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 finelyrndivided particles on screens with relatively large openings. Agglomeration of fine particles into largerrnparticles by the use of flocculants reduces the amount of materials passing through the filter screensrn.Nevertheless, the large quantities of water passing these screens still contain significant amounts of very finernchemically active solids (particularly cement and fine silica) that must be promptly recovered and reused.rnPrompt recovery of these solids in the water clarification and solids recovery system associated with thernHatschek system requires the use of chemical flocculants which agglomerate these fine solids intornhydraulically coarser particles that settle rapidly to a position from which they are returned to the feed of thernmachine.rnEffective flocculants for cement based materials are anionic in nature and their chemistry is surprisinglyrnsimilar to effective dispersents and super-plasticisers used in concrete. This paper discusses the chemistryrnand the properties required of suitable flocculants contrasting these with those required of super-plasticisers.rnCritical factors within the Hatschek machine itself including the delivery system for slurry to the Hatschekrnmachine vats and delivery of flocculant to the system are also discussed and recommendations are made.rnThe results disclosed here were obtained during the optimisation of the operation of a recently commissionedrn4-vat Hatschek machine installed. Several flocculants were evaluated before and after changes were made tornthe feed system with various molecular weights (MWt?s) and charge densities.rnSeveral practical methods for the selection of flocculants were evaluated but it was found that these did notrnreliably predict their performance and that the best method of evaluation was through limited trials on thernHatschek machine itself.