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Properties of heat treated diatomite Aota, John Junpachi 1985

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PROPERTIES OF HEAT TREATED DIATOMITE by JOHN JUNPACHI AOTA B.E. Kyushu I n s t i t u t e o f Technology, Japan A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES THE DEPARTMENT OF METALLURGICAL ENGINEERING We ac c e p t t h i s t h e s i s as conforming to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September, 1985 © John J u n p a c h i A o t a , 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission f o r extensive copying of t h i s t h e s i s f o r scholarly purposes may be granted by the head of my department or by h i s or her representatives. I t i s understood that copying or p u b l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of M e t a l l u r g i c a l E n g i n e e r The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date October 1, 1985. i i ABSTRACT The density, strength and thermal conductivity of diatomite compacts were determined a f t e r heating the specimens i n the temperature o range between 200 and 1050 C. The l i n e a r dimensional change of these specimens on heating was also measured. A l l properties changed during heating with the increase i n f i r i n g temperatures and e s p e c i a l l y above o 800 C. The reduction of density, strength and thermal conductivity of o d i a t o m i t e compacts on f i r i n g at up to 600 C can be at t r i b u t e d to the mass loss due to decomposition of the constituents such as hydrated s i l i c a , o carbonates, e t c . Heating these compacts above 600 C resulted i n shrinkage with a concomitant increase i n bulk density, strength and thermal conduc-t i v i t y . Attempts were made to evaluate the fundamental factors which may be contributing to th i s change of properties during heat treatment. Analyses of data have been made using a s i n t e r i n g model, which assumes that a f t e r decomposition diatomite p a r t i c l e s are composed of a large number of microspheres and that the change of properties ( i . e . , increase i n density, strength and thermal conductivity) can be att r i b u t e d to the growth of i n t e r p a r t i c l e contact areas ( s i m i l a r to neck growth during s i n t e r i n g ) . Equations for shrinkage, thermal conductivity and strength are a l l related to the bulk density and weight loss of the compacts, and are given by: i i i F or s h r i n k a g e , AL = 1 PBo W . L 3 C p D W ' o o For t h e r m a l c o n d u c t i v i t y , K = N a Ks [ ( „ ) - 1] + K P B O W F o r s t r e n g t h , P r Wr, 2 / 3 S = c: [(-*-?-) - 1] + s n P B Q W o The v a l i d i t y o f t hese e q u a t i o n s has been t e s t e d w i t h the e x p e r i -mental d a t a of s h r i n k a g e , b u l k d e n s i t y , weight l o s s , s t r e n g t h and thermal c o n d u c t i v i t y o f pure d i a t o m i t e compacts. The good agreement between the e x p e r i m e n t a l r e s u l t s and the e q u a t i o n s may be i n d i c a t i v e t h a t i t i s the i n c r e a s e i n i n t e r p a r t i c l e c o n t a c t a r e a s d u r i n g the heat treatment which r e s u l t s i n the i n c r e a s e i n s t r e n g t h , b u l k d e n s i t y and thermal conduc-t i v i t y . The g e n e r a l a p p l i c a b i l i t y o f the e q u a t i o n s , developed i n o r d e r to e x p l a i n the change i n p r o p e r t i e s of pure d i a t o m i t e compacts, was f u r t h e r t e s t e d w i t h e x p e r i m e n t a l d a t a o b t a i n e d from a commercial brand o f d i a t o m i t e i n s u l a t i n g b r i c k s . i v TABLE OF CONTENTS 1.0 Introduction 1 1.1 Insulating Refractory Bricks 1 1.2 Diatomite and Diatomite I n s u l a t i n g Bricks 2 1.3 L i t e r a t u r e on the Fired Properties of Diatomites 6 2.0 Objectives of the Research 11 3.0 Experimental 12 3.1 Raw Materials 12 3.2 Sampling and Grinding 12 3.3 Equipment 16 3.4 Specimen Preparation 26 3.4.1 Shapes and Sizes of Specimens 26 3.4.2 F i r i n g 27 3.5 Test Procedures 28 3.5.1 Shrinkage Measurements 28 3.5.2 Density Measurements 28 3.5.3 Thermal Conductivity Measurements 31 3.5.4 Strength Measurements 31 4.0 Results and Discussion on Properties of Fi r e d Diatomite 33 V 4.1 Experimental Results on the E f f e c t of Grinding and Compacting 33 4.2 Experimental Results on Shrinkage and Density 36 4.2.1 Shrinkage and Density Changes of Diatomite 36 4.2.2 Mineralogical Changes of Diatomite 38 4.3 Discussion on Sintering of Diatomite 46 4.3.1 Si n t e r i n g of Diatomite 46 4.3.2 Weight Changes of Diatomite 47 4.3.3 True Density Changes of Diatomite 48 4.4 Development of Shrinkage Equations 50 4.4.1 Development of Density Equations 61 4.4.2 Testing of shrinkage and Density Equations 65 4.5 Results and Discussion on Thermal Conductivity 65 4.5.1 Experimental Results of Thermal Conductivity 69 4.5.2 Development of Thermal Conductivity Equations 69 4.5.2.1 Correction for Mineral Transformation 77 4.5.3 Testing of Thermal Conductivity Equations 78 4.5.4 Por o s i t y E f f e c t on Thermal Conductivity 84 4.6 Results and Discussion on Strength 94 4.6.1 Experimental Results of Strength 94 4.6.2 Development of Strength Equations 94 4.6.3 Testing of Strength Equations 102 4.6.3.1 Experimental Values of "C " 102 4.6.3.2 Evaluations of Results with Strength Equations 104 5.0 V e r i f i c a t i o n of