Theoretical basis of silver electrolytic refining
Silver electrolysis is now using silver nitrate electrolyte without exception. Electrolytic silver alloy was charged in a silver nitrate electrolyte (or coarse silver) silver the anode and cathode plates or other nitric acid insoluble material. After the current is supplied, the silver and base metal impurities of the anode are dissolved, and pure silver is precipitated at the cathode. The electrolysis process can be considered to be carried out in an electrochemical system of Ag (cathode) | AgNO 3 , HNO 3 , H 2 O, impurities | Ag, impurities (anode).
During electrolysis, the reaction on the cathode is the process of silver precipitation:
Ag + +e 
Ag
It should be noted, however, that in addition to the reaction of precipitation of silver on the cathode, the following harmful reactions that consume electrical energy and nitric acid may occur:
H + +e H 2
2NO 3 - +10H + +8e N 2 O↑+5H 2 O
NO 3 - +4H + +3e NO↑+2H 2 O
NO 3 - +2H + +e NO 2 ↑+H 2 O
NO 3 - +3H + +2e HNO 3 +H 2 O
Since these reactions occur, it is often necessary to add nitric acid to the solution; however, in terms of power consumption, due to the dissolution of components such as copper in the anode, only silver is precipitated on the cathode to compensate for the electric energy consumption.
On the anode, oxidative dissolution of silver and base metal impurities occurs. Silver is not mono-oxidized into monovalent silver ions, and when the current density is small, it may be partially oxidized to half-valent silver ions. The half-valent silver ions can be decomposed by themselves to form monovalent silver ions, and a metal silver atom is separated into the anode mud, which affects the plating yield.
Ag-e Ag +
2Ag-e Ag 0.5 +
Ag 0.5 + Ag↓+Ag +
However, since the anode plate contains other metal impurities, in addition to the oxidation of the metal silver on the anode, the base metal such as copper is also oxidized to enter the solution. The oxidation of metals such as silver and copper on the anode is accomplished by a series of reactions:
NO 3 - -e NO 2 +[O]
2Ag+[O] Ag 2 O
Ag 2 O+2HNO 3 2AgNO 3 +H 2 O
2NO 2 +H 2 O HNO 3 +HNO 2
HNO 2 +[O] HNO 3
MeO+2HNO 3 Me(NO 3 ) 2 +H 2 O
In addition to the formation of nitric acid and nitrous acid, some of the nitrogen dioxide generated in the reaction is volatilized and lost during the process.
In gold and silver alloy anodes, since gold atoms can replace several silver atoms in the alloy and maintain the original silver character, gold can form a uniform and tightly bonded alloy with silver in any proportion. Moreover, since gold is insoluble in silver electrolysis, when gold atoms in the alloy are more than a certain degree, silver atoms are wrapped to mask (not dissolve) silver. Therefore, the gold content in the alloy should not be too high. In addition to gold, silver, the anode usually contain 1% to 3% or more of copper, bismuth, antimony, lead, selenium, and other impurities and a small amount of platinum group metal.
During silver electrolysis, the behavior of the elements on the anode is related to their potential and concentration in the electrolyte and whether they will hydrolyze. The table below lists the standard potentials for metals.
Table Standard potential of metal at 25 ° C
element | cation | Potential ∕V | element | cation | Potential ∕V |
Zinc | Zn 2 + | -0.76 | arsenic | As 3 + | +0.30 |
iron | Fe 2 + | -0.44 | copper | Cu 2 + | +0.34 |
nickel | Ni 2 + | -0.25 | copper | Cu + | +0.52 |
tin | Sn 2 + | -0.14 | silver | Ag + | +0.80 |
lead | Ph 2 + | -0.126 | palladium | Pd 2 + | +0.82 |
hydrogen | H + | ±0.00 | platinum | Pt 2 + | +1.20 |
antimony | Sb 3 + | +0.10 | platinum | Pt 4 + | +1.29 |
bismuth | Bi 3 + | +0.20 | gold | Au 3 + | +1.50 |
In the process of silver electrolysis, according to the nature and behavior of each element, they can be divided into:
(1) Zinc, iron, nickel, tin, lead, and arsenic which are electrically negative than silver. Among them, the content of zinc, iron, nickel and arsenic is extremely small, which has little effect on the electrolysis process. During the electrolysis process, they all enter the electrolyte in the form of nitrates, and gradually accumulate to contaminate the electrolyte and consume nitric acid. However, in general, it does not affect the quality of electrolytic silver. Tin is in the form of stannate into the anode mud. Part of the lead enters the solution, and another part is oxidized to form PbO 2 into the anode mud. A small amount of PbO 2 adheres to the surface of the anode plate and is difficult to fall off. Therefore, when there are more PbO 2 , it will affect the dissolution of the anode.
