large plate electrowinning and automatic plate stripping, the advanced purification process of solution was designed. Through the deep purification process consisting of neutralization indium precipitation and low iron purification, high temperature antimony salt nickel and cobalt removal, copper slag chlorine removal, low temperature zinc powder replacement copper and cadmium removal, fine purification, calcium and magnesium removal, the contents of harmful impurities such as iron, indium, copper, cadmium, nickel, cobalt, fluorine and chlorine in the solution are strictly controlled to ensure that the quality of the deeply purified zinc sulfate solution is qualified and can meet the needs of continuous and stable production. The energy-saving effect of advanced purification process is obvious, and the average DC power consumption per ton of electric zinc is 2880kWh, reaching the advanced level of the same industry in China. The good effect of advanced purification process ensures the successful application of OPL and large plate zinc electrowinning, and improves the technology and equipment level of zinc smelting industry, which has important popularization and application value.


Introduction
Oxygen pressure leaching (OPL) is a hydrometallurgical process with short flow and enhanced leaching, which has the characteristics of strong adaptability of raw materials, environmental friendliness and good comprehensive recovery effect [1][2] .OPL of sulfuric acid system can be used to leach and recover zinc, copper, nickel, cobalt, molybdenum and other metals from concentrates and secondary materials [3][4][5] .In the domestic zinc smelting industry, OPL zinc smelting technology has been applied in lead and zinc smelting enterprises such as Zhongjin Lingnan Danxia Smelter, Hulunbeier Chihong Mine Limited Company and Western Mining Industry [6][7] .

Deep Purification
As a zinc hydrometallurgy technology, the main processes of OPL also include leaching, purification and electrowinning.Compared with traditional technology, deep purification of OPL solution has become an important guarantee for continuous and stable production.Mainly because: (1) Compared with the traditional "roasting-leaching" zinc smelting technology, OPL eliminates roasting and adopts direct leaching of zinc sulfide concentrate.The cancellation of roasting shortens the technological process and thoroughly solves the problem of sulfur dioxide emission in traditional zinc smelting technology, but at the same time, it also cancels the open circuit path of fluorine, chlorine and other impurity elements in smelting flue gas, which is easy to cause the accumulation and over-standard of fluorine and chlorine impurities in zinc hydrometallurgy production.
(2) The leaching rate of copper, cadmium, cobalt, nickel, indium and other valuable metals in the concentrate is improved by means of enhanced leaching in OPL.For example, the leaching rate of copper is increased from about 40% to over 92% in conventional method, and the leaching rate of indium which is difficult to be leached in conventional method is also over 90% [8] .More associated valuable metals enter the leaching solution, and the impurity content of the leaching solution increases accordingly.
(3) The application of large plate electrodeposition and automatic zinc stripping technology puts forward higher requirements for the quality of zinc sulfate solution [9] .The application and development process of large plate electrowinning and automatic zinc stripping technology in China are almost synchronized with OPL zinc smelting technology, and the two technologies represent the new progress of electrowinning and leaching respectively.As an intermediate process of zinc hydrometallurgy, advanced purification combines OPL with large plate electrowinning to form a complete technical process of clean zinc smelting.The traditional electrodeposition period is 24 h, but the electrodeposition period of automatic zinc stripping technology for large plate zinc electrodeposition needs to be extended to more than 36 h, so that the zinc sheet can reach a certain thickness and meet the requirements of automatic zinc stripping; However, with the prolongation of electrowinning period, the adverse effects of impurity elements such as nickel, cobalt, copper, cadmium, iron, indium, magnesium, fluorine and chlorine in OPL solution on zinc electrowinning process are further enlarged.Therefore, it is of great significance to design a reasonable deep purification process of zinc sulfate solution, so as to reduce the impurity content to the allowable range, and effectively enrich valuable metals such as nickel, cobalt, copper, cadmium and indium in the removal process, thus realizing the comprehensive recovery of secondary resources.
See Table 1 for the composition requirements of zinc sulfate solution for large plate electrowinning and automatic zinc stripping.

