Technischer Bericht NTB 92-06

Herstellung von Bentonit-Granulat mit hoher Schüttdichte zur Bohrlochabdichtung

As part of a feasibility study, compacted pellets and granulate were produced from a Na-bentonite (MX-80, Wyoming), initially in the laboratory and later on a larger scale. The aim of the experiments was to find a technique for manufacturing disperse, pourable bentonite products with variable maximum grain size and a high bulk density of > 1.5 g/cm3 (related to dry mass). This bulk density is required in order to achieve the necessary material properties when using bentonite products to seal boreholes or other underground cavities.

Compact ion tests using a hydraulic press with closed mould provided the optimum conditions for uniaxial pressing:

Pressing power:≥ 200 MPa


Moisture content:~ 1 %

Compacted bentonite pellets with densities of 2.1 to 2.2 g/cm3 and good compressive strength and abrasion resistance can be produced under these conditions; these also withstand high stresses during handling.

A dense, pourable granulate can be produced from the bentonite by way of pulverization, provided the pulverization conditions are adjusted to the physical properties of the pellets.

Pouring tests with bentonite granulate and bentonite pellets showed that, in terms of the preferred grain size distribution functions for producing optimum packing densities (e.g. the Fuller curve), bulk densities of > 1.5 g/cm3 cannot be achieved. The highest bulk density values result if a coarse-grained fraction in the form of pellets is combined with dust-free, fine-grained granulate which acts as a filling material. With maximum pellet dimensions of 10 to 50 mm, the granulate should have a grain size of 0.63 – 3 mm. The optimum quantity ratio of pellets to granulate is around 70:30.

Under such conditions, bulk density values of 1.6 to 1.7 g/cm3 can be achieved if the bentonite products are poured individually and alternately from separate containers. After a layer of pellets has been poured, the voids are filled with the fine-grained pourable granulate.

A roller (cylinder) press is a suitable machine for manufacturing high-quality bentonite pellets. A particular advantage of this type of press is the favourable pressure distribution on compaction which allows high densities to be achieved. The shape and size of the pellets can be varied by changing the pressing segments.

The granulate which acts as a filling material (0.63 – 3 mm) can be produced commercially from breaking up or abrading briquettes, or from whole pellets, using a special granulate mill. A method is suggested for the production of the granulate products.