MICROFILTRATION OF VERY DILUTE COLLOIDAL MIXTURES

The microfiltration behaviour of very dilute (ca. 10 ppm) mixtures of gold (50 nm) and latex (I mu m) colloidal suspensions has been investi gated for a range of conditions (varying transmembrane pressure, and s tirring) in a batch cell using hydrophobic GVHP and hydrophilic GVWP m icrofiltration (MF) membranes. The flux and retention for the hydrophi lic GVWP were similar for stirred and unstirred conditions. For the hy drophobic GVHP stirring produced slightly lower Aux and higher retenti ons. The hydrophilic (GVWP) membrane showed greater flux and lower ret ention than the hydrophobic (GVHP) membrane in both stirred and unstir red conditions. This provides evidence of the reduced interaction betw een the colloids and the hydrophilised membrane. Transmission of the f ine gold sol through the hydrophobic GVHP membrane was dependent on th e presence of the micron-sized latex particles. In the absence of the latex the retention gold sol was essentially complete due to the rapid formation of a ''self-rejecting'' cake layer of colloidal gold for bo th stirred and unstirred conditions. In the presence of the large late x particles retention was as low as 60% (transmission 40%). The latex cake has limited ability to retain the Sold sol and also disturbs the formation of the ''self-rejecting'' colloid cake layer. For the sol/la tex mixture chosen the ''cake'' resistance is determined by the resist ance of the latex particles at 100 and 200 kPa applied pressure. Howev er at 50 kPa the ''cake'' resistance is four times that obtained at 10 0 kPa. Thin-section electron micrographs show that under these conditi ons the fine sol particles become entrapped in the voids of the latex cake, dramatically increasing resistance.