Deposits of Bentonite are mostly exploited by mining. Having an approximately 30% content of moisture, still, the extracted bentonite is noticeably solid. Initially, the material is sliced or crushed and activated by adding Na2CO3 (soda ash), if required. Afterward, Bentonite is dried, using forced drying or the air, to touch the required moisture. EUBA (European Bentonite Association) provides information of 15% moisture content in bentonite which is already dried, WMA (Wyoming Mining Association) in the range of 5% to 20%, and Chinese producers and exporters in the range of 6% to 12%.
With respect to the final purpose, bentonite is either screened in the form of granule or milled into the form of powder and superfine powder. For the specific usages, bentonite gets purified by eliminating the associated gangue minerals or processed with acids to create acid-activated bentonite, or processed with organics to make organoclays. Below Figures show bentonite processing.
SME Mineral Processing Handbook explains a very simple method for processing.
First, by drying in the rotary kilns to touch the moisture for 5% to 8%, bentonite is processed.
Second, it is grounded in high-speed roller mills.
Third, it is classified into the size grades through sieves and cyclones.
Fourth, using ring roller mills machines, feed size of 50 mm is grinded to different sizes as fine as 0.03 mm, or to granulation 3.3 mm containing a minimum of fines.
Fifth, wet fed bentonite containing moisture of 15% to 32%, is fluid bed dried to the moisture content of 7% to 11%. Fuller’s earth having a moisture content of 47% is fluid bed dried to 30% to 35% moisture.
Sixth, the Negative pressure air system ends in dust-free conveying. Release and elimination of foreign materials like silica through supporting bottom discharge is achieved at the same time with grinding.
The most important feature of montmorillonites, to regain interlayer water and to expand, is lost if heated (dried) in a temperature higher than 105 °C to 125 °C for Li montmorillonite, 300 °C to 390 °C for H or Ca montmorillonite, and 390 °C to 490 °C for Na montmorillonite. It is safe for the drying temperatures to be up to 205°C for most of the Sodium bentonites. Bentonite may be dried and wetted many times repetitively without losing swelling properties.
In drying bentonite commercially, experience has shown that regaining of interlayer water is not easy in practice if the last trace of interlayer water is eliminated, but on condition that some water remains among the layers, swelling is mostly relatively easy.
Overall
Bentonite Processing is quite simple, though, because of its high affinity to absorb moisture; special attention is needed so as to choose the appropriate size reduction device. If the raw bentonite has considerable content of moisture, primary size reduction in a crusher that employs effect as the key mechanism of comminution might not be effective. Generally, the good choice for the primary crushing of wet and sticky material is the roller crusher which has specially designed toothed rollers that ripped large Lumps of clay in small pieces for more processing. However, jaw crusher and cone crusher may even be applied.
The next actions in bentonite processing are drying in rotary kiln and milling. The order of steps rests on the choice of the mill. If the mill is reactive to the moisture, (for example if milling of the wet material is not enough), the crushed material is dried before the milling. If the mill is of the ring roller mill type or fluid energy mill type that utilizes a fluidized bed or air stream to stimulate drying and conveying, produced material might be of acceptable wetness for the ultimate product. If not, bentonite might be dried after the milling.
Sources:
hrcak.srce.hr
Rudarsko-geološko-naftni zbornik Journal, Vol. 24, 2012