Equations with Data of Commercial Diatomite Bricks 107 v i 5.1 R e s u l t s and D i s c u s s i o n on S h r i n k a g e and D e n s i t y o f Commercial D i a t o m i t e B r i c k s 109 5.1.1 E x p e r i m e n t a l R e s u l t s of S h r i n k a g e and D e n s i t y on Commercial D i a t o m i t e B r i c k s 109 5.1.2 T e s t i n g of S h r i n k a g e and D e n s i t y E q u a t i o n s w i t h Commercial D i a t o m i t e B r i c k s 112 5.2 R e s u l t s and D i s c u s s i o n on Thermal C o n d u c t i v i t y o f Commercial D i a t o m i t e B r i c k s 112 5.2.1 E x p e r i m e n t a l R e s u l t s of Thermal C o n d u c t i v i t y on Commercial D i a t o m i t e B r i c k s 112 5.2.2 T e s t i n g of Thermal C o n d u c t i v i t y E q u a t i o n s w i t h Commercial D i a t o m i t e B r i c k s 112 5.3 R e s u l t s and D i s c u s s i o n on S t r e n g t h o f Commercial D i a t o m i t e B r i c k s 117 5.3.1 E x p e r i m e n t a l R e s u l t s o f S t r e n g t h on Commercial .Diatomite B r i c k s 117 5.3.2 T e s t i n g of S t r e n g t h E q u a t i o n s w i t h Commercial D i a t o m i t e B r i c k s 117 6.0 Summary and C o n c l u s i o n s 121 Appendix: The R e s u l t s o f the E x p e r i m e n t s C a r r i e d out by E.L. C a l a c a l 124 R e f e r e n c e s 132 • i i LIST OF TABLES I. Typical Properties of Insulating Refractory Bricks 3 II. Chemical and Physical Properties of Quesnel Diatomites 15 III. True Density Changes of Diatomites on Firing 42 IV. X-ray Diffractometer Analyses of Diatomites 43 V. d-Spacings of Tridymite and Clay Minerals 44 VI. The Calculated Values of — (from the shrinkage data) 67 R VII. Geometric Parameters for Various Packings 79 VIII. The Calculated Values of "N" 81 IX. The Calculated Values of "k " 83 s X. Equations for Thermal Conductivity of a Porous Body 89 (25) XI. Geometric parameters (B) of various packing (Kakar) v 99 XII. The Calculated Values of "C" 103 XIII. Properties of Commercial Diatomite Bricks 108 v i i i LIST OF FIGURES 1. Quesnel diatomite: f o s s i l i z e d diatoms 5 2. Quesnel Diatomite P i t 13 3. Lumps and saw-cut specimens of Quesnel diatomite 14 4. Laboratory extruder used for extrusion 17 5. Tinius Olsen Compressive Testing Machine 18 6. Ferro Enamel E l e c t r i c Furnace 20 7. Thermal conductivity apparatus 21 8. Set-up of the thermal conductivity apparatus 21 9. D e t a i l s of the brass plate and disk for the thermal conductivity apparatus 23 10. Methods and positions for the dimensional measurements of bricks 29 11. The e f f e c t of compaction on the shrinkage behaviour 34 12. The e f f e c t of compaction on the bulk density of f i r e d specimens 35 13. The experimental shrinkage data of pure diatomite specimens .... 37 14. The experimental weight loss data of pure diatomite specimens .. 39 i x 15. The experimental bulk density data of pure diatomite specimens 40 16. Fores i n diatomite compact 49 17. The proposed model of the diatomite s h e l l wall 51 18. The SEM photo of diatomite s h e l l wall (from Calacal) 51 19. Diatom showing minute c e l l u l a r structure 53 20. Shrinkage of diatom s h e l l on f i r i n g , maintaining the o r i g i n a l geometric shape 55 21. A schematic i l l u s t r a t i o n of the weight loss of diatomite 57 22. The geometry of Frenkel's Model for the i n i t i a l stage of s i n t e r i n g by viscous flow 59 23. The geometry of unit c e l l incorporating of a microsphere 60 24. The experimental shrinkage data of pure diatomite specimens, o normalized to the 600 C f i r i n g 66 25. The t h e o r e t i c a l shrinkage plot as a function of r e l a t i v e density and weight l o s s . Experimental shrinkage data are also included 68 26. The experimental thermal Conductivity Data of Pure Diatomite Specimens 70 27. The geometry of two spheres i n contact and the equivalent e l e c t r i c a l network ( a f t e r Ramanan) 72 X 28. Spheres in a two dimensional cubic array and an equivalent e l e c t r i c a l network (after Ramanan) 72 29. Heat flow through a sphere in a simple cubic model 73 r 30. The experimental data and the t h e o r e t i c a l (simple cubic packing) thermal conductivity plot 85 31. The experimental data and the t h e o r e t i c a l (orthorhombic packing) thermal conductivity p l o t 86 32. The experimental data and the t h e o r e t i c a l (b.c.c. packing) thermal conductivity plot 87 33. The e f f e c t of porosity on the thermal r e s i s t i v i t y of ( 31) graphite (a f t e r Wagner) v y 90 34. Dimensionless thermal conductivity vs. f r a c t i o n a l porosity (32 ) for p o l y c r y s t a l l i n e graphite ( a f t e r Rhee) 90 35. The experimental data of thermal conductivity vs porosity 91 36. The experimental r e l a t i o n s h i p between r e l a t i v e density (Ap/p Q) change and thermal conductivity (AK/K q) change 93 37. The experimental compressive strength data of pure diatomite specimens 95 38. A model for the breakage of br i c k s during the compressive strength test 96 39. The experimental data and the predicted strength 105 40. The e x p e r i m e n t a l s h r i n k a g e d a t a of commercial d i a t o m i t e b r i c k s . 