SELECTION AND PERFORMANCE OF FLOCCULANTS FOR HATSCHEK MADE AIR CURED FIBRE CEMENT

摘要

The manufacture of fibre cement by the Hatschek process involves the filtration of dilute slurries of finely divided particles on screens with relatively large openings. Agglomeration of fine particles into larger particles by the use of flocculants reduces the amount of materials passing through the filter screens .Nevertheless, the large quantities of water passing these screens still contain significant amounts of very fine chemically active solids (particularly cement and fine silica) that must be promptly recovered and reused. Prompt recovery of these solids in the water clarification and solids recovery system associated with the Hatschek system requires the use of chemical flocculants which agglomerate these fine solids into hydraulically coarser particles that settle rapidly to a position from which they are returned to the feed of the machine. Effective flocculants for cement based materials are anionic in nature and their chemistry is surprisingly similar to effective dispersents and super-plasticisers used in concrete. This paper discusses the chemistry and the properties required of suitable flocculants contrasting these with those required of super-plasticisers. Critical factors within the Hatschek machine itself including the delivery system for slurry to the Hatschek machine vats and delivery of flocculant to the system are also discussed and recommendations are made. The results disclosed here were obtained during the optimisation of the operation of a recently commissioned 4-vat Hatschek machine installed. Several flocculants were evaluated before and after changes were made to the feed system with various molecular weights (MWt?s) and charge densities. Several practical methods for the selection of flocculants were evaluated but it was found that these did not reliably predict their performance and that the best method of evaluation was through limited trials on the Hatschek machine itself.