DEFLOCCULATION

Chemical/physical process that leads to sedimentation phenomena of the solid parts of a colloidal system. More precisely, the solid phase tends to separate, producing flocs in suspension.

The surfaces of solid particles dispersed in colloidal solutions are usually marked by the same charge, thus preventing their agglomeration. However, the balance may be affected by, for example, a pH variation, a temperature change or by the use of coagulating agents, leading to particle aggregation.

In ceramics, flocculation phenomena of ceramic suspensions can be handled in many different ways, to reach a good applications as well as a proper development of the industrial process. The actions may change according to the different parameters of the production process and to the rheology values of the suspension.

In this regards thee are the most important processes to solve the problem.

1. ELECTROSTATIC REPULSION FOR CATION EXCHANGE

The multivalent cations – with a double positive charge (such as calcium and magnesium) or with triple positive charge (such as iron or titanium) – are characterized by a very high charge, able to break the strong negative charge that is on the micelle’s surfaces. The negative charge is nevertheless important to keep the repulsive force between the micelles, ensuring their flow. The addition of monovalent cations within the system (such as sodium) allows the replacement of the multivalent positive charges with weaker electric charge (CATION EXCHANGE). This specific cationic exchange decreases the positive charge on the micelles without altering the negative ones, producing a de-agglomeration and therefore a decrease in the system’s viscosity.

2. STERIC REPULSION

It is produced by using polymeric dispersants made of inactive molecular chains which contain functional groups able to interact with clays and raw materials. The edges of these molecules interact with the slurry’s suspended particles, increasing their distance. Polymeric dispersants, in other words, bind to particles (through the functional group) placing their “tail” at the edge: this position produces the proper distance to let the particles flow one on top of the other without electrostatic interaction. This process decreases in turn the suspension's viscosity value.

3. COMPLEXATION

Complexing agents are formed by particular chemical molecules provided with functional groups with atoms (such as oxygen or nitrogen) that provide the system with a very negative electronic charge. This negative charge attracts multivalent positive charge (such as calcium, magnesium, iron or titanium) more than monovalent charges.

In short: as soon as the complexing agents are added to the slurry, they release sodium (monovalent cation), preferably attracting multivalent cations. This process results in the elimination of the multivalent charges from the system, putting back into circulation monovalent charges: this process facilitates the cationic exchange, thus increasing the double electric layer and the reduction of the viscosity value.