The settling of a synthetic sludge and purified kaolinite in various liquid phases was studied in order to determine the collcidal forces of particle interaction responsible for flocculation. Hydration forces seem to be important in the interactions of quartz particles. More detailed experimental work has been done with settling and flocculation of kaolinites. These studies have shown that the kaolinite gel structure is likely due to the electrical structure within individual particles rather than interactions between surface layers. If these phenomena are found to extend to real sludges, it mal alter the conventional concepts for sludge stability.
Hot water extraction tailing sludges, while different from each other, have a common property to gelation, rather than to c compact settling (1). Real sludges represent a certain type of concentrated slurries, with a total solid phase concentration as high as 30% w/w. However, as indicated by Schutte (l), "Iong term settling of diluted sludges shows that there are structural units in sludge that are not broken up by dilution with mile agitation" The search for the structural units responsible for this flocculated, yet ‘dispersed’, state of sludges is the main goal of this study.
Semi-quantitative X-ray diffraction of sludge solids indicates quartz and clays as the principle minerals which dominate the contents of the solid phase (1).
There are indications that quartz and clays demonstrate non-DLVO behavior in colloidal stability experiments. Like amorphous colloidal silica (2), quartz-in-water dispersions show stability in concentrations of NaCl as high as 1.0 M (3). Such effects of increased colloidal stability in concentrated electrolyte solutions are visibly related to hydration forces (4). This phenomenon is thought to be relevant to clays as well (5).
A theory dealing with electric double layers, including the effect of hydration forces, has been developed (6). In the framework of this theory, hydratien forces can influence the distribution of ions in an electric double layer and affect repulsion by lowering the total ionic concentration within the double layers, extending the range of their mechanical action. This effect becomes noticeable if both the ionic strength of electrolyte and electric surface potential are sufficiently high. The first condition is satisfied in claims of increased stability due to hydration forces (2, 3, 4). The second condition was left undetermined because the surface potential of colloid particles is an almost unmeasurable value, especially in high ionic strength media.
The typical ionic strength of sludge supernatant is about 5.10−2 eq/L, making the DLVO theory, alone, hardly sufficient for the explanation of sludge stability.To be certain that other forces are significant in the stability and gelation of sludges, it was decided to test sludge solids for colloidal stability in conditions where the DLVO theory is definitely inapplicable.
To meet these conditions, testing was done in highly concentrated salt solutions and non-polar solvents, along with tests in low concentration electrolytes where DLVO forces are definitely present.