Coal Dust And The Metal Casting Process
Coal dust - typically high quality milled anthracite - has long been used in metal casting to enhance surface finish, prevent wetting and improve releasability. Anthracite, alongside many other materials for casting and foundry applications, is supplied in the highest quality by African Pegmatite.
Metals are cast in foundries by melting them into a liquid form and pouring the molten material into a mold. Thereafter, the mold is removed and the metal allowed to return its solid form as it cools off, although in some cases the metal is left to cure in the mold. By far, the majority of the metals processed in foundries are aluminium and cast iron. Although castings of bronze, brass, steel, and zinc are also done in foundries.
The process of metal casting starts with the physical conversion of the chosen metal into its molten form. The process takes place in a furnace and begins by charging the furnace with external scraps, internal scraps, virgin material, and alloying elements. Virgin material is the pure form of the primary metals used in manufacturing a specific alloy or metal and sold in commercial quantities, these are typically ores. Internal scraps include any defective castings, gates or risers produced within the factory. External scraps, on the other hand, are materials produced during punching, machining, or forging.
After melting the charge materials, refining takes place. The purpose of refining is to eliminate deleterious substances from the molten material and avoid casting defects. As the charge materials undergo melting, materials are added to meet up with industry or internal specifications. Fluxes are used in the separation of metal from slag, and degassers ensure the removal of dissolved gasses from metals. Combined, these produce a metal that is best suited to casting and its final application once cooled.
Variety of furnaces for heating metals exists. However, the choice of a furnace is influenced by the amount of alloy produced. Electric arc furnaces, cupolas, and induction furnace are used in the case of ferrous metals. On the other hand, metal casters would prefer reverberatory or crucible furnace for non-ferrous materials such as brass, bronze, and aluminum castings. One common feature of these furnaces is that they are lined on the inside by refractory materials. Refractory materials are key to the ongoing success of the modern foundry, providing insulation qualities which can lead to lower fuel costs.
Degassers such as chlorine, nitrogen, helium, and argon are used in removing contaminant gases present in non-ferrous metals. Conversely, carbon monoxide is used in degassing ferrous materials including iron and steel. Hydrogen is the most harmful gas that requires degassing. Hydrogen is formed from the reaction between materials or water vapor or machine lubricants. A high hydrogen concentration indicates an increased porosity of the metal which weakens its mechanical properties. In the case where porosity still exists following degassing, porosity and leak paths are sealed through the process of vacuum impregnation.
The metal casting process also involves creating patterns made out of wood, plastic, metal, or wax. The various processes involved in the construction of molds is determined by the size and complexity of the casting, the metal to be poured, type of foundry, and the number of parts to be produced. The mold processes used in metal casting are die-casting, billet casting, sand casting, lost foam casting, v-process casting, and ceramic mold casting amongst others. Many foundries seek to use reusable molds as this cuts down on production costs massively and reduces waste.
The next step that follows after the melting and the mold-making process is pouring the molten metals into the mold. In foundries operating on the traditional model, the molten metals are poured into the molds with ladles. Advances in technology have allowed for the use of automatic pouring machines and robots in pouring molten metals. However, pouring can also be achieved with gravity or the aid of vacuum or pressurized gases. Continuous casting setups may be used, employing tunishes, refractory-lined ladles and other apparatus. Such innovations vastly improve the efficiency of the modern foundry.
The molten metal is allowed to assume a solidified state before attempting to remove it from the mold. Shaking and tumbling are the primary means by which the solid metal is removed from the mold, particularly those made through sand casting. During the process of casting, heads, runners, gates, and risers are formed. These are removed using cutting torches, bandsaws, or ceramic cutoff blades in a process known as degassing.
Use Of Foundry Coal Dust In Sand Casting
A popular technique used in sand casting is green sand. It is a mixture of silica sand, chromite or zircon sand, bentonite, water, inert sludge, and coal dust produced from the pulverization of coal.
The use of coal dust is typically in the form of anthracite - a higher quality coal. Historically, lower grade and/or more bituminous forms of coal would be used, to the determinant of the environment. Bituminous coals emit more toxic gases when burned.
Greensand is not in itself wet but denotes its wet state – a feature of “greenwood.” The amount of coal dust in green sand never exceeds 5% of the total mixture and undergoes partial combustion(pyrolysis) in the presence of molten metal, releasing an off-gassing vapor.
However, green sand is not used in the casting process for non-ferrous metals due to the presence of coal dust as an additive. Coal dust gives off carbon monoxide which results in the oxidation of the metal. Aluminum, for instance, uses olivine sand in the place of coal dust as an additive.
Greensand possesses certain characteristics augmented by the addition of coal dust. These properties include:
- Refractoriness: This refers to the ability of green sand to resist high temperatures without getting deformed. Steel requires molding sand that can withstand a temperature of 1500oC whereas a temperature value of 650oC is needed for aluminum alloys. Foundry sands with reduced refractoriness will melt and fuse with the casting. Coal dust, as an additive, has a fusion temperature of more than 1600o Hence, the temperature at which molding sand is increased with the addition of coal dust to the mixture.
- Surface finish: A better surface finish is achieved with finer particles. Unfortunately, finer particles indicate an increased permeability but an improved surface finish. A better surface finish means less machining is required post-casting.
- Permeability: The ability of molding sand to deplete the available gases is referred to as permeability. Gases formed during the pouring process such as hydrogen, nitrogen, steam, and carbon dioxide results in casting defects, including blowholes and gas holes. Carbon monoxide is passed over steel and iron castings to remove unwanted gases and prevent oxidation. The combustion of foundry coal dust results in the release of carbon monoxide – which as was mentioned earlier is needed in degassing castings of ferrous materials. Carbon monoxide is also produced during the gasification of coal dust where it is passed through oxygen and steam under high temperature and pressure.