110 41. The e x p e r i m e n t a l b u l k d e n s i t y d a t a o f commercial d i a t o m i t e b r i c k s I l l 42. E v a l u a t i o n of the s h r i n k a g e e q u a t i o n u s i n g the e x p e r i m e n t a l d a t a of commercial d i a t o m i t e b r i c k s 113 43. The e x p e r i m e n t a l thermal c o n d u c t i v i t y d a t a of commercial d i a t o m i t e b r i c k s 114 44. E v a l u a t i o n of the thermal c o n d u c t i v i t y e q u a t i o n u s i n g the e x p e r i m e n t a l d a t a of commercial d i a t o m i t e b r i c k s 116 45. The e x p e r i m e n t a l s t r e n g t h d a t a of commercial d i a t o m i t e b r i c k s .. 118 46. E v a l u a t i o n o f the s t r e n g t h e q u a t i o n by u s i n g the e x p e r i m e n t a l d a t a o f commercial d i a t o m i t e b r i c k s 120 xii ACKNOWLEDGEMENT The a u t h o r i s g r a t e f u l f o r the most generous amount o f a t t e n t i o n , a d v i c e and encouragement g i v e n by h i s t h e s i s d i r e c t o r Dr. A . C D . C h a k l a d e r t h roughout t h i s work. S i n c e r e g r a t i t u d e i s a l s o extended to the s t a f f o f the Department o f M e t a l l u r g i c a l E n g i n e e r i n g and f e l l o w g r a d u a t e s t u d e n t s f o r much h e l p f u l a d v i c e . The deepest a p p r e c i a t i o n I s e x p r e s s e d t o Mr. D. A l b o n , R e s e a r c h D i r e c t o r and Mr. J . L . W i l l i a m s , the P r e s i d e n t o f C l a y b u r n I n d u s t r i e s L t d . who i n c o u n t l e s s ways h e l p e d t h i s r e s e a r c h s t u d y . F i n a n c i a l a s s i s t a n c e from the Research C o u n c i l o f B r i t i s h Columbia and C l a y b u r n I n d u s t r i e s L t d . i n the form of a G.R.E.A.T. award i s g r a t e f u l l y acknowledged. 1 1.0 INTRODUCTION The p r i m a r y f u n c t i o n o f h i g h temperature f u r n a c e s , b u i l t o f r e f r a c t o r y m a t e r i a l s , i s to c o n f i n e and u t i l i z e h eat energy. F u r n a c e s , however, l o s e a c o n s i d e r a b l e p r o p o r t i o n o f the s u p p l i e d heat energy t h r o u g h the r e f r a c t o r y w a l l , which then d i s s i p a t e s i n t o the s u r r o u n d i n g s . Such a heat l o s s can be m i n i m i z e d by a l a y e r o f i n s u l a t i n g r e f r a c t o r y b r i c k s which p r o v i d e s a d e s i r e d temperature drop a c r o s s a f u r n a c e w a l l . 1.1 I n s u l a t i n g R e f r a c t o r y B r i c k s I n s u l a t i n g r e f r a c t o r y b r i c k s d e r i v e t h e i r low t h e r m a l c o n d u c t i v i t y from the a i r e n c l o s e d i n t h e i r p o r e s ^ ^ . The i n s u l a t i n g e f f e c t i s p r i n c i -p a l l y due to the p r e s e n c e o f a s e r i e s o f a i r spaces between an a l t e r n a t e s e r i e s o f s o l i d b o u n d a r i e s . A c c o r d i n g l y , the more a i r p r e s e n t , and the l e s s s o l i d , the lower the thermal c o n d u c t i v i t y . A i r spaces i n the b r i c k s a r e produced by I n c o r p o r a t i n g one o r more d i f f e r e n t m a n u f a c t u r i n g t e c h n i q u e s . The t e c h n i q u e s a r e (1) u s i n g porous raw m a t e r i a l s such as f u s e d alumina b u b b l e s , expanded v e r m i c u l i t e , d i a t o -m i t e , l i g h t weight a g g r e g a t e s made from f i r e c l a y , e t c . , (2) u s i n g combus-t i b l e a d d i t i v e s w i t h i n b r i c k s and s u b s e q u e n t l y b u r n i n g them o f f d u r i n g the f i n a l f i r i n g s t a g e and (3) foaming c l a y s l i p s and s t a b i l i z i n g them d u r i n g the moulding p r o c e s s . In most cases the amount o f pores t h a t can be g e n e r a t e d i s l i m i t e d , as the i n t r o d u c t i o n o f a l a r g e volume f r a c t i o n o f pores r e s u l t s i n a b r i c k of poor m e c h a n i c a l s t r e n g t h . 2 In s p i t e of the d i f f i c u l t y o f making these b r i c k s t h e r e i s a wide range of i n s u l a t i n g r e f r a c t o r y b r i c k s c u r r e n t l y a v a i l a b l e i n the market. T y p i c a l p o r o s i t y v a l u e s of i n s u l a t i n g r e f r a c t o r y b r i c k s commonly used i n (2) i n d u s t r i e s a r e shown i n T a b l e I . D i a t o m i t e i n s u l a t i n g b r i c k s occupy a s i g n i f i c a n t segment o f i n s u l a t i n g b r i c k usage, because they are i n e x p e n -s i v e and have r e l a t i v e l y h i g h s t r e n g t h w i t h low t h e r m a l c o n d u c t i v i t y v a l u e . 1.2 D i a t o m i t e and D i a t o m i t e I n s u l a t i n g B r i c k s (3) D i a t o m i t e i s a s i l i c e o u s r o c k o f s e d i m e n t a r y o r i g i n . Diatom s k e l e t o n s , the f o s s i l i z e d remains of m i c r o s c o p i c s i n g l e - c e l l e d a q u a t i c p l a n t s , a r e the main component of d i a t o m i t e . The l i v i n g diatom organism has the c a p a c i t y to e x t r a c t s i l i c a from i t s a q u a t i c h a b i t a t and to form a s h e l l s t r u c t u r e which c o n s i s t s m a i n l y o f a b i o l o g i c a l l y p r e c i p i t a t e d amorphous s i l i c a . When the organism d i e s , i t s i n k s as a sediment. Under f a v o r a b l e c o n d i t i o n s r e s i d u e s accumulate and upon subsequent compaction these become p o t e n t i a l d i a t o m i t e d e p o s i t s . The term d i a t o m i t e i s used f o r the m i n e r a l r o c k d e p o s i t whose main c o n s t i t u e n t i s diatomaceous s i l i c a , the s u b s t a n c e o f s i l i c a s h e l l i t s