Bentonite: Applications With Coal As Bentonite-Clay In Foundry Applications, And Others

Far from use solely to make bricks, bentonite is a form of clay that has found numerous uses over its thousands of years of use - but perhaps most notably in refractories from the 1900’s onwards, alongside powdered coal amongst a wealth of other applications. Common amongst them is the need for the best quality bentonite clay and anthracite coal - such as those available from African Pegmatite.

Introduction

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.

natural bentonite deposit

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.

sand casting with bentonite
sand mold with bentonite

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.

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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).

bentonite used to make these bricks

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.

Other Uses Of Bentonite And Coal

Bentonite-anthracite (where anthracite is a superior type of coal) mixtures have long been used for oil separation and removal from liquids. A key concept that makes bentonite a viable material for this area is that it is an excellent sorbent in and of itself(20). The sorbent nature leans on the replacement of the Na or Ca counterion with the nitrogen on a quaternary amine. This enhances organophillicity (see later). Bentonite is said to be moderately selective for organic materials including benzene, crude oil and petroleum products. As anthracite has a similar density to bentonite clay, they mix well, with anthracite preventing any advanced absorption into the already swelling bentonite clay. Liquid absorption of the system, however, diminishes significantly at high temperatures(21).

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(22). 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(23). This has potential applications in oil spill and general industrial clean up. 

The fact that bentonite anthracite mixtures are highly organophilic explains their broad efficacy and efficiency in the oil separation and clean up space. Ease of use comes into play, too, with the macro scale particle sizes of 0.85 to 2.36 mm pre-swelled meaning that once swelled and full of contaminants, simple size exclusion filtration is more than sufficient to remove everything(24).

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(25) 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(26). In other similar studies, bentonite and coal have been shown to be good sorbents for a variety of liquid based contaminants arising from waste streams and landfill - but authors noted that there is no effect of the carbon type material on its own, rather it requires the bentonite to be present to effectively absorb the contaminants(27). This behaviour has been adopted elsewhere with effective results against leaching lead showing the efficacy of the bentonite coal system(28).

In terms of industrial and mining clean up, bentonite-anthracite mixtures find extensive uses especially in the area of acid mine drainage - a common problem facing the mineral mining sector. Groundwater can easily become contaminated with various tailings and runoffs from mining operations and must be prevented from entering local water courses. Groundwater near coal mines in sub-Saharan Africa has been shown to reach toxic and harmful concentrations of 1,300 mg L-1 of iron. Simple filtration of the water using bentonite clay, anthracite and fly ash has been shown to dramatically reduce iron levels in water(29).

Mine tailings are not limited to relatively easy to deal with iron, however. Other heavy metal ions - including sulfides - are known pollutants and also need to be removed. Research has shown that bentonite anthracite filtration is effective at removing greater than 80% of iron species (including sulfides) in one study using a bentonite anthracite filter column(30). Although it did not perform as well as commercial grade zeolite filters, the industrial wastewater was largely stripped of contaminants when a combined bentonite-anthracite filter column bearing a surface area of approximately 1 m2 g-1 was used. The authors noted a particularly good performance in terms of removals of any residual organic materials, where it did outperform the zeolite.

For such installations, addition of as little as 10% bentonite clay by weight to a sand and coal mixture is effective at enhancing the levels of absorbency of the clay-sand-coal system. It was noted that the anthracite present limited the swelling capabilities of the mixture - which was deemed as beneficial(31).

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(32). Sometimes overlooked as a conventionally behaving clay, bentonite can be formed into bricks and monoliths just like any other clay type material. When combined with ferrochrome production wastes and a source of carbon (for example, anthracite powder), ceramic refractory materials can be produced that are able to withstand temperatures some 200 to 300 °C higher than when ‘normal’ clays are used - this is most likely due to the effect of the bentonite and residual chrome materials from the ferrochrome waste(33).

In many cases, a set up combining bentonite-anthracite filtration alongside another method (such as a zeolite or ground glass) may be most suitable.

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Summary

  • 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

The combination of bentonite and coal (or anthracite) is highly effective across a range of settings from metal casting to industrial clean up. African Pegmatite is a leading supplier of anthracite, bentonite clay and a host of other minerals suited and tailorable to a variety of applications. Boasting the broadest experience and the finest in house technologies, African Pegmatite is the preferred industrial partner.

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Reference

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