Typical leaching solution
A zinc smelting project adopts two-stage OPL process.The leaching solution produced by it contains impurity elements such as nickel, cobalt, copper, cadmium, indium, fluorine, chlorine and magnesium.The composition of the leaching solution is shown in Table 2.The electrowinning process of this process is 3.2 m 2 In the process of large plate electrowinning and automatic zinc stripping, the leaching solution needs deep purification.2 are typical components of two-stage countercurrent low acid OPL solution.The advantages of two-stage countercurrent low acid OPL are low acidity and iron content of leaching solution, and only less neutralizer is needed.During the leaching process, most of the iron dissolved in the leaching process precipitates into the slag in the autoclave with the final acid reduced to about 10 g/L, and the impurities such as arsenic which can co-precipitate with iron are basically removed from the solution.However, residual iron remains in the solution, while impurity elements such as nickel, cobalt, copper, cadmium, indium, fluorine, chlorine and magnesium basically remain in the leaching solution.
3.2.Harm of impurity elements 3.2.1.Ni, Co, Cu, Cd.Nickel and cobalt cause corrosion of zinc sheet in electrowinning, resulting in burning plate, and black spots or horn-shaped round holes are corroded on zinc sheet.
Copper is a positively charged metal, which is easy to precipitate on the cathode, and does not redissolve after precipitation, thus reducing the quality of precipitated zinc.At the same time, copper can form a micro-battery with zinc, which makes zinc redissolve and forms a round "burning plate" with irregular periphery.
Because the precipitation voltage of cadmium is relatively positive than that of zinc, it is easy to precipitate at the cathode, which reduces the quality of zinc sheet.

Fe, In.
Iron and indium generally do not precipitate at the cathode, which will not affect the quality of precipitated zinc, but will reduce the current efficiency and increase the power consumption.

Ca, Mg.
Calcium and magnesium are more negatively charged than zinc, and will not precipitate at the cathode or oxidize at the anode during electrodeposition.However, when the content of calcium and magnesium ions in the solution is too high, it will bring other adverse effects: 1) Increase the density, viscosity and resistance of the solution, reduce the current efficiency of zinc electrowinning and increase the power consumption.
2) The sulfate solubility of calcium and magnesium is relatively small.In the solution circulation system, when the temperature of electrolyte decreases, it will precipitate from the solution, which will cause scaling of equipment and pipelines in the system.The crystallization of calcium magnesium sulfate is difficult to remove, which causes equipment damage and pipeline blockage in severe cases, and brings great harm to normal production.

F, Cl.
Fluorine and chloride ions corrode the electrode plate, which causes trouble to zinc electrowinning production.
Fluoride ion corrodes cathode aluminum plate, which makes it difficult for zinc sheet to peel off from cathode plate.Aluminum is a more active metal than zinc.It can be used as a electrode in acidic electrolyte mainly because of the existence of a corrosion-resistant oxide film on the surface.The existence of oxide film makes the zinc sheet easily peeled off from the electrode plate.However, when fluorine ion corrosion destroys the oxide film of cathode aluminum, the precipitated metal zinc will form a solid solution with aluminum and tightly combine together, which makes it difficult to strip zinc.Modern zinc electrowinning production adopts large plate and mechanical zinc stripping technology.In order to ensure uninterrupted zinc stripping, the concentration of fluoride should not exceed 10mg/L.
Chloride ion corrodes anode lead plate, shortens the service life of anode, increases the lead content of precipitated zinc, and reduces the product quality.After coating, a layer of PbO was formed on the surface of anode plate2 Protective film.Chloride ion has a small radius, which easily penetrates into the anode to react with lead, corrodes the anode plate and increases the lead content in the electrolyte.
Therefore, the impurities such as nickel, cobalt, copper, cadmium, iron, indium, calcium, magnesium, fluorine and chlorine in the OPL solution will have adverse effects on the product quality, current efficiency and automatic zinc stripping in the subsequent zinc electrowinning process, and must be removed from the solution.At the same time, valuable metals such as nickel, cobalt, copper, cadmium and indium are effectively enriched in the removal process to realize comprehensive recovery.

