(70dt) Improvement of the Flowability of Cohesive Bulk Solids with Surface Active Agents: Modelling and Influence of the Relative Humidity

Bulk solids are an important part of the manufacturing industry where they are processed, produced, stored and handled. Especially when bulk solids are handled, humidity adsorbed from the surrounding atmosphere plays an important role: The adhesion forces between the particles encreases and the flowability diminishes. This work presents a model to calculate the flowability of cohesive bulk solids at varied air humidity. Moreover, it considers also a surface coating of the particles with surface active agents that ensure the control of the flowability by hydrophobizing the surface.

For this investigation small glass beads and limestone powder (natural or hydrophobized with surfactants), were sheared in a ring shear cell. By varying the humidity of the ambient air during the shear tests, the water adsorption on the bulk solids surface could be varied and the influence on different bulk solid properties like porosity å, wetting angle á and contact angle è, the angle of internal friction öi and also the interparticle adhesion forces FH could be registered.

The adsorption mechanism of humidity from the ambient air, in general water, on the particle surface depends on the relative humidity level. At low relative humidity (RH < 50-65%) of the surrounding atmosphere, the moisture adsorbs on the whole surface of the particle and establishes an adsorption layer (adsorption layer model). At higher relative humidities (RH > 50-70 %), the adsorption of water takes place preferably in the particle-particle-contact and a water bridge is formed (liquid bridge model). The turnover from the adsorption layer model to the liquid bridge model could be characterized by a water layer thickness of 2 nm.

The performance of the existing models to calculate the flowability of humid cohesive bulk solids is not very high. The presented model calculates the flowability index ffC with respect to the relative humidity level (adsorption layer model and liquid bridge model). Moreover, it allows a more detailed calculation of the flowability in the turnover region of the two water adsorption models. It also considers for the first time significant material data like the contact angle è and the wetting angle á and includes the effects of a surface coating like modified porosity and moisture content on the adhesion forces in the particle contact to achieve a better fitting.

Furthermore, it could be shown that the improvement of the flowability by surface coatings is independent of the surface structure of the coated material but depends strongly on the chemical composition of the used surface active agent.