Bentonite: Applications With Coal As Bentonite-Clay In Foundry Applications, And Others
Bentonites are aluminium phyllosilicate clays that are comprised mostly of montmorillonite. Montmorillonite itself is a type of dioctahedral smectite - the crystal structure of which is a layer bearing octahedral geometry sandwiched between two layers of tetrahedral geometry. Types of bentonite are demarcated by the name of the most prevalent metal in them, for example sodium bentonite (Na-bentonite) and calcium bentonite (Ca-bentonite). Na-bentonite is one of the most common, and is prized for its ability to swell, tolerance of high temperatures and is regarded as generally a better binder(1). Na-bentonite originates from volcanic ash that was deposited in marine environments long ago. Ca-bentonite, on the other hand, is not known for its swelling properties and itself derived from volcanic ash deposited in freshwater environments(2). With regards to water, structural water is eliminated from bentonites by heating in the range of 400 to 500°C, and the crystal structure is changed completely beyond 900°C. In comparison to the chemically similar kaolin, bentonite is regarded as much more easily molded and sintered.
Bentonites have a wide variety of uses from cosmetics to detergents, and desiccants to fertilizers. Here, we will focus mostly on the use of bentonite in foundry applications, especially when it has been mixed with coal. Typically with bentonite clays, the coal of choice is anthracite. Itself a widely used material, outside of fuels it has found use in foundries and other high temperature environments.
Bentonite-coal mixtures are inexpensive, which in addition to their wide use detailed below, adds to their attractiveness in industrial settings.
Use Of Bentonite In Foundry Environments With Coal
Green And Dry Sand Castings
It is stated that roughly 70% of ferrous castings are made from green moulding sand(3). In green sand castings, the sand is held together with bentonite/clay and water as binders; it is said to be ‘green’ as the molds are largely reusable and recyclable. This green moulding sand contains in the region of to 10% by mass of bentonite clay, up to two percent carbon (i.e. coal), up to 5% water with the balance being sand. Montmorillonite is said to be key in the casting production process, particularly with deference to mold regeneration(4).
One study looking into the flowability of green sand for casting applications found that bentonite is essential in allowing for good sand flow (and thus a better casting mold) and that the ideal ratio of bentonite to carbonaceous material is 3:1(5) and a water content of no more than 3%. Other studies have found optimal bentonite loadings of 5% by mass; giving a compression strength of 53 kN/m2 whilst remaining easy to handle(6).
Hot and dry strength are key requirements for green sand casting, and therefore sodium bentonite is the preferred iteration of the clay(7). In the casting process, only very small amounts of montmorillonite become decomposed, and bentonite containing in excess of 70% montmorillonite by mass is preferred for casting applications. Carbonaceous materials such as coal are used in casting mold sands as additives, and are mostly burned off. The reason for using the carbonaceous material (coal) is to ensure a better surface finish on the metal by preventing molten metal mixing with sand in a process called wetting. Typically for green sand, the coal is in dust form and is sourced from anthracite - one of the forms of coal with a higher carbon and lower sulfur content.
In castable refractories, where bentonite-coal has been used, thermal expansion properties have been seen to be enhanced. Resistance to thermal shock has also been increased(8) in a calcined refractory.
With regards to dry sand castings, the principles are largely the same except recyclability is not prioritised. Calcium bentonite can be used in sand casting applications, but they are more susceptible to erosion and are more prone to scabbing and other defects caused by expansion(9). Overall, it can be said that the addition of bentonite to green and dry sand casting molds increases its flowability and compressive strength, whilst retaining high heat tolerance. It is estimated that industrial casting uses in excess of 25% of the global production supply of bentonite every year(10). Wet compressive strength (i.e. pre-curing or resting of the mold) is higher with increased quantities of bentonite and coal(11) - which is a useful property should any kind of extrusion or mechanical compaction process be employed in the mold formation.
Both sand and bentonite clay can be reclaimed from foundry processes(12). Using an ‘advanced oxidation’ process, reclamation and recycling back of used bentonite is enhanced(13). The process takes spent bentonite and sand from the casting process, treats it oxidatively and using this methodology removes thermally destroyed bentonite in the waste channel. In comparison to a traditional system, this advanced oxidation treatment sends only 3% bentonite (by weight) to waste, compared to 30-50%. Such a process is advantageous as it reduces the amount of fresh bentonite required for ‘make up’ when forming a new casting mold.
Other Bentonite-Coal Uses
In the patent literature, bentonite alongside coal dust has been used to repair blast furnace linings(14) and damaged portions of runners through furnaces or cupolas(15). Both methods, in essence, use bentonite-coal along with other materials such as graphite and calcium chloride to produce a putty which is applied to the break, and cures rapidly in situ. These repairs are designed to be small scale.
Use Of Bentonite In Refractory Environments Without Coal
Pelletisation of iron ore is one of the key uses for bentonite clays in refractories without coal. The bentonite is employed as a flux, making firing a less energy intensive process, and allows for the oxidation of magnetite to hematite ahead of the blast furnace(16). Pelletisation makes for a more efficient melting process due to a larger surface area to volume ratio, and takes place in a tunnel kiln, leading to ball shaped pellets.
In the production of fire clay bricks, just a 1.5% by weight addition of bentonite to the usual materials increased compressive strength significantly - it is thought that bentonite increases coagulation of the mixture. The increased porosity and inner surface area leads to improved refractory performance(17). Furthermore, in the production of predominantly kaolin-based refractory bricks, the addition of bentonite in proportions of 2% has been shown to increase both strength and density of bricks produced - whilst reducing cost(18).
Other Uses Of Bentonite And Coal
Bentonite-anthracite (where anthracite is a type of coal) mixtures have long been used for oil separation and removal from liquids. One study used a mixture of 30% by weight bentonite clay and 70% coal dust to remove oil from synthetic oils and oil/water emulsions with efficiencies of up to 98%. Such properties are due to the porosity profile of bentonite, and the fact that it is organophilic(19). Anthracite has a similar bulk density to bentonite clay and slows the premature absorption of oils into the swelling clay. Such absorbance is only effective at low temperatures - the ability to absorb liquids drops away quickly following heating even past a modest 100°C(20). This has potential applications in oil spill and general industrial clean up.
On large scales, mixtures of bentonite and coal are used as landfill linings and toppers. Following from the aforementioned absorbent behaviour of bentonite-anthracite mixtures, in a landfill, it is prized for its ability to absorb harmful materials which leach out of the rubbish pile, such as heavy metals(21) including cadmium, lead and nickel. In this study, bentonite was used in a ratio of 2:1 to coal, and altogether used alongside sand. Crucial in such applications is the resistance of the overall material to pressure, and it has been found that sand-bentonite-coal exhibits good compressive strength and high density when all three components are used together(22).
In the production of coal briquettes, addition of kaolin and especially bentonite clays have been found to elevate the ash fusibility temperature, rendering gasification more efficient(23).
- Bentonite is a type of clay that is prized for its porosity, workability and non-toxic nature
- In foundry applications, it is used with coal (bentonite-coal) in sand/green sand molding for metal castings; where it enhances the refractory properties of the mold whilst also increasing strength and flowability
- Bentonite and coal can be used together for small-scale furnace repair
- For refractory applications without coal, bentonite is used in iron ore pelletisation and in fire clay brick production to increase strength
- Other applications with coal include oil separation processes in oil spills and industrial clean up; landfill linings and enhancing the production of coal briquettes
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