Purification method 3.3.1. Fe removal and In precipitation.
The removal methods of iron and indium are similar, but iron can only be removed as impurities, while indium needs to be recovered.Therefore, iron removal and indium precipitation should be carried out step by step to avoid the mixing of iron and indium, so as to improve the enrichment rate of indium.At present, the methods of removing iron from solution mainly include precipitation, ion exchange or extraction.Among them, precipitation method is more suitable for iron removal in zinc hydrometallurgy.
The iron content in OPL solution can be controlled below 2 g/L, and most of the iron content in OPL solution is Fe 3+ Exist in form, a small amount with Fe 2+ Morphological existence.Fe 3+ And Fe 2+ The precipitation methods of Fe are different, and Fe 3+ The hydrolysis precipitation method is directly adopted, and the reaction formula is: Fe2(SO4)3+6H2O=2Fe (OH))3↓+ 3H2SO4 Fe 2+ Can be oxidized to Fe 3+ Re-hydrolysis precipitation, or goethite precipitation, the reaction formula is: FeSO4+1/2O2+H2 O = FeOOH↓ + H2SO4 The precipitation of indium also adopts hydrolysis precipitation, and the reaction formula is In2(SO4)3+6H2O=2In (OH))3↓+ 3H2SO4 Whether in dilute solution or concentrated solution, Fe 3+ , Fe 2+ , In 3+ The precipitation conditions are different.In the process of decreasing acidity and increasing pH of solution, when acidity is 2~3 g/L, Fe 3+ firstly begin to precipitate.When the pH of the solution rises to about 3.0, In 3+ Hydrolysis precipitation began to occur; When the acidity of the solution continues to decrease and pH rises to about 5.5, Fe 2+ It is possible to precipitate.Therefore, the process of iron removal and indium precipitation can be divided into three steps: high valence iron precipitation, indium precipitation and low valence iron precipitation, so as to obtain indium enriched slag with less iron and higher indium grade.
In the process of iron removal and indium precipitation, the residual impurities such as arsenic and antimony in the leaching solution are also precipitated and removed.

Removal of Ni and Co.
Cobalt and nickel are the most difficult impurities to remove from zinc leaching solution.There are two commonly used purification methods: one is electrochemical method, that is, displacement precipitation method; The second is chemical precipitation, such as cobalt precipitation with xanthate and cobalt precipitation with naphthol.Because xanthate and naphthol will bring organic contamination to the solution, which is not conducive to the subsequent electrodeposition, displacement precipitation is a reasonable method for removing nickel and cobalt [10] 。 Displacement is an oxidation-reduction reaction in which the added metal loses electrons and is oxidized, while the precipitated material gains these electrons and is reduced.It is often used to separate other elements from the main metal solution [11] .The displacement reaction can be divided into two steps: 1) Anodic reaction (metal zinc is oxidized to Zn 2+ Transfer to solution); 2) Cathode reaction (impurity metal ions are reduced to metal and precipitated).
Zn is more active than most metal impurities in leaching solution, and Zn 2+ Entering the solution will not cause secondary pollution, so zinc powder is selected as displacement agent.In order to make the replacement process have enough thermodynamic driving force, it is necessary to add activator to realize the replacement precipitation of nickel and cobalt with zinc powder.Commonly used methods include arsenic salt purification method, antimony salt purification method.Compared with the arsenic salt purification method, SbH3 is easy to decompose than AsH3, and the toxic gas is relatively small, so the antimony salt purification method is preferred based on safety considerations.The reaction formula is: The antimonide of nickel and cobalt is more stable than nickel and cobalt themselves.In the presence of antimony salt, the replacement of nickel and cobalt by zinc powder has greater electromotive force and equilibrium constant in thermodynamics, and the reaction is more efficient and rapid 3.3.3.Removal of Cu and Cd.When copper and cadmium are removed by zinc powder replacement, copper precipitates preferentially.However, excessive zinc powder is needed in the process of cadmium removal to meet the purification requirements.The reaction formula is as follows: Zn + Cu 2+ =Zn 2+ + Cu ↓ Zn + Cd 2+ =Zn 2+ + Cd ↓

