Chapter 1:
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There are different types of furnace crucibles made from different properties. The most popular and commonly used furnace crucibles are; graphite crucible, clay graphite crucible, silicon carbide crucible, platinum crucible, porcelain crucible and metal crucible. These crucibles are termed according to the raw materials they are made from.
The main market for furnace crucibles is the foundry industry used for melting and testing metal. Crucibles are also used for chemistry research. With the growing metal and metal production industry, the need for specialized metal and alloy casting is increasing day by day and durable and efficient furnace crucibles are needed to carry out melting and casting activities.
1.1, Graphite Crucible
Graphite furnace crucibles are refractory containers specially shaped for metallurgical operations. Graphite furnace crucible is primarily made of graphite material that is typically grayish-black in colour, opaque, and has a radiant blaack sheen. The properties of the graphite furnace crucible distinguishes it from other types of furnace crucibles.
The graphite material that makes up the graphite furnace crucible itself is tightly fused together and no part becomes loose when metal is being melted, therefore, your metal is protected from contamination. Graphite furnace crucibles can withstand high melting temperatures of metal up to degree celsius.
Graphite furnace crucibles have good resistance to chemical corrosion and thermal shock. There is a good demand for graphite furnace crucibles based on the assessment and growth in foundry and casting industries. Graphite crucibles are used to melt ferrous, non ferrous metals and alloys can be melted and smelted using a graphite furnace crucible.
Graphite crucibles prevent contamination of molten metal when melting and holding metals. Other properties of the graphite furnace crucible include:
Thermal stability: The graphite furnace crucible is able to withstand the changing temperatures of metals during the melting and holding process.
Corrosion resistance: The uniform and dense design of the graphite furnace crucible can effectively prevent corrosion of the graphite furnace crucible.
Impact resistance: The impact strength that graphite furnace crucible can bear is very high, therefore, any further process can be done easily.
Acid resistance: Special materials such as flux and additives are added to molten metal in the crucible in order to improve the melting quality of such metal, this means that graphite crucible needs to be able to withstand the effects of additives and fluxes, this therefore greatly extends the life of graphite crucible.
High thermal conductivity: The high content of fixed carbon ensures good thermal conductivity of the graphite furnace crucible, and reduces the time of dissolution or melting, significantly reducing fuel consumption, or other energy consumption.
Metal pollution control: The material composition is strictly controlled to keep the metal from being polluted by the graphite crucible.
Quality stability: High pressure forming method and quality assurance system fully ensures the high quality molten metal.
Pure graphite furnace crucible is made of 99.9% of graphite. Its features include excellent thermal stability and great heat transfer performance for fast heating and cooling and corrosion resistance to strong acid and alkali. Pure graphite furnace crucible is used to melt and cast metals such as gold, silver brass, aluminium, copper, zinc, etc. Pure graphite furnaces cannot be heated by a carbon furnace or charcoal kiln, it will oxidize and crack if any of these furnaces is used.
Pure graphite starts to oxidize at 400 degree celsius and they withstand temperatures degree celsius when used in an air electric furnace. The withstand temperature in a vacuum or gas protection furnace is degree celsius. Graphite furnace crucible is generally used in alloy tool steel smelting and smelting of nonferrous metals and their alloys. Graphite is more durable than ordinary material products, which makes graphite furnace crucible to have a long service life under correct use.
Silicon carbide graphite furnace crucibles are high quality melting crucibles made out of silicon carbide and graphite. Typical silicon carbide characteristics include: low density, high strength, good high temperature strength (reaction bonded), oxidation resistance, excellent thermal shock resistance, high hardness and wear resistance, excellent chemical resistance, low thermal expansion, refractory material (high melting point) and high thermal conductivity. Silicon carbide possesses interesting electrical properties due to its semiconductor activities.
Silicon carbide is formed in two ways, reaction bonding and sinistering. Each forming method greatly affects the end microstructure. Reaction bonded silicon carbide is made by inflitering compacts made of mixtures of silicone carbide. And carbon with liquid silicon. The silicon reacts with the carbon forming more silicon carbide which bonds the initial silicon carbide.