(2) Electron ratio silver positive platinum and platinum group metals. These metals generally do not dissolve and enter the anode slime. When the content is high, it will stay on the surface of the anode, hinder the dissolution of the anode silver, and even cause the passivation of the anode, so that the electrode potential of the silver rises, which affects the normal progress of the electrolysis. However, in fact, a part of platinum and palladium enter the electrolyte. Part of the palladium enters the electrolyte because palladium is oxidized to PdO 2 ·nH 2 O at the anode, and the newly formed oxide is easily soluble in HNO 3 . Platinum also has similar behavior. In particular, when a higher concentration of nitric acid, an excessively high electrolyte temperature, and a large current density are employed, the amount of palladium and platinum entering the solution increases. Since the potential of palladium (0.82 V) is similar to that of silver (0.8 V), when the concentration of palladium in the solution is increased (some people think: 15 to 50 g/L), it precipitates with the silver at the cathode.
(3) A compound that does not undergo an electrochemical reaction. Such compounds usually have Ag 2 Se, Ag 2 Te, Cu 2 Se, Cu 2 Te and the like. Because of their small electrochemical activity, they do not change during electrolysis and fall off into the anode mud as the anode dissolves. However, when metal selenium is present in the anode, it can be dissolved together with silver and precipitated at the cathode in the weakly acidic electrolyte. However, in a solution with high acidity (maintained at about 1.5%), the selenium in the anode does not enter the solution.
(4) Copper, bismuth, and antimony whose potential is close to that of silver. These metals are the most harmful to electrolysis.
In the electrolysis process, a part of the basic salt [Bi(OH) 2 NO 3 ] is introduced into the anode mud, and the other part is nitrated into the solution. After the ruthenium and osmium accumulate in the solution, they will precipitate on the cathode, which deteriorates the quality of the electrolytic silver. When electrolyzed under low acid conditions, the cerium nitrate in the solution will hydrolyze to form a basic salt precipitate, which will affect the quality of the electrolytic silver powder.
The content of copper in the anode is usually the most, often up to 2% or more. During the electrolysis process, copper enters the solution as copper nitrate, which turns the color of the electrolyte blue. Since the potential of copper is lower than that of silver by more than half, and the concentration of copper in the silver nitrate solution can be precipitated at the cathode, in the case of normal electrolysis, the possibility of copper being precipitated at the cathode is not large. However, when concentration polarization occurs, or the electrolyte is stirred poorly, and the silver ions sink sharply, causing the ratio of silver to copper in the electrolyte to exceed 2:1, copper will precipitate on the upper portion of the cathode. And during the movement of copper ions to the cathode in the solution, Cu 2 + may be reduced to Cu + , and Cu + may be oxidized to Cu 2 + at the anode, and the current is consumed in vain. In particular, when an anode having a high copper content is electrolyzed, since only silver is precipitated in the cathode, and each 1 g of copper is dissolved in the anode, 3.4 g of silver is precipitated in the cathode, which easily causes a sharp drop in the concentration of silver ions in the electrolyte. There is a danger of copper being precipitated in the obsolete cathode. When electrolyzing an anode with a high copper content, a part of the electrolyte containing a large amount of copper should be taken out frequently, and a part of the silver nitrate liquid having a high concentration should be added. It should be noted, however, that it is also advantageous to maintain a certain concentration of copper in the electrolyte during the silver electrolysis process because copper can increase the density of the electrolyte and reduce the sedimentation rate of the silver ions.
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