Removal of F and Cl.
Chlorine mainly comes from concentrate, neutralizer zinc calcine or zinc oxide dust.The methods used to remove chlorine from solution are silver sulfate precipitation, copper slag removal of chlorine and ion exchange [12] .Among them, copper slag dechlorination method is that copper and copper ions react with chloride ions in solution to transform copper into insoluble cuprous chloride precipitation and remove it.In practical production, sponge copper (Cu 30%) slag produced in the process of recovering cadmium from copper and cadmium slag treated by comprehensive recovery process of zinc hydrometallurgy can be used as chlorine precipitator.Chlorine removal by copper slag dechlorination method, the reaction formula is as follows: Cu(Copper sponge)+ 2Cl -+Cu 2+ =Cu2Cl2↓ Because the solution will be contaminated by copper ions after chlorine removal, in order to avoid repeated copper removal, chlorine removal should be carried out before purifying copper and cadmium.
Fluorine is removed by adding lime to convert it into calcium fluoride.Lime is also a kind of neutralizer, which is needed in the process of indium precipitation and iron removal.Therefore, a small amount of lime is mixed into the neutralizer zinc calcine to achieve the purpose of fluorine removal.

Remove Ca and Mg.
Calcium and magnesium are removed by cooling method.According to the different solubility of calcium and magnesium ions at different temperatures, when calcium and magnesium are close to saturation, the temperature of solution is reduced from 50 ℃ to 35 ℃ by forced cooling, so that calcium and magnesium ions are removed by crystallization of calcium sulfate and magnesium sulfate.

Process flow design
Impurities elements such as Fe 3+ , Fe 2+ , Cu 2+ , Cd 2+ , Co 2+ , Ni 2+ , Mg 2+ , In 3+ , Cl -, F -in the leaching solution, must be removed according to the requirements of deep purification.According to the principle of impurity element removal, the deep purification process of OPL solution is designed, as shown in Figure 1.The main processes include neutralization precipitation indium and low iron purification, nickel and cobalt removal by high temperature antimony salt, chlorine removal by copper slag, copper and cadmium removal by zinc powder replacement at low temperature, deep purification and calcium and magnesium removal by cooling method.

