Cu and Cd cementation cobalt-nickel Method A except replacing the purified metal thermodynamically substituted by another metal in the aqueous process for the replacement. From thermodynamics, only the more negatively charged metal can be used to replace the more positively charged metal in the solution. For example, metal zinc can be used to displace copper in solution:

Zn+Cu 2+ ==== Zn 2+ +Cu ↓

Therefore, the order of substitution is determined by the potential order of the metals in the aqueous solution, and the magnitude of the displacement tendency is determined by their potential difference. This can be illustrated by thermodynamic calculations.
From the thermodynamic analysis, it can be seen that the replacement of Cu, Cd, Co, and Ni by zinc powder can be thoroughly purified, and the ion activities of Cu, Cd, Co, and Ni can be 10 - 38 , 10 -11.63 of Zn ion activity, respectively. , 10 - 16.81 , with 10 -17.69 times.
B. Kinetics of displacement purification. It is easier to purify Cu by zinc powder replacement, and it is not easy to purify and remove Co and Ni. It is easy to precipitate Cu with a theoretical amount of zinc powder, and Cd can be removed with several times the theoretical amount of zinc powder, but it is difficult to remove Co to meet the requirements of zinc electrowinning with even a hundred times theoretical amount of zinc powder. The reason why Co is difficult to remove is that many documents at home and abroad are explained as having a high overvoltage when Co 2+ is reduced and precipitated, and there is also a problem of reaction rate.
The rate of displacement reaction can be understood as the rate of dissolution of a negatively charged metal in a solution containing a positively charged metal ion and can be expressed by the following formula:
Dc A
- —— = k — c
Dt V

Where k is the rate constant;
A - the contact area with the solution;
V - the volume of the solution;
C——the concentration of positively charged metal ions;
t - reaction time.
The above formula is obtained:

V 1 c 2
k = - - · - ln —
A t c 1


Where c 1 is the concentration before the reaction of the positively charged metal ion;
c 2 - the concentration after the reaction of the positively charged metal ion for t time.
The rate of the replacement process may be diffusion control or chemical reaction control. Research confirmed that the reaction

Zn+Cd 2+ ==== Cd+Zn 2+

At 50 °C , when the rotational speed is below 250r/min, the displacement reaction rate constant k is proportional to the rotational speed n. When the rotation speed is above 250r/min, the displacement reaction rate remains unchanged. It is shown that at low rotational speeds, the displacement reaction proceeds in the diffusion zone and at high rotational speeds the reaction proceeds in the kinetic zone. The relationship between the rate of displacement reaction and temperature: (is in the range of 25~85 °C)

1350
Lgk = 13.54 - ———
T

The activation energy = 4.95 x 5650 J/mol = 23.14 kJ/mol, ie the reaction has no pure diffusion characteristics. For the replacement of tin by zinc powder, the temperature is higher, and the reaction rate is of course larger, but the overvoltage of hydrogen is lowered, that is, the hydrogen is precipitated at the same time as the replacement, and the rate of displacement reaction is reduced. Therefore, in general, cadmium is operated at a low temperature (40 to 50 ° C), and zinc powder of 3 to 6 times the amount of cadmium is used. [next]
Cobalt is an inert metal and it is more difficult to replace it with zinc powder.

Zn + Co 2+ ==== Co + Zn 2+

In order to increase the rate of zinc substitution of cobalt, the temperature is increased to 80-90 ° C. In addition, since the super-voltage of cobalt precipitation is relatively large, and the over-voltage of hydrogen is relatively low, if cobalt is precipitated by zinc powder alone, hydrogen is mainly emitted in the cathodic reaction of the electrochemical corrosion system, and the reaction of replacing cobalt is naturally very high. slowly, the as 2 0 3 was added at the time of replacing cobalt, CuSO 4 is antimony potassium tartrate, or activators, reducing electropositive metal out of the cobalt compound to accelerate the formation of multi-displacement reaction. In addition, studies have confirmed that the use of Sb-containing and Pb-containing zinc powder has greater activity, because Sb can reduce the precipitation overvoltage of Co, and Pb forms a convex and concave surface on the zinc powder, which can prevent the re-dissolution of cobalt.
C replacement purification process At present, zinc powder is used for deep purification of zinc sulfate solution at home and abroad. There are basically two types of process flow, one is the first hot purification (high temperature) and then the cold purification (low temperature) process, also known as positive Purification; the other is the process of hot purification (high temperature) after cold purification (low temperature), also known as reverse purification or reverse purification. In general, the factory uses two purification processes. In order to further achieve deep purification, some factories at home and abroad also use three-stage or four-stage purification processes.
The process of positive purification and reverse purification of zinc sulphate solution is shown in the next two figures.