e l f . The terms diatomaceous e a r t h and k i e s e l g u h r a r e synonymous w i t h d i a t o m i t e . Other names such as i n f u s o r i a l e a r t h , t r i p o l i and t r i p o l i t e a r e o b s o l e t e t o d a y . TABLE I Typi c a l Properties of Insulating Refractory Bricks A B C D E Material Diatomite F i r e c l a y F i r e c l a y Anorthite Alumina Bubble o Service l i m i t , C 900 1300 1500 1300 1800 Porosity, % 71 71 72 81 63 Crushing Strength, kg/cm 34 28 16 10 65 Thermal Conductivity Kcal/mh C 0.14 0.22 0.22 0.14 0.61 (W/m.K) (0.16) (0.26) (0.26) (0.16) (0.71) 4 ' M i n e r a l o g i c a l l y d i a t o m i t e , a form of h y d r a t e d s i l i c a r e s e m b l i n g o p a l i n e , has a unique p h y s i c a l s t r u c t u r e which c o n t a i n s i n h e r e n t l y a l a r g e volume f r a c t i o n o f p o r e s . No o t h e r n a t u r a l o r s y n t h e t i c a l l y produced m a t e r i a l has been found to have the same p h y s i c a l c h a r a c t e r i s t i c s ( F i g u r e 1 ) . D i a t o m i t e , b e i n g a m i n e r a l r o c k d e p o s i t , always c o n t a i n s such i m p u r i t i e s as o r g a n i c m a t t e r , o x i d e s of aluminum, i r o n and t i t a n i u m a l o n g w i t h compounds o f sodium, p o t a s s i u m and c a l c i u m . Because o f the c h e m i s t r y o f d i a t o m i t e m i n e r a l s c o n t a i n i n g m a i n l y S i G ^ , i t i s h i g h l y s u i t a b l e f o r the manufacture o f low temperature i n s u l a t i n g r e f r a c t o r i e s . D i a t o m i t e i n s u l a t i n g b r i c k s a r e manufactured from m i x t u r e s o f raw d i a t o m i t e and sawdust, and sometimes some p l a s t i c c l a y s a r e a l s o added i n t o the m i x t u r e s . The p o r o s i t y o f d i a t o m i t e i n s u l a t i n g b r i c k , which I s due to the i n h e r e n t p o r o s i t y o f d i a t o m i t e i t s e l f , can be f u r t h e r a d j u s t e d by c h a n g i n g the amount o f sawdust and f i r e c l a y i n the m i x t u r e . The moulding c o n d i t i o n s , such as the m o i s t u r e c o n t e n t o f the b r i c k and n a t u r e o f the e x t r u d i n g vacuum a l s o c o n t r i b u t e to the p o r o s i t y o f the f i n a l p r o d u c t s . I n r e c e n t y e a r s the I n d u s t r i a l use o f d i a t o m i t e i n s u l a t i n g b r i c k has i n c r e a s e d s i g n i f i c a n t l y due both to t o d a y ' s h i g h energy c o s t and t o the use of h i g h o p e r a t i o n a l temperatures to improve the p r o d u c t i v i t y o f i n d u s t r i a l f u r n a c e s . S a l e s i n Canada and the w e s t e r n U n i t e d S t a t e s a r e e s t i m a t e d a t 8 m i l l i o n bricks/annum, w i t h a market v a l u e e x c e e d i n g F i g u r e 1 Q u e s n e l D i a t o m i t e ; f o s s i l i z e d d i a t o m s . (800X) 6 $5 m i l l i o n . I n the N o r t h American C o n t i n e n t t h e r e a r e o n l y two major s u p p l i e r s o f d i a t o m i t e i n s u l a t i n g b r i c k s , Skamol Skarrehage M o l e r v a e r k A/S o f Denmark and C l a y b u r n R e f r a c t o r i e s L t d . , i n A b b o t s f o r d , the l a t t e r u t i l i z i n g l o c a l d i a t o m i t e from Q u e s n e l , B.C. To be s u c c e s s f u l i n s a l e s o f d i a t o m i t e i n s u l a t i n g r e f r a c t o r i e s , one o f the most i m p o r t a n t elements i s to have the c a p a b i l i t y o f m a n u f a c t u r i n g these p r o d u c t s w i t h h i g h s t r e n g t h and low t h e r m a l c o n d u c t i v i t y v a l u e . These r e q u i r e m e n t s appear to be c o n t r a d i c t o r y , as h i g h p o r o s i t y , n e c e s s a r y t o a c h i e v e a low t h e r m a l c o n d u c t i v i t y , r e s u l t s i n poor s t r e n g t h o f the b r i c k . I t Is not s u p r i s i n g t h a t these r e q u i r e m e n t s have been d i f f i c u l t t o a c h i e v e . I t i s , however, hoped t h a t w i t h a fundamental u n d e r s t a n d i n g of the s i n t e r i n g b e h a v i o u r o f powder compacts, i t might y e t be p o s s i b l e to d e v e l o p the d e s i r e d h i g h s t r e n g t h i n a b r i c k and s t i l l m a i n t a i n the t h e r m a l c o n d u c t i v i t y v a l u e a t a low l e v e l . 1.3 L i t e r a t u r e s on t h e F i r e d P r o p e r t i e s o f D i a t o m i t e One o f the most i m p o r t a n t p r o p e r t i e s o f d i a t o m i t e b r i c k s i s the t h e r m a l c o n d u c t i v i t y , which l a r g e l y depends on the amount o f a i r space w i t h i n the b r i c k . S i m i l a r l y , s t r e n g t h - the o t h e r i m p o r t a n t p r o p e r t y a l s o depends on the e x t e n t o f p o r o s i t y c o n t a i n e d w i t h i n the b r i c k s . The more the p o r o s i t y , the lower the thermal c o n d u c t i v i t y and the s t r e n g t h . I t i s g e n e r a l l y c o n s i d e r e d t h a t thermal c o n d u c t i v i t y and s t r e n g t h a r e (4) p r o p o r t i o n a l l y r e l a t e d to the d e n s i t y of d i a t o m i t e compacts. B a r r e t t , however, noted t h a t d i a t o m i t e b r i c k s showed remarkable d e v i a t i o n s from a l i n e a r r e l a t i o n s h i p between thermal c o n d u c t i v i t y and d e n s i t y owing to the d i f f e r e n c e i n s t r u c t u r e and pore s i z e i n d i a t o m i t e b r i c k s . 