Properties of Clay-Water System

Transcription

1 Properties of Clay-Water System ก - Part III Ceramic raw materials 1

2 ท าไมต องเข าใจพฤต กรรมของด นก บน า To understand the behavior of clays กก 0.2 µm (Plasticity) Cation exchange reactions ก ก Deflocculation and flocculation of clays ก Ceramic raw materials 2

3 Disordered structure Broken bonds Cause of cation exchange ท าไมถ งม การแลกเปล ยนประจ Typical values of Cation Exchange Capacity for various clays Clay Coal measure fireclay Ball clay China clay Brick clay Bentonite Halloysite Vermiculite Cation exchange capacity (meq/100 g) Ceramic raw materials 3

4 Cation exchange reactions ปฎ ก ร ยาการแลกเปล ยนประจ ท ผ วด น การแลกเปล ยนประจ ท ผ วด น X-clay + Y + Y-clay + X + Depends on: the nature of ions X and Y their relative concentration nature of clay any secondary reactions ก H > Al > Ba > Sr > Ca > Mg > NH 4 > K > Na > Li Ca-clay + NaOH Na-clay + Ca(OH) 2 sol. Ca-clay + Na 2 CO 3 Na-clay + CaCO 3 insoluble Ca-clay + Na 2 SiO 3 Na-clay + CaSiO 3 insoluble Ceramic raw materials 4

5 geoweb.tamu.edu/.../grossman/groundwater.html Ceramic raw materials 5

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7 Methylene Blue Absorption Techniques ( ) ก ก Methylene blue cationic thiazine dye Methylene Blue Index (MBI) ก ก ก ก ก ก ก ก ก ก ก MBI = F x (cc) millequivalents of methylene blue per 100 g. clay F = 100 x ก ก (m 2 /g) = MBI x A x 6.02 x 10-2 A = 130 square angstroms ( ) Ceramic raw materials 7

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9 Methylene Blue absorption: Halo effect Ceramic raw materials 9

10 Deflocculation and flocculation of clays ก ก ก Deflocculation: precipitation and replacement mechanism example: Ca-clay + NaCO 3 or sodium oxalate or sodium phosphate H-clay + sodium hydroxide Calgon (sodium hexametaphosphate) and Dispex (sodium polyacrylate): ก Polyanions ก ก กก Flocculation: edge- to-face Flocculation Deflocculation Ceramic raw materials 10

11 Behavior of clay in water Fluocculation Generally acidic condition Surface electrostatic charge Face to edge attraction Viscosity relatively high Ceramic raw materials 11

12 Behavior of clay in water Deflocculatation Generally alkaline conditions Surface electrostatic charges neutralized No longer attracted Viscosity relatively low Ceramic raw materials 12

13 ph and viscosity curves for hydrogen ball clay. Solid line = viscosity, broken line = ph Edge and face charges on (a) H-clay (acid environment), (b) Na-clay (acid environment), (c) Na-clay (alkaline environment) Ceramic raw materials 13

14 Causes of flocculation ก ก ก Natural clay, where the predominant exchangeable cation is calcium, are normally flocculated. Salts of polyvalent cations are more effective flocculants than those of monovalent cations. Thus, the deflocculation of a natural clay may be inhibited by the presence of soluble salts, especially sulphates of calcium, magnesium or iron, Addition of barium carbonate may effectively remove the sulphates: CaSO 4 + BaCO 3 CaCO 3 (insoluble) + BaSO 4 Deflocculation can often subsequently be achieved by the addition of sodium carbonate, sodium silicate or both. Ceramic raw materials 14

15 Plasticity of Clays Ceramic raw materials 15

16 Flow curves of various clays ก Montmorillonite has the smallest amount of bottom curvature and highest yield value, despite the high water content. China clay shows the greatest deviation from the Bingham behavior. High plasticity : a high yield value and a small degree of bottom curvature กก ก ก ก กก Montmorillonite > Ball clay > Fireclay > China clay Ceramic raw materials 16

17 Theory of Plasticity ความเหน ยวเก ดจากอะไร Clay particles in plastic mass carry the normal negative charges and surrounding them is a film of adsorbed water molecules, bonded by electrostatic forces. bound water present in the films free water in excess of that required to form films thickness of water film depends on the surface density of charge and on the exchangeable cations present. The amount of bound water is greater, the greater the surface area on which it can be adsorbed. small particle size high plasticity thin platy or fibrous particles high plasticity Ceramic raw materials 17

18 Effect of exchangeable ions on the flow curve ประจ ท ม การแลกเปล ยนท ผ วด นม ผลอย างไรต อความเหน ยวของด น Ca-clay Na-clay Ca-clay: strongly agglomerated and packing is random and very loose has a high yield value and require a high water content to flow. Na-clay: denser, low yield value and required very little water to flow. Ceramic raw materials 18

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