[next]

Both of the above processes can achieve deep purification of the solution, which needs to be selected according to the specific conditions of each plant.
D. Safety issues for displacement purification The highly toxic gas substance AsH 3 or SbH 3 is often precipitated in displacement purification.
Under the condition of zinc powder displacement purification, AsH 3 or SbH 3 may be precipitated by the elements As, Sb and HAs0 2 , HSb0 2 , and their potentials are:

As + 3H + + 3e ==== AsH 3 ↑


As can be seen from the above, the possibility that the elements As, Sb generate AsH 3 and SbH 3 is much more likely than that generated from HAsO 2 and HSb0 2 . Under the condition of zinc powder replacement (aZn 2+ = 1, P AsH3 = P SbH3 = 101.32 kPa), ASH 3 is generated from the elements As, Sb, and the equilibrium pH of SbH 3 is 2.61, 3.29, when the pH value increases, the equilibrium P AsH3 and P SbH3 decreased accordingly. When the pH was 5, P AsH3 decreased to 1.36 x 10 -5 kPa, and P SbH3 decreased to 0.802 kPa. The equilibrium pH of AsH 3 generated from HAs0 2 and HSb0 2 is 9.81, and the equilibrium pH of SbH 3 is generated to be 10.5. That is, under the actual solution pH, AsH 3 and SbH 3 gas must be generated. Because AsH 3 and SbH 3 are highly toxic substances, they are extremely harmful to the human body. Therefore, strict replacement and protection measures must be taken during replacement operations.
In the industrial production, the cobalt removal method removes Co in addition to Co and β -naphthol in addition to Co. The basic principle of the removal of cobalt from xanthate is to use the potassium citrate C 4 H 9 0CSSK or sodium citrate C 4 H 9 OCSSNa to react with the trivalent cobalt in the solution to form an insoluble cobalt citrate precipitate. The main reaction is:

4KC 4 H 9 OCSS+2CoS0 4 ←→ 2Co(C 4 H 9 OCSS) 2 +2K 2 S0 4

In the widely used zinc powder replacement cobalt removal process, the zinc consumption is often 10-20 times of the theoretical amount, the working time is long, the process is generally carried out at high temperature (>80 ° C), and often the toxic gas AsH 3 or SbH 3 Precipitation, new purification and cobalt removal methods are still being sought at home and abroad. Thus with sodium boron tetrahydro (NaBH 4) reduction purification of zinc sulfate solution to remove the nickel, cobalt exploration.
The stability of NaBH 4 in aqueous solution decreases as the pH decreases and the temperature increases. In order to stabilize NaBH 4 and maintain the pH value during the purification process, a solution containing a certain amount of NaOH and NaBH 4 (for example, a solution containing 2.2 g/L NaBH 4 and 2.3 g/L NaOH) should be prepared at room temperature for reduction. Agent. The reaction of reducing nickel, cobalt and copper, tin, lead , antimony, iron and other metal ions is: [next]