Removal of Fe and In
The supernatant containing about 10 g/L of acid produced in the OPL process is pre-neutralized with zinc calcine as neutralizer.The neutralization temperature is 80 ~85 ℃, the neutralization reaction time is 1.5 ~2 h.Fe 3+ will hydrolyze and precipitate when the solution is pre-neutralized to the acidity is 2 ~3 g/L, and the Fe 3+ form neutralizing slag with excessive calcine and insoluble substances in calcine, but indium will not form hydroxide precipitate and still exist in solution in the form of ions.
The neutralized pulp is pumped into a neutralization thickener for preliminary liquid-solid separation, and the underflow of the thickener is further separated by a filter press.The filtrate and thickened supernatant are pumped to the indium precipitation tank for neutralization and indium precipitation.After slurry ball milling, the filter residue is pumped back to the second stage leaching of OPL to recover the residual zinc in the residue.
Indium precipitation adopts neutralization indium precipitation process with calcine as neutralizing agent.When calcine slurry is added into the indium deposition tank, the neutralization temperature is controlled at 70 ~75 ℃, the neutralization reaction time is controlled at 1.5 ~2h, and the pH value of the solution after indium deposition is controlled at about 3. Indium in the solution will precipitate into the slag in the form of hydroxide.Indium slag can be obtained by liquid-solid separation of the slurry.Liquid-solid separation adopts box filter press, and the filter cake of the filter press is indium slag, which is transported to indium recovery system to produce indium ingots.The solution enters the low iron purification process.
In the process of neutralization and indium precipitation, iron in solution, especially Fe 3+ Most of them were also hydrolyzed and precipitated, and the iron content of the solution decreased to 0.1 g/L, which was mainly Fe 2+ However, the iron content in the solution required by electrowinning process is less than or equal to 10mg/L, so the solution after indium precipitation still needs to be further purified with low iron.For low iron purification, a mechanical stirred tank was used.By adding calcine into the iron removal tank, the pH value of the solution was adjusted to about 5.5, and oxygen was blown into the solution to remove Fe 2+ Oxidation to Fe 3+ .The reaction temperature was controlled at about 80 ℃, and the continuous reaction was carried out for 2 ~3 hours.After iron removal, the content of Fe in the solution decreased from 0.1 g/L at the initial stage to less than 10mg/L at the end stage.Meanwhile, impurities such as As, Sb and F in the solution also co-precipitated with Fe into the solid phase.
After iron removal, the slurry is separated from liquid and solid by a box filter press, and then sent to a ball mill for ball milling together with neutralized slag, and then sent to a second-stage leaching feeding tank for OPL to recover zinc.The filtrate is pumped to the high temperature nickel and cobalt removal process.
In the conventional zinc hydrometallurgy production system, the process flow of removing Cu and Cd at low temperature, Ni and Co at high temperature, and Cu and Cd at high temperature is generally adopted.According to the characteristics of the technological process: after iron removal, the liquid temperature reaches about 80℃.If Cu and Cd are removed first, the solution temperature should be reduced to about 55 ℃ .However, when Ni and Co are removed by replacement in the second stage, the solution needs to be heated to 80~90℃, and then cooled down in the third stage.This is not conducive to the use of energy.Therefore, the technological process of removing nickel and cobalt with high temperature antimony salt, copper and cadmium with low temperature zinc powder and copper and cadmium with zinc powder is adopted.

Removal of nickel and cobalt
The impurities such as Fe, As and Sb in the solution have been removed by neutralization.Before electrodeposition, the impurities such as Cu, Cd, Ni and Co in zinc sulfate solution need to be further removed.
The liquid after iron removal at about 80℃ is heated by a spiral plate heat exchanger, and the heated solution at 90 ℃ flows into a continuous displacement purification stirred tank, and the slurry zinc powder and antimony salt are added to remove nickel and cobalt.The reaction temperature in the control tank for removing nickel and cobalt is 85~90 ℃, and the reaction time is about 2.5~3h.The purified solution is continuously fed into a nickel and cobalt removal filter press for filtration, and the filtered filtrate flows into an outdoor filtrate storage tank, and then is pumped to remove copper and cadmium for purification and chlorine for purification.The reaction temperature of removing Cu and Cd by zinc powder replacement is 55~60 ℃, and that of removing Cl from copper slag is 40 ℃, so the filtrate after removing Ni and Co should be cooled by blast air cooling tower first.The filter residue enters the slurring tank under the filter press respectively, and the washing water is added into the slurring tank for slurring, and the slurring residue slurry is sent to pickling.
The pickling of nickel-cobalt slag adopts intermittent operation, adds waste electrolyte in the pickling tank, adjusts the acidity of the solution to a pH value of 2~3, and directly heats it with a steam pipe in the tank.The temperature is controlled to be about 70 °C, so that the zinc in the slag is dissolved, and the metal zinc in the slag is recovered.The filter residue after pickling is used as the raw material for the comprehensive recovery of nickel and cobalt.

Removal of Cl
The main task of the dechlorination step is to remove chloride ions from neutral solutions.Continuous dechlorination operations are based on the open circuit of part of the solution for dechlorination, and the chlorine content in the decontaminated solution determines the amount of open circuit to the dechlorination section.According to the chlorine content in the solution, after removing nickel and cobalt, 1/3 of the solution needs to enter the dechlorination process.
In a continuously stirred reaction tank, copper slag precipitated chloride ions from the neutral stock solution at a temperature of 40℃.After filtration, the filter slag is sent to the comprehensive recovery system of copper and cadmium slag, and the filtrate enters the low temperature zinc powder to replace and remove copper and cadmium.