7 A l i t e r a t u r e s u r v e y r e v e a l s t h a t i n 1941 O l i v e r and R i g b y s t u d i e d the e f f e c t o f i n c r e a s i n g heat t r e a t m e n t on the p r o p e r t i e s o f a diatomaceous I n s u l a t i n g b r i c k but t h e r e has been no r e c e n t p u b l i c a t i o n on t h i s s u b j e c t and no p u b l i s h e d d a t a on the change of p h y s i c a l p r o p e r t i e s such as s t r e n g t h , c o n d u c t i v i t y e t c . t h a t o c c u r s i n d i a t o m i t e compacts on f i r i n g . There are o n l y a few r e p o r t e d s t u d i e s on the m i n e r a l o g i c a l changes and the pore s i z e changes i n d i a t o m i t e compacts on f i r i n g . One o f t h e s e s t u d i e s , which i s perhaps the most e x t e n s i v e , Is t h a t of C a l a c a l ^ ^ f o r h i s d o c t o r a l d i s s e r t a t i o n i n the U n i v e r s i t y o f Washington, S e a t t l e , U.S.A. He c o n s i d e r e d s i n t e r i n g as a t h e r m a l p r o c e s s which r e s u l t e d i n the r e d u c t i o n of the s u r f a c e a r e a of f i n e d i a t o m i t e powder. He a n a l y z e d the s i n t e r i n g c h a r a c t e r i s t i c s o f d i a t o m i t e compacts by measuring the s u r f a c e a r e a of the compacts a f t e r f i r i n g them at e l e v a t e d t e m p e r a t u r e s . H i s e x p e r i m e n t s i n v o l v e d the f i r i n g of d i a t o m i t e compacts (12.65 mm d i a m e t e r x 0.2 to 0.3 mm t h i c k ) under i s o t h e r m a l c o n d i t i o n s a t t e m p e r a t u r e s between 700 and 1300 C, and then measuring the s u r f a c e a r e a , the midpore d i a m e t e r and the t o t a l pore volume of specimens a f t e r f i r i n g . M e rcury p o r o s i m e t r y was used to o b t a i n d a t a of pore s i z e - as s m a l l as 120 A. N i t r o g e n a b s o r p t i o n was used to back up s u r f a c e a r e a d a t a measured by Hg p o r o s i m -e t r y . C a l a c a l d i d not attempt to u t i l i z e e x i s t i n g s i n t e r i n g models because of the s p e c i f i c g e o m e t r i c c o n d i t i o n s n o r m a l l y Imposed on these models. P a r t i c u l a r l y the d a t a o b t a i n e d i n s i n t e r i n g d i a t o m i t e can not f u l f i l the b a s i c assumptions used i n d e r i v i n g s i n t e r i n g e q u a t i o n s . These b a s i c assumptions a r e : (1) The c o n s e r v a t i o n of the mass of s o l i d p a r t i c l e s d u r i n g s i n t e r i n g , and (2) the c o n s e r v a t i o n of the volume of s o l i d p a r t i c l e s d u r i n g s i n t e r i n g . 8 C o n s e q u e n t l y , he developed a s i n t e r i n g e q u a t i o n o f d i a t o m i t e compacts by u s i n g the v o l u m e t r i c changes of c y l i n d r i c a l pores, and r e l a t i n g them to the changes o f the s u r f a c e area o f d i a t o m i t e compacts as f o l l o w s : The t o t a l s u r f a c e a r e a o f a number of c y l i n d r i c a l pore i s A = 2PnrA where A = t o t a l s u r f a c e area P = number of pores r = average pore r a d i u s X = pore l e n g t h The t o t a l volume (V) of the c y l i n d r i c a l pore i s , V = Pur X The t o t a l pore volume, the pore r a d i u s and the t o t a l s u r f a c e a r e a can be r e l a t e d by the f o l l o w i n g e q u a t i o n . A V 2PTirfl ?%T2X 2 r (1.1) The n a t u r a l l o g a r i t h m of Eq. 1.1 i s , InA = ln2 + InV - l n r 9 D i f f e r e n t i a t i o n of t h i s e q u a t i o n w i t h r e s p e c t to time g i v e s ]_dA = l _ d V _ j _ d r A dt V dt r dt U * ' The e q u a t i o n p r e d i c t s the change i n s u r f a c e a r e a w i t h change i n pore r a d i u s and pore volume. A f t e r i n t r o d u c i n g the c a p i l l a r y f o r c e a c t i n g i n the pore and then on i n t e g r a t i o n , the f i n a l form of Eq. 1.2 (when t h e r e i s no change i n the number of pores d u r i n g s i n t e r i n g ) can be e x p r e s s e d by r - r - i j t (1.3) o I n And when pore c o a l e s c e n c e o c c u r s , the f o l l o w i n g e q u a t i o n i s a p p l i c a b l e ; 1 A m-1 1 (T 2-) = ~r + K't (1.4) m-1 A m-1 where K = K A ' m _ 1 = I X i 2 r] r ' o m = the exponent i n the Eq. = -KA™, which was proposed by K u c z y n s k i f o r the r i p e n i n g of p o r e s ^ ^ y = s u r f a c e t e n s i o n n = v i s c o s i t y C a l a c a l d i d not use these e q u a t i o n s f o r a n a l y s i s o f the s i n t e r i n g c h a r a c t e r i s t i c s o f d i a t o m i t e but he a p p l i e d t h e s e e q u a t i o n s when e v a l u a t i n g the h i g h temperature v i s c o s i t y o f d i a t o m i t e . 