- Collapsibility: This refers to the sand’s ability to strip off the solidified metal casting with ease. Increased adherence to the metal casting exists with molding sands having low collapsibility. Foundry coal dust help to increase the collapsibility of green sand at knockout, since this substance burns out during the casting process.
Other features of molding sands that include as greensand are cohesiveness, superior chemical inertness with regards to the metal being cast, and availability/cost of the molding sand. It is noteworthy that chrome-containing greensand is more expensive than non-chrome sand and is thus typically reserved for higher end castings or where maximum temperature tolerance is required. The addition of coal dust/anthracite to a greensand casting mold is associated with modest gains in compressive strength due to preferential associations with the binding agents.
Preventing Foundry Burn And Surface Defects In Ferrous Castings
Assuming an otherwise optimal casting process, one of the major drawbacks of sand and greensand castings is the potential for surface defects. Surface defects can come in many different shapes and sizes, resulting from slightly different causes, with many grouped under the broad term ‘foundry burn’:
Burn on is a defect that is caused by the shallow penetration of molten metal into the sand bulk, which occurs when the mold gets sufficiently hot that partial decomposition of the (oftentimes clay based) binder occurs. The result is molten metal flows into the hot sand, where it solidifies. Particularly serious cases of purn on are where localised overheating (from the metal, causing ‘hot spots’ in the sand) allowing deep flows into the sand mold. These phenomena result in a casted product that is not only harder to remove from the mold, but one that requires post-casting machining to remedy the metal protrusions. Additionally, a mold that has suffered from burn on cannot often be reused.
Other, related, impacts include ‘burn in’ which differs from burn on in the size and distribution of the surface defects and is more correctly known as fusion. Sintering of the clay binders and of the silicate binders can also occur, causing the formation of iron silicates in the case of an iron or steel casting. Silicates themselves can become sintered and melt, allowing more penetration of metal into the mold. In this scenario, sand grains can fuse and meld into each other. They then deposit on the casted surface and are very difficult to remove.
Coal dust in the sand as an additive is key in the prevention of these defects occurring. The aim is to prevent molten metal from penetrating into the sand and reacting with the compounds distributed within it. When coal dust (or any other carbonaceous material) located in the sand is heated sufficiently, it will pyrolyse. Pyrolysis of these carbon compounds produces a modest amount of carbon monoxide gas and a thin layer of carbon between the molten metal and the sand mold. Together, these provide an effective barrier between metal and sand, thereby preventing foundry burn. The gas layer is largely considered a secondary protective measure, compared to the carbon. This is an unusual example of non wetting behaviour, unexpected as carbon is soluble in many metals at such high temperatures.
Although modest pressure increases will be noticed using coal dust/anthracite in a greensand casting scenario, they are well within tolerances and as such there is no risk to the mold or the casing itself. The pressure increase is due to water evaporation from the sand itself, in addition to gases evolved in pyrolysis. Hydrogen (mentioned earlier) is the only potential problem gas, which can penetrate the metal if present in sufficient quantity, owing to its small atomic radius.
Iron oxide formation can be another cause of surface defect formation, which is prevented by the carbon monoxide generating a reducing environment, as a direct result of the pyrolysis. Reducing environments are also provided by hydrogen gas evolution from pyrolysis. Overall, a reducing atmosphere is beneficial as it can aid in the prevention of silicate and other oxide formation.
Coal dust and anthracite are not used in non-ferrous castings as their in situ pyrolysis resulting in carbon monoxide evolution is not useful. Alternative methods to prevent surface defects like burn on, burn in and penetration can be considered such as non-coal derived organics and the selection of alternative sand types.
Properties Of Coal Dust Suppliers For Foundry Purposes
- Volatile content: The quality of the surface finish for a casting done with foundry coal dust as an additive depends on a high amount of volatile matter. It is recommended that foundry coal dust have a minimum of 30% volatile matter.
- Ash content: Coal dust low ash content, not exceeding 12% is recommended from coal dust suppliers.
- Sulfur content: A high sulfur content may lead to casting defects. Coal dust suppliers should limit the sulfur levels to 1% at most.
- Chlorine content: The chlorine content should also be reduced to the barest minimum. An excess of chlorine may also result in casting defects.
In general, higher quality coals such as anthracite make the best coal dust - often with low volatile, ash, sulfur and chlorine contents. Also, a mixture of foundry coal dust and clay may be used to line the bottom of a cupola furnace. Exposure to high-temperature results in the decomposition of coal dust and clay becomes slightly crumbled. Consequently, open holes are formed, through which the molten metals are tapped during the metal casting process in a furnace.
- Coal dust, often anthracite, has long been used in metal casting applications
- Most commonly used in greensand casting, anthracite is used to prevent wetting and to ensure a better surface finish
- Wetting, and other operational problems, cause surface defects such as burn on, oxidation or the proliferation of metal burrs
- Surface defects need to be manually removed by machining, increasing time and labour costs
- Modern foundrymen seek to eliminate the need for post-casting machining, and aim for a reliable, scalable system - which can be achieved with coal dust/anthracite
African Pegmatite is a leading supplier of minerals and materials for a wealth of foundry settings. Broad experience, far reach and in-house milling make African Pegmatite the ideal partner for the provision of coal dust and anthracite for any industrial process.