The sintered silicon carbide is produced from pure silicon carbide powder with nonoxide sintering aids. Conventional ceramic forming processes are used and the material is sintered in an inert atmosphere at temperature reaching up to degree celsius or higher.
Both forms of silicon carbide are highly wear resistant with good mechanical properties.
The raw materials, graphite and silicon carbide with additives such as ferro silicon, ferro manganese etc, are graded to the required sizes and mixed with bounding materials like molten pitch or other suitable synthetic resins in suitable proportions. The kneaded homogenous mixture is charged under a specified technical process. While this operation is carried the total mass of mixture in the mould is kept in uniform hot condition to avoid manufacturing defects while pressing.
This furnace crucible is mostly used for melting non-ferrous precious metals such as brass, copper, nickel, chromium as well as their alloys. The silicon carbide furnace crucible is inspected for lamination, cracks, etc. and then placed in a kiln for firing at - degree celsius under reducing atmosphere to avoid oxidation of graphite and carbon. To create a reducing atmosphere, the silicon carbide graphite furnace crucibles are loaded into specific fireboxes tightly packed with reducing agents like coke/coal dust, sometimes graphite powder.
The main components of clay graphite furnace crucible are natural flake graphite and clay. Clay graphite furnace crucible contains 30% to 50% of graphite, the refractoriness of clay graphite crucible is similar to clay bricks, but thermal shock resistance and chemical corrosion resistance of clay graphite furnace crucible is significantly better than clay bricks. Clay graphite furnace crucibles are used in high temperature melting of iron, steel, copper alloys or melting of precious metals which can handle large batches of material.
Their thick walls can handle direct heat and open flame foundries. They are mostly used with gas or propane and medium/high heat furnaces and kilns. The clay graphite furnace crucible can withstand a temperature of 850 to degree celsius. The clay graphite also needs to be preheated or tempered at 300 degrees celsius for an hour before melting can be done with it. If tampering is not done and metals are melted directly, it can cause damages to the clay graphite furnace crucible.
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The performance characteristics of the clay graphite furnace crucible are; controlled electrical resistivity, good thermal conductivity, good resistance to chemical corrosion, high refractoriness and good oxidation to resistance.
In melting with a clay graphite furnace crucible, light metals should be charged first before heavier metals, the light metals serve as a cushion for the heavier materials.
1.2, Quartz Crucible
Quartz crucibles have many desirable properties including high chemical purity, high corrosion resistance, high melting point, extreme hardness, low coefficient of thermal expansion, excellent electrical insulation, good corrosion resistance to ensure product quality and reliability, etc.
Metals with very high melting points such as palladium and platinum are usually melted with the quartz crucible. Platinum has a melting temperature of degree celsius while palladium has a melting temperature of degree celsius. The best type of crucible that can melt and hold these high melting temperature metals is the quartz crucible simply because it can stand high and varied use. Quartz crucible can also be used in alloy steel smelting, and smelting of nonferrous metals and their alloys. Quartz crucible can be used for special alloy refining.
1.3, Metal Crucible
Metal crucible may be in the form of steel, which can be used to melt metals whose temperature is less than degree celsius, because steel melts between to degree celsius.
Homemade steel crucibles can be used to melt metals such as aluminum and zinc, because these metals melt at a temperature well below the melting temperature of steel. However flaking (scaling) of a steel crucible interior surface is a drawback. This scale can contaminate the melt and slender the walls of crucible rather quickly. Coating of marcote-7 can be applied to the crucible to provide some level of protection between the steel and the metal being melted. Steel crucibles will work for starters in home foundry who don&#;t mind dealing with the scaling.
1.4, Porcelain Crucible
The term porcelain refers to a wide range of ceramic products that have been fired at high temperature to achieve vitreous or glassy qualities such as translucence and low porosity. Porcelain is a ceramic material made by heating ceramic raw materials, generally including clay in the form of kaolin, in a kiln to temperature between to degree celsius.
