Use Of Bentonite In Foundry Environments With Coal
Green And Dry Sand Castings
The most wide ranging and common use for bentonite anthracite mixtures is in the foundry casting sector. 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. It has been reported that the proportion of the additives (i.e. bentonite and anthracite) have a profound impact on the overall process of forming pellets, informing properties such as cold compressive strength and porosity amongst others(17).
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(18).
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(19). As early as in the 1970s USSR, brick production facilities had been experimenting with partial replacement of conventional clay and sand in kaolin containing bricks for high performance and decorative applications. Authors reported density and strength increases in the bricks produced was on the order of 19% - all whilst reducing overall cost in the hundreds of thousands of rubles, a significant value at the time. The same principles continue to be applied today.
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