The cementation process is already known from the ancient times of human culture but the early knowledge about the process was spread in Europe at the beginning of the Middle Ages. At that time, the process was used by alchemist mainly for the performing of the miracle of metals transmutation. From several decades cementation is commonly used in industry for recovery of metals, removal of metal ions from dilute wastes and for purification of solutions. Cementation is also a widely spread method of purification of various electrolytes in electrolytic production of nickel or zinc. 

The silver ion cementation on copper was introduced in practice to the recovery of silver from industrial baths, based on sulphuric acid and copper sulphate, used for electrowinning and electroplating of copper. The final industrial product of copper electrowinning should not contain even traces of silver. Otherwise, the presence of silver in the material affects strongly various properties of pure copper (e.g. mechanical properties) and impedes the process of wires formation. Although, the process has been used in practice for a long time, the theoretical background of cementation still remains questionable. This heterogeneous reaction is especially complicated by the fact that the reduction of metal ion and oxidation of the metal base occur at the same time on the same surface (Fig. 1). The direct coupling of both half-cell reactions involves a formation of cathodic and anodic sites on the surface of the metal base. Additionally, various valence states of copper ion ( Cu²⁺ i Cu⁺) in acidic sulphate solutions and the presence of oxygen influence strongly the mechanism of the process.

Fig. 1 Cementation of noble metal ion (M^m+) on the surface of sacrificial metal (N)

The silver ion cementation on copper in acidic sulphate solutions follows first-order kinetics and the overall process is controlled by mass transfer of Ag⁺ to the reaction surface. The presence of oxygen in the system modifies strongly not only the mechanism of the process but also the morphology of silver deposit.  The mechanism of the process consists from two stages. In the first stage of the reaction, Cu⁺ ions appear in the solution independently of the presence or absence of oxygen.  In oxygen-free solutions, Cu⁺ ions generated in the first stage can be transferred partially to the bulk of the solution (reaction 2b in Fig. 2B) and just there can react with the another silver ion with a creation of colloidal silver.  

A)  

B)

Fig.2 Mechanism of silver ion cementation on copper in the oxygen saturated (Fig. 2A) and oxygen-free (Fig. 2B) solutions

The different mechanism of the process results in a different  morphology of the cemented silver deposit. The composition of an electrolyte, especially an initial silver ion concentration and copper sulphate concentration have a huge impact on the silver morphology. SEM images in Fig. 3 show formation of cathodic sites on the reacting surface in the cementation process in the oxygen-free 0.5 M sulphuric acid.

 A)  B)  

C)   D)  

Fig .3 Growth of the cathodic site in the cementation process conducted in the oxygen-free 0.5 M H₂SO₄ containing 20 mg/L Ag⁺

The growing dendrite behaves as a cathodic site with a relatively surface area and promotes a creation of an anodic site in a close neighbourhood. Such an anodic side is visible in Fig. 4 as a crack encircling the protrusion. Anodic sites develop their working surface area in the copper material just under the deposited silver, with a formation of deep cavities.

Fig. 4 The anodic site formed in the cementation process conducted in the oxygen-free 0.5 M H₂SO₄ containing 20 mg/L Ag⁺