10 C a l a c a l c o n c l u d e d t h a t : (1) I g n i t i o n l o s s e s of d i a t o m i t e s r e s u l t e d i n pore c r e a t i o n due to gas escape d u r i n g the i n i t i a l s t a g e s of s i n t e r i n g . (2) The compaction p r e s s u r e a f f e c t e d the midpore d i a m e t e r and t o t a l pore volume of s i n t e r e d d i a t o m i t e but d i d not i n f l u e n c e the s u r f a c e a r e a measured by a H g - p o r o s i m e t e r . Whether the powder was s i n t e r e d b e f o r e compaction o r compacted b e f o r e s i n t e r i n g , had no e f f e c t on the s u r f a c e a r e a o f p o r e s , w h i c h r a n g e d i n s i z e f r o m 120 A (0.012 urn) t o 10^ A (100 urn) i r r e s p e c t i v e l y . (3) S u r f a c e a r e a measurements by H g - p o r o s i m e t r y a l o n e was found not to be a p o w e r f u l t o o l i n s i n t e r i n g s t u d i e s , u n l e s s I t was used i n c o n j u n c t i o n w i t h some o t h e r method c a p a b l e of measuring the s u r f a c e a r e a of v e r y f i n e p o r e s , such as n i t r o g e n a b s o r p t i o n . (4) The r e s u l t s o b t a i n e d by H g - p o r o s i m e t r y showed t h a t t h e r e must be f o u r s t a g e s i n the s i n t e r i n g of d i a t o m i t e . These s t a g e s a r e : F i r s t s t a g e : S u r f a c e a r e a , midpore d i a m e t e r and t o t a l pore volume i n c r e a s e . Second s t a g e : S u r f a c e a r e a i n c r e a s e and midpore diameter and t o t a l pore volume d e c r e a s e . T h i r d s t a g e : S u r f a c e a r e a d e c r e a s e , midpore d i a m e t e r i n c r e a s e and t o t a l pore volume d e c r e a s e . F o u r t h s t a g e : M e l t i n g . Some of C a l a c a l ' s e x p e r i m e n t a l d a t a a r e shown In the Appendix o f t h i s t h e s i s . 2.0 OBJECTIVES OF THE RESEARCH The p r o j e c t was aimed a t i n v e s t i g a t i n g the p h y s i c a l p r o p e r t y changes of d i a t o m i t e compacts t h a t o c c u r on h e a t i n g to e l e v a t e d temper-a t u r e s and c o r r e l a t i n g these changes to the m i c r o s t r u c t u r a l changes t h a t a l s o o c c u r d u r i n g s i n t e r i n g . T h i s was done i n o r d e r to u n d e r s t a n d the o r i g i n o f the s t r e n g t h and the thermal c o n d u c t i v i t y i n d i a t o m i t e i n s u l a t -i n g b r i c k s . The p r o c e d u r e s adopted f o r t h i s study were: (1) Measurement of s h r i n k a g e a l o n g w i t h weight and d e n s i t y changes o f compacts on h e a t i n g . T h i s was to a l l o w a n a l y s i s of the s i n t e r i n g c h a r a c -t e r i s t i c s o f d i a t o m i t e . (2) True d e n s i t y measurement and X - r a y d i f f r a c t i o n a n a l y s i s to i d e n t i f y m i n e r a l o g i c a l changes i n the d i a t o m i t e . (3) D e t e r m i n a t i o n of c o l d c r u s h i n g s t r e n g t h and t h e r m a l c o n d u c t i v i t y v a l u e s o f d i a t o m i t e compacts and then the c o r r e l a t i o n o f these p r o p e r t i e s w i t h s i n t e r i n g . These measurements were c a r r i e d out on specimens f i r e d a t p r e -o d e t e r m i n e d temperatures between 200 and 1050 C. The range o f t e m p e r a t u r e t h a t i s commonly used i n m a n u f a c t u r i n g d i a t o m i t e i n s u l a t i n g b r i c k s i s 800 o to 1000 C. 12 3.0 EXPERIMENTAL 3.1 Raw Materials The source of diatomite used in this series of experiments is a Quesnel deposit owned by Clayburn Refractories Co., Abbotsford, B.C. The Quesnel Diatomite pit is on a h i l l lying on the west bank of the Fraser River seventeen miles downstream from the city of Quesnel, B.C. The pit is approximately 20 feet deep and 150 feet wide. The colour of diatomite changes locally from white to light grey and to brown. Preliminary investigations indicated that the white material was the purest, and therefore, this material was used for a l l experiments. Photographs of the deposit site and the diatomite lumps are shown in Figures 2 and 3. The chemical analyses of three types of diatomite found in the deposit are shown In Table 2. Data of X-ray diffraction analyses of the white diatomite are also included in Table II. 3.2 Sampling and Grinding Lumps of white diatomite were picked up from the Quesnel quarry site and brought back to the Abbotsford plant. After being dried over-o night in a convection type laboratory dryer at 100 C, the lumps were ground using a small laboratory jaw crusher, then hand-screened through a eight-mesh Tyler Screen. The minus eight-mesh diatomite powder was stored in plastic bags for future use. Small 2 1/2 x 1 1/2 x 3/4 - i n . (63 x 38 x 19 - mm) specimens were also saw-cut from dry lumps to investigate the effects of grinding and compacting the diatomite, which was necessary for preparation of test specimens. 13 Figure 2 Quesnel Diatomite P i t . Figure 3 Lumps and saw-cut specimens of Quesnel Diatomite. 15 TABLE II Chemical and Physical Properties of Quesnel Diatomite Sample No. 1 2 3 Colour White Light Grey Brownish Grey Chemical Analysis (wt%) S i l i c a Si0 2 83.67 75.32 74.22 Alumina A 12°3 7.12 9.50 9.90 Iron F e2°3 1.41 3.81 1.45 Titania T i 0 2 0.30 0.46 0.37 Calcium CaO 0.50 0.85 3.55 Magnesium MgO 0.15 1.46 1.40 L.O.I. 4.97 6.08 7.44 Total 98.16 97.48 98.33 Loose Bulk Density Ab/ft 17.9 22.6 25.2 (g/cm3) (0.287) (0.362) (0.404) X-ray Mineral Analysis Major Phase Amorphous - -Minor Phase Quartz Trace Possibly clay minerals having -d = 9.6A Note: Loose bulk density was measured at the Clayburn plant by the standard quality control procedure which uses a measuring cylinder to determine the volume of dry diatomite powder. 16 3.3 Equipment The preparation of test specimens and a l l the physical tests were carried out in the R & D and Quality Control Laboratory of Clayburn Refractories Co., Abbotsford. The X-ray determination of phases and S.E.M. studies were made in the Metallurgical Engineering Department at the University of British Columbia. The equipment used for these experiments are listed below. Photographs of some of the equipment are included in this thesis as these are not available at U.B.C. 3.3.1 Dryer The dryer used was a model LEB 2-20 made by Despatch Ind. Ltd. 3 ° which has a 23 cubic foot capacity (0.65 m ) and a 204 C max. temperature li m i t . 