4MeS0 4 +NaBH 4 +lONaOH ==== 4Me+4Na 2 S0 4 +Na 3 B0 3 +7H 2 0

The reduction method can deeply purify and remove nickel, cobalt and other impurities higher than zinc potential, and the process is simple and convenient, and the reduction reaction can be rapidly carried out at a lower temperature, the working time is only l0 min, and the nickel and cobalt in the reducing precipitate are high. , easier to handle. Although the price of sodium tetrahydroborate (NaBH 4 ) is relatively high, the precipitation of 1 t of impurities consumes only 0.15 t of NaBH 4 and is therefore economically conceivable compared to the displacement method.
In the purification of fluorine and chlorine wet zinc, the main source of fluorine is brought into the leachate when treating fluorine-containing zinc oxide powder and sublimate smoke. When the fluorine content in the zinc electrowinning solution is high, it will cause difficulty in exfoliating zinc. For this reason, in general, when treating zinc oxide having a high fluorine content, it is necessary to perform pre-baking to remove fluorine and then leaching. A domestic factory uses a multi-hearth furnace to roast zinc oxide to remove fluorine. At present, there are few methods for removing fluorine from the solution, and the known methods are as follows.
A The use of strontium salts to remove fluorine from solution is based on the principle that fluorine and hydrazine form a poorly soluble compound to precipitate. However, barium salts are expensive and should not be used industrially.
B Add a small amount of lime milk to remove fluorine during the leaching process. The principle is that fluorine and calcium form a poorly soluble compound calcium fluoride (CaF 2 ).
In the wet zinc smelting process, chlorine is leached in these materials due to the treatment of zinc calcine, various soot, zinc oxide and other zinc-containing materials (such as mold slag and galvanized slag) containing a certain amount of chlorine. In the process, almost all of the solution enters. At the same time, because the entire system uses a lot of tap water, it also brings in a certain amount of chlorine. The presence of chlorine affects the zinc electrowinning process, causing corrosion of the lead anode and equipment, and the lead content of the electrowinning liquid is increased, so that the quality of zinc precipitated from the cathode is lowered. In addition, the precipitation of C1 2 deteriorates labor conditions and affects environmental protection. There are many methods for removing chlorine in wet zinc smelting. Among them, the fire method generally adopts multi-hearth furnace roasting method to remove chlorine. The wet method often uses silver sulfate precipitation method, copper slag dechlorination method, ion exchange method and alkali washing and removing chlorine method. Wait.
The basic principle of chlorine removal by silver sulfate precipitation is to remove silver chloride from the chloride salt in the solution to form a poorly soluble silver chloride precipitate to remove chlorine. Its reaction formula is as follows:

Ag 2 SO 4 + 2NaCl ==== Na 2 SO 4 + 2AgCl ↓

This method has a good dechlorination effect, but its use in production is limited because it is expensive and the recovery rate of silver is low.
The basic principle of chlorine removal by copper slag is to use copper and copper ions (Cu 2+ ) to interact with chloride ions (C1 - ) in solution to form insoluble copper chloride (Cu 2 C1 2 ) precipitates. Removal in solution such as:

