The principle of gold and silver ingot casting - metal protection and deoxidation

When gold or silver metal is melted in the air, it can dissolve a large amount of gas. These gases are released when the metal condenses, which causes difficulties in production operations and causes loss of metal.

When silver is completely melted in the air, it can dissolve about 21 times the volume of oxygen. These oxygens evolve to form "silver rain" when the metal condenses, causing splashing of the fine silver beads. If the oxygen is too late to be released, defects such as shrinkage, pores, and pitting may be formed in the silver ingot. In particular, in the case of ingot casting of alloy materials, in order to obtain a good quality ingot, it is necessary to protect the alloy liquid surface from oxidation and to prevent the alloy from being saturated with gas. A protective solvent is often added for this purpose. It forms a protective shell on the surface of the alloy.

According to practice, when the temperature of molten metal silver increases, the solubility of oxygen in silver decreases (Fig. 1). In order to reduce the difficulty in silver casting, the temperature of the silver liquid should be increased before casting, and a layer of reducing agent (such as charcoal) should be placed on the surface of the silver to remove oxygen. A piece of pine is also added to the charge and burned as the silver melts to remove some of the oxygen. Before the casting, the wooden stick was used to stir in the silver liquid, and the effect was good. In foreign countries, there is also melting of silver in a vacuum. In the past, some mints added a little copper , zinc or cadmium to the silver to deoxidize. In order to obtain easily rolled silver, silver in the United Kingdom was slightly higher than the liquid temperature was increased to 1300 ℃, iron block and put in silver, then at 1200 ~ 1300 ℃ is cast into the mold preheated to 200 ℃ in. In some factories, when adding wood blocks, at the same time, before the casting, a small amount of residual residue from the liquid surface is removed from the mouth to the back, and a piece of graphite is placed in the mouth (sawed from the waste graphite crucible), and then to the liquid. Add a large bowl of grass ash and cast it. In this way, it is possible to burn and remove part of the oxygen, and to heat and adsorb the residual surface of the liquid to improve the quality of the ingot.

Figure 1 Solubility of oxygen in silver at different temperatures

Gold is more inspiratory, and when molten in air, gold can dissolve 33 to 48 volumes of oxygen, or 37 to 46 times of oxygen. However, since the casting temperature of gold is generally high and is carried out in the open flat mold, the mold is often preheated to 160 ° C or above, and the gas is easily released. In order to ensure that the ingot surface is flat, some factories also take measures such as watering the ingot surface or covering the wet paper to accelerate the surface first.

As for the flue gas, since the molten gold or silver raw materials are relatively pure, the flux and the oxidizing agent are not added in a large amount, and although a small amount of carbon dioxide, chlorine dioxide or the like is present, there is no significant influence on the ingot.

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