3.3.2 Laboratory Extruder A Vac-Aire Experimental Auger Machine Model 492H made by International Clay Machinary Co. was used for fabricating the specimens (Figure 4). 3.3.3 Tinius Olsen Testing Machine A Tinius Olsen 300,000 lb. Super "L" Compressive Testing Machine was used for compaction of the specimens and for testing the strength of Figure 4 Laboratory extruder used f o r e x t r u s i o n . 18 19 3.3.4 Kiln A Ferro-Enamels Electric Furnace Model #2 with Automatic Controller was used for f i r i n g specimens (Figure 6). The k i l n has 30 cm W x 30 cm D o x 33 cm H inside capacity with 1260 C max. working temperature and uses a type "R" platinum-rhodium thermocouple for temperature detection and o control. The temperature variation during holding periods was +4 C. 3.3.5 Thermal Conductivity Apparatus The apparatus is essentially a home-built model based on the device designed by Blakeley and Cobb^^ for low temperature thermal conductivity measurements. The apparatus uses the difference in temperature (1) of the ai r , (2) of a dull black surface on the top of the test brick which i s radiating and convecting freely to Its surroundings and (3) of the hot face. These give a measure of heat flowing through the test brick. A schematic diagram of the apparatus is shown in Figure 7 and the overall view in Figure 8. A brass block 9 x 4.5 x 0.25 - i n . (229 x 114 x 6-mm) with a 2-in. (50.8 mm) hole at the center acts as the guard ring of the calorimeter. A brass disc 1/4-in. (6.3 mm) thick x 1 15/16-in. (49.2 mm) diameter is held in this 2-in. (50.8mm) hole by insulating fibres rammed Into the 1/32-in. (0.8 mm) gap. A 3/32-in. (2.4 mm) hole extends horizon-t a l l y along the minor axis of the block to the center of the brass disc. This hole receives a copper-constantan thermocouple so that the junction is at the center of the disc (Figure 9). The surface of the block, Figure 6 Ferro Enamels E l e c t r i c Furnace 21 Figure 7 Low temperature thermal c o n d u c t i v i t y apparatus. (A) thermocouples, (B) t e s t b r i c k , (c) brass p l a t e , (D) diatomite b r i c k , (E) diatomite powder, (F) heating element Figure 8 Set-up of the thermal c o n d u c t i v i t y apparatus. 22 including the center disc, i s painted with a heat resistant f l a t black o paint ("Tremclad" High-heat Enamel-650 C manufactured by Tremco Ltd. Toronto). The center disc acts as a calorimeter, whereas the rest of the block acts as a guard ring as mentioned before. The heater for the hot face of the test brick was made by winding nichrome ribbon in the grooves of a firebrick t i l e 9 x 4 1/2 x 3/4-in. (229 x 114 x 10-mm) which i s covered by a 9 x 4 1/2 x 3/8-in. (229 x 114 x 9-mm) firebrick l i d . A platinum-rhodium thermocouple is placed on the top of the heater to record the hot face temperature of the test brick. The apparatus was placed in a 20 L x 16 W x 7 H-in. (508 L x 206 W x 178 H-mm) insulating box lined with diatomite bricks. The gap between the box and test brick was f i l l e d with calcined diatomite powder. The temperatures of the hot face and the cold face of the test brick were detected by the thermocouple as mentioned before and i t was assumed that the heat passing through the test brick escaped from the blackened surface of the plate and that, at least in the central portion of the test brick, the heat flow i s linear. Since the thickness of the brick and the temperature drop across i t are known, the conductivity can then be readily calculated from the heat flow equation. The apparatus was placed on a bench in a closed room to maintain a s t i l l atmosphere. The electric current for the heating element i s regulated by a controller made by Robert L. Stone Co., Texas, U.S.A. The air temperature was measured by a mercury thermometer placed two feet away from the apparatus. 63 -H k ( i n m m ) F i g u r e 9 D e t a i l s of the b r a s s p l a t e and d i s k f o r the t h e r m a l c o n d u c t i v i t y a p p a r a t u s . 24 The current to the heater was switched on and the readings of air temperature, hot face temperature and cold face temperature were taken after thermal equilibrium was attained. Blakeley and Cobb stated that i t took 127 minutes for a 6.20 cm thick brick to attain thermal equilibrium. In the present experiments the readings were taken 24 hrs after the elect r i c current was switched on. The temperature gradient across the sample was calculated from the readings of the hot and cold face temperatures and the brick thickness. The heat flow through the brick was derived from the difference in temperature between the central disc of the block surface and the air as follows: The heat loss (H) from the surface is the sum of the losses due to convection (H c) and to radiation (H r). Taking the values of G r i f f i t h and Davis for the heat loss from a horizontal plane surface facing upward, H = H + H c r H = 2.19 (T -T ) 1 , 2 5 c c a T +273 4 T +273 4 H r = A- 8 2 K -W- 5 " <-nrao-) 1 where T and T are the cold face temperature and the air temperature in c a degree centigrade, respectively. Heat transfer equation i s , where, 2 q = heat flow (= heat loss, H) (Cal/cm .S) 2 A = area of heat flow (cm ) o K = thermal conductivity (cal/cm.S C) o T, = hot face temperature ( C) h o T c = cold face temperature ( C) X£-X^ = thickness of specimen (cm) T.