Cu + Cu 2+ + 2Cl - ==== Cu 2 Cl 2 ↓

The copper slag used may be copper slag produced by two stages of purifying copper cadmium, or copper slag produced by recovering cadmium from copper cadmium slag.
The basic principle of the ion exchange dechlorination method is to use a resin exchangeable group to undergo a displacement reaction with the ions to be removed in the electrowinning solution, 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. . In a battery with chlorine content of up to 260~370mg/L, the domestic 717 strong alkaline anion exchange resin was used to remove chlorine, and good results were obtained.
Domestic 717 resin, originally chlorine type, was treated with 1.5% sulfuric acid to convert it to sulfuric acid type. When zinc electrowinning fluid is used as the exchange liquid, the ion exchange potential is higher than the low-valent ion, ie, SO 4 2- >Cl - >F - , and the ion exchange method can achieve satisfactory results by removing fluorine chlorine by ion exchange. However, when a strongly basic quaternary amine type anion resin is used in a high concentration neutral solution (containing SO 4 2- up to 200 g/L), chloride ions exhibit greater exchange capacity, so exchange with resin, chloride ions will replace Sulfate (SO 4 2- ) on the resin, and chlorine is removed from the solution. The resin was then rinsed with a 1.5% sulfuric acid solution to obtain regeneration. The ion exchange dechlorination method removes chlorine by roasting, and has the advantages of simple equipment, low investment, good working conditions and no influence on the recovery of rare metals. [next]
Calcium and magnesium in the purification of calcium and iron wet zinc smelting solutions are brought into the system from zinc concentrates and auxiliary materials in the smelting process. Calcium magnesium salts into a solution of zinc hydrometallurgy system, can not be cleaned of other general purification method of Cu, Cd, Co and the like, will continue to accumulate in the circulating solution throughout the wet system until it reached saturation. Calcium and magnesium salts are abundant in solution, which brings some adverse effects to wet zinc smelting, such as:
(1) Calcium and magnesium salts enter the wet zinc smelting solution system, correspondingly increase the bulk density of the solution, and increase the emulsion of the solution, which makes the liquid-solid separation and filtration of the leaching slurry difficult. When CaSO 4 and MgSO 4 crystallized on the filter cloth, the pores of the filter cloth were also clogged, and the filtration could not be performed.
(2) In the solution circulation system, when the local temperature drops, Ca 2+ and Mg 2+ are crystallized by CaS0 4 and MgS0 4 respectively, and are deposited in the outer casing of the device which is easy to dissipate and the metal pipe conveying the solution, and the crystallization It will become a hard solid, causing equipment damage and pipeline blockage. In severe cases, it will cause production stoppage, which will bring great harm to the wet smelting process.
(3) In the zinc electrowinning solution, when the calcium and magnesium salts are high, the electric resistance of the electrolyzed liquid is increased, and the current efficiency of the zinc electrowinning is lowered. Based on the above hazards, the removal of excessively dissolved calcium and magnesium is a common problem encountered by every wet zinc smelting plant. The Changsha Research Institute of Mining and Metallurgy combined the raw materials used in the Xichang smelting plant to study the pretreatment of zinc concentrate containing calcium and magnesium. Gained effective results. At present, there are two commonly used methods for purifying calcium and magnesium.
One is to remove magnesium before baking. Some foreign wet zinc smelting plants, when the zinc sulfide concentrate contains 0.6% of Mg, the Mg is removed by the dilute sulfuric acid washing method, and the chemical reaction formula is

MgC0 3 +H 2 SO 4 ==== MgS0 4 +H 2 0+CO 2

Mg0+H 2 SO 4 ==== MgS0 4 +H 2 0

Mg was allowed to enter the washing solution with MgS0 4 to be excluded. This method effectively removes magnesium from the zinc sulfide concentrate. However, due to the addition of a process, it will inevitably lead to the loss of valuable metals. If the zinc oxide concentrate contains Zn0, ZnC0 3 , this part of zinc also enters the pickling solution during pickling, which is difficult to recover and will be lost.
Secondly, the solution can be concentrated to remove calcium and magnesium. The principle of removing calcium and magnesium by the cooling solution method is based on the difference in solubility of Ca 2+ and Mg 2+ at different temperatures. When the calcium and magnesium contents are close to saturation, the forced cooling is used at the normal working temperature, and Ca 2+ and Mg 2+ are used. It will precipitate out as CaS0 4 and MgS0 4 , which will reduce the content of Ca 2+ and Mg 2+ in the solution.
In industrial production, a blast air cooling tower is used, and the new liquid after removing Cu, Cd, Co, etc. will be placed in a large new liquid storage tank when it is lowered to 40~50`c in the cooling tower. Naturally, it slowly cools. At this time, calcium and magnesium salts are crystallized and deposited on the inner wall and the bottom of the tank. As time increases, the entire wall of the tank and the bottom of the tank form an integral massive crystal. The crystals are periodically removed to achieve the purpose of removing calcium and magnesium.
There are also some factories that add purified liquids other than Cu, Cd, and Co to the air cooling tower of the waste effusion, and cool the CaS0 4 and MgS0 4 crystals on the cooling tower tray together with the waste effusion. Precipitated in the bottom of the cooling tower. There are also wet zinc smelting plants that use a portion of the new liquid to produce zinc sulphate by-products, from which a portion of the calcium and magnesium in the system can be split.
references:
1 Mei Guanggui, Wang Derun, Zhou Jingyuan, Wang Hui, and wet smelting zinc . Changsha: Central South University of Technology Press, 2001
2   Peng Rongqiu editor of non-ferrous metallurgy manual extraction (zinc, cadmium, lead, bismuth volume) Beijing: Metallurgical Industry Press, 1992

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