Removal of Cd
After mixing the solution from nickel and cobalt removal and copper slag dechlorination, the solution temperature is about 55 ℃ , and it first flows into multiple series of copper and cadmium removal stirring tanks to form continuous purification.The addition of zinc powder is also in the form of pulp, and the residence time of the material in the tank is about 1.5~2h.The purified slurry is sent to a chamber filter press for filtration.The filtrate flows into the filtrate storage tank located outdoors, and then pumps the fine purification process.After the filter slag is slurried, it pumps the copper-cadmium slag pickling tank.
The pickling of copper-cadmium slag is to reduce the zinc content in the slag, and it also adopts intermittent operation.The process flow is similar to that of nickel-cobalt slag pickling.After pickling and filtration, the filter residue is sent to the copper-cadmium residue recovery system to recover copper and cadmium.

Deep purification
In order to meet the requirements of mechanized zinc stripping on the impurity content of the solution, the solution purified by removing copper and cadmium is then purified by adding zinc powder.The purification step is a process step to ensure safety and quality.Mainly, the cadmium leaked in the first two purification steps will be removed in this purification stage.
Fine purification is also low temperature purification, the purification temperature is~50 ℃, the reaction time is about 1.5~2h, and multiple mechanical stirring tanks are used for continuous operation.Zinc powder was added in the fine purification tank to further remove copper and cadmium.The copper-cadmium slag obtained by fine purification contains more zinc powder, and after slurring, it is returned to remove copper and cadmium for purification.The filtrate is pumped into the cooling tower to cool down and remove calcium and magnesium.

Removal of Ca and Mg
The removal of calcium and magnesium is realized by using the solubility of calcium sulfate to decrease with the decrease of temperature.The temperature of refined purified filtrate is about 50 ℃.After cooling by cooling tower, the temperature drops to about 35 ℃, calcium and magnesium ions form crystals and precipitate, and liquid-solid separation is carried out by thickener.The underflow discharged by thickener is pumped into calcium and magnesium slag slurry tank, and waste electrolyte is added into the tank to dissolve zinc sulfite crystals, and the pulp is filtered by filter press.The clear liquid of thickener contains about 155 g/L of zinc, which is sent to zinc electrowinning process as qualified zinc sulfate solution.

Removal Efficiency
After deep purification, the removal efficiency of harmful impurities is shown in Table 3.

Conclusion
The main results are as follows: 1) The content of impurities in OPL solution is higher than that in conventional zinc hydrometallurgy because of its complex composition.Through reasonable process design, the advanced purification process consisting of neutralization indium precipitation and low iron purification, high temperature antimony salt removal of nickel and cobalt, copper slag removal of chlorine, low temperature zinc powder replacement removal of copper and cadmium, fine purification, calcium and magnesium removal can ensure the production of qualified zinc sulfate solution, which can meet the actual production requirements of large plate zinc electrowinning.In solutionCl -, F -The content is stably controlled below the limit (Cl -< 200mg/LF -< 20mg/L), continuous production can be realized by mechanical zinc stripping operation.The average DC power consumption per ton of electric zinc is 2880kWh, reaching the advanced level of the same industry in China.
2) The deep purification of OPL solution is not only a process of impurity removal, but also a process of enrichment of associated valuable metals.The enrichment rate of valuable metals in indium slag, copper-cadmium slag, nickel-cobalt slag and other purified slag is high, which realizes the high value of purified slag.
3) The advanced purification process has achieved good application effect, ensured the successful application of zinc OPL and large plate zinc electrowinning, and improved the technology and equipment level of zinc smelting industry, which has important application value.

Table 2
Removal efficiency of harmful impurities, %