Induction melting furnace smelting of silver and its alloys
Characteristics of silver and its alloys
Silver is a precious metal with a melting point of 960.8Y and a density of 10.49g/cm3. It does not oxidize at room temperature. Pure silver is silvery white. It can form an alloy with any proportion of gold or copper. When the alloy contains gold or copper proportions As it increases, the color turns yellow. When silver is eutectic with aluminum and zinc, it is also very easy to alloy. Among all metals, silver has the best conductivity.
When silver is smelted in a general metallurgical furnace, it will oxidize and become volatile. But when there is splashed metal (splashed metal refers to the low-priced metals that coexist and exist as impurities in the ore, concentrate and intermediate products of metallurgical plants of gold, silver and tong group metals, mainly including copper, lead, zinc The silver oxide is quickly reduced. Under normal smelting (furnace temperature 1100-1300^), the volatilization loss of silver is about 1% or less, but when the oxidation is strong, there is no covering agent on the molten silver, and the charge contains more lead, zinc, monuments, fetters, etc. When the metal is volatilized, the loss of silver will increase.
When silver melts in the air, it can absorb approximately 21 times its own volume of oxygen, which is released when silver is condensed to form a boiling state, commonly known as “silver rain”, which will cause the splash loss of fine silver beads.
Silver casting process
The final step of silver purification and refining is to melt the high-purity silver powder or silver plate refined by electrolytic or chemical methods, and then cast into ingots or pellets that meet national standards or other specifications.
The induction melting furnace used for the noble casting of gold and silver. The capacity can be selected according to the daily processing capacity of gold and silver, usually about 50~200kg. If there are special needs, a larger induction melting furnace can also be used for induction melting. The main points of the technical operation of furnace smelting silver are as follows.
AAdd proper amount of flux and oxidant
Generally, saltpeter and sodium carbonate or saltpeter and borax are added. The amount of flux and oxidant added varies with the purity of the metal. Such as smelting electrolytic silver powder containing more than 99.88% silver, generally only add 0.1% -0.3% sodium carbonate to oxidize impurities and dilute the slag; while smelting silver with higher impurities, you can add appropriate amount of saltpeter and borax to strengthen Oxidize a part of the impurities to make slagging and remove. At the same time, the amount of sodium carbonate should be appropriately increased. The amount of oxidant should not be too much, otherwise the crucible will be strongly oxidized and damaged.
After the smelting process of oxidation and slagging, the silver grade of the cast ingot is higher than that of the raw material silver, so it is necessary to add an appropriate amount of protective flux and oxidant.
B Strengthen the protection and deoxidation of silver
When silver melts in the air, it can dissolve a large amount of gas, which is released when it is condensed, which brings difficulties to the production operation and causes the loss of metal.
When silver melts in the air, it can dissolve approximately 21 times the volume of oxygen. This oxygen is released when the metal is cold, forming a “silver rain”, causing the splash loss of fine-grained silver. If the oxygen is too late to release, defects such as shrinkage holes, pores, and pitted surfaces are formed in the silver ingot.
In actual operation, when the temperature of the molten silver increases, the solubility of oxygen in the silver decreases. In order to reduce the difficulty of casting, the temperature of the silver liquid should be increased before casting, and a layer of reducing agent (such as charcoal, plant ash, etc.) should be covered on the surface of the silver liquid to remove oxygen. There is also a piece of pine wood added to the charge, which is mainly burned with the melting of silver to remove part of the oxygen. There is also the use of wooden sticks to agitate the molten liquid before casting to achieve the purpose of deoxygenation.
C master the pouring temperature
When the silver metal is cast, the increase in metal temperature is helpful to reduce the amount of gas dissolved, and the overheated metal is poured into the mold, and the condensation rate is slow, which is beneficial to the release of gas and reduces the defects of the ingot. Usually the casting temperature of silver should be 1100-1200T; o
D mold wall should use paint, pouring operation should be reasonable
When the silver ingot is cast, use ethane or petroleum (heavy oil or diesel) flame to smoke a thin layer of smoke evenly on the inner wall of the mold, and the use effect is good.
In addition, the quality of the casting operation has a lot to do with the quality of the ingot. For vertical mold casting, the liquid flow must be stable, the flow must be in the center, and the material must not be scattered and the inner wall must not be washed. Start a trickle, and then rapidly increase the liquid flow until the metal surface is filled with about three-fifths of the mold height, and gradually slow down to allow the gas to be fully discharged. When pouring to the gate, pay attention to replenishing the flow until the solution is not pumped in. For the open integral flat mold, the operation is relatively simple, as long as the mold is placed on a horizontal surface, the ground scroll is perpendicular to the long axis of the mold, and the molten metal is evenly poured into the core of the mold. In order to protect the inner wall of the mold, the position where the molten metal is poured must be constantly changed during casting to prevent the center of the mold from being corroded into a pit.