High thermal shock resistance of porcelain crucibles is attributed to the presence of mullite or cordierite structures. Hence, porcelain crucibles are classified as mullite crucibles. They are used in induction furnaces for melting glass and metals and in most cases, it is suggested for safe use up to degree celsius. The porcelain crucible is usually a cup-shaped container used in laboratories for heating purposes. The apparatus contain chemical compounds for heating at extremely high temperatures.
1.5, Which Material of Crucible is most Suitable for Precious metal Smelting Furnace
The different types of crucibles can be used in different melting applications based on the melting and smelting temperature. The modern furnace crucible is highly heterogeneous, graphite-based composite material, which relies on its composition and control of graphite structural alignment to achieve the desired performance.
Furnace crucible materials range from silicon carbide, graphite, clay graphite, porcelain, quartz, etc. They offer many different performance characteristics since each application presents a complex set of temperature, chemical and physical parameters which define the technical boundaries within which the furnace crucible has to be designed to operate. Before selecting the right crucible furnace for smelting, the following should be considered.
The metal, alloy or ore you want to melt or hold will determine the temperature range within which your furnace crucible. Crucibles should not be heated above their melting temperatures, this can lead to an irrevocable damage of the furnace crucible.
At the same time, operating below the furnace crucible&#;s lower temperature limit can also lead to problems during melting. Melting and holding practices should be taken into consideration before furnace crucible setup and use. If your smelting requires superheating, you need to take higher metal temperatures into account.
The ability of a furnace crucible to handle the rate of temperature change is as important as its minimum and maximum temperature limits. Melting requires constant temperature changes and the furnace crucible you choose should be a to withstand the frequent furnace crucible temperature changes, i.e resistant to thermal shock. Some crucible types are much better at handling rapid temperature change than other types.
The high carbon content in a graphite furnace crucible contributes to its high thermal conductivity and non-wettability. When the graphite in the graphite furnace crucible forms a directionally oriented matrix, the crucible also provides high thermal shock resistance. This is critical to foundry operations where temperature changes by hundreds of degrees within a few seconds.
If your furnace is always charged with molten metal then it does not require a furnace crucible that is designed to be highly resistant to physical damage. On the other hand, if your precious metal ingots or ores make up the bulk of your charge, and they are not carefully lowered into the furnace, through an automatic loading system, a crucible that is mechanically strong and able to withstand physical shock is highly recommended.
Crucibles that have high carbon content and a directionally oriented graphite structure are capable of such excellent impact resistance. A furnace crucible with a durable glaze is also recommended. Damage to the glaze from rough handling can lead to oxidation damage of the furnace crucible. Extruded aluminium ingots often have sharp edges that cut deeply into a furnace crucible&#;s body.
All furnace crucible models have some level of resistance to corrosion and chemical attack. Most of these fluxes and other metal treatments used in smelting and melting nonferrous metals are highly corrosive and require a furnace crucible that offers a high level of resistance to chemical attack.
This resistance is best impacted by both a consistently dense furnace crucible material structure and a durable protective glaze. For example, to separate gold from heavy material, the enriched pre-concentrate is placed in a furnace gold crucible with borax and baking powder or ammonium chloride and heated at a temperature of degree celsius.
This causes the oxidic materials such as limonite, ilmenite, etc to melt. In the solid liquid system which is created, liquid gold concentrates at the bottom of the crucible underneath tha slag. The melting temperature of gold is degree celsius.
If your smelting requires the use of corrosive metal treatments, a furnace crucible that offers protection the appropriate level of protection against these agents is needed.
Another factor that should be considered when planning and checking your crucible requirements based on your furnace&#;s specifications is power availability. In many locations, power for melting or holding might not be available at all times or might be absurdly expensive at certain times or at certain levels. If this is the case at your facility, it may be particularly important to select an energy efficient furnace crucible.
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