-T h c — — — = T grad i.e. temperature gradient in specimen V X1 T grad A T grad A Using "A" for a unit area, . . . • , v S Heat loss (H) Thermal conductivity (K) = T e m p e r a t u r e g r a d l e n t (T grad) T +273 4 T +273 4  2.19<T e-T a) 1- 2 5 + 4.82[(-£ I 5 B r) - j - ^ - ) ] V T c 26 Accordingly, thermal conductivity can be calculated from the measurements of (1) the thickness of specimen, X^-X^, (2) hot face temperature, T h, (3) cold face temperature, T and (4) ambient temperature, T . C A 3.3.6 X-ray Diffraction and Scanning Electron Microscope X-ray diffraction patterns of raw diatomite and diatomites after f i r i n g were obtained using a Norelco X-ray diffractometer with a Cu target o and Ni f i l t e r at 40 KV - 15 mA and a speed of 1 29/min. For microphoto-graphs an ETEC-Autoscan scanning electron microscope was used. 3.4 Specimen Preparations 3.4.1 Shapes and Sizes of Specimens The minus eight-mesh dry diatomite was thoroughly mixed with 50 weight % (wet base) of water, then extruded through a Vac-Aire Auger Machine. Due to a lack of lubrication between diatomite particles, the extrusion was d i f f i c u l t . Although the mix appeared to be too dry to be extruded, the product from the die mouth was excessively wet and soft, but despite these d i f f i c u l t i e s complete extrusion was achieved. The specimens had many extrusion cracks and the corners of the specimens were rough. It was, however, the original plan to fabricate specimens by re-pressing the extruded bricks because the laboratory extruder was too small to make specimens large enough for the experiment. The extruded bricks were crushed by hand to approximately ~ 5 mm size then stored in plastic bags to keep the moisture i n . 27 The specimen sizes that were fabricated by re-pressing were 9 x 4 1/2 x 2 1/2-in. (229 x 114 x 63-mm) f u l l size bricks to determine thermal conductivity and 7.5 x 1.2 x 1.0-in. (190 x 30 x 25-mm) small size bricks to determine shrinkage, density and strength. A steel box was set on the Tinius Olsen Compressive Testing Machine, the box was then f i l l e d with extruded material and 300 p . s . i . 2 (21.1 kg/cm ) uni a x i a l pressure was applied to the 9 x 4 1/2-in. (229 x 114-mm) top face. The steel box was then turned over and 150 p . s . i . 2 (10.5 kg/cm ) uniaxial pressure was applied to the bottom face of 9 x 4 1/2-in. (229 x 114-mm). The bricks, after being removed from the steel box, appeared to be uniformly pressed. A large number of f u l l size and small size bricks were prepared by this method. The green bulk densities 3 of bricks were 80.8 +0.2 p.c.f. (1.294 + 0.003 g/cm ). The bricks were air-dried at room temperature. 3.4.2 Firing o After being air-dried the bricks were placed in the dryer at 100 C. Several f u l l size and small size bricks were fired in the electric furnace at each pre-determined temperature. Eleven different f i r i n g temperatures o were selected between 200 and 1050 C. o The temperature was r a i s e d at a rate of 5 C/min. up to the designated temperature and held at the temperature for 24 hours. After the holding period the kiln was switched off and allowed to cool down to room temperature in 18 hours. 28 Some of the specimens cut from the dry lumps obtained d i r e c t l y from the quarry (see page 12), were also f i r e d along with the laboratory-prepared specimens. This was done to see the e f f e c t of grinding and compacting the diatomite on t h e i r f i r i n g behaviors and other properties. 3.5 Test Procedures 3.5.1 Shrinkage Measurements The dimensions of the bricks were measured with a micrometer to an accuracy of + 0.001 i n . (0.0025 cm.). To obtain reasonable data on the dimensions, the length of the b r i c k was measured at the four corners and at the center along the l o n g i t u d i n a l axis and the average of the f i v e measurements was used as the length of the b r i c k (Figure 10). The other dimensions - width and thickness were measured by the same method. The dimensions of the b r i c k were measured both before and a f t e r f i r i n g . The l i n e a r shrinkage at the f i r i n g temperature was c a l c u l a t e d by, ^ L o r i g i n a l length (L ) - f i r e d length (L) Shrinkage (—) = o r l g l n a l l e n g t h ( L ° }  3.5.2 Density Measurement A) Bulk Densities Bulk density was calculated using the formula below. Length —H F i g u r e 10 Methods and p o s i t i o n s f o r t h e d i m e n s i o n a l measurements of b r i c k s . 30 B u l k d e n s i t y (o ) = " e i g h t o f b r i c k B u l k d e n s i t y tPg/ v o l u m e o f b r l c k weight o f b r i c k l e n g t h x w i d t h x t h i c k n e s s o f b r i c k The measurement of the weight o f the b r i c k was d i f f i c u l t s i n c e the d i a t o m i t e b r i c k q u i c k l y absorbed m o i s t u r e from the e n v i r o n m e n t . I n i t i a l l y the b r i c k s were weighed and approximate v a l u e s were o b t a i n e d . The b r i c k s were then d r i e d i n a d r y e r and were p l a c e d i n a d e s i c c a t o r o v e r n i g h t t o c o o l . The d i g i t a l w e i g h i n g b a l a n c e was s e t a t the p r e d e t e r m i n e d a p p r o x i m a t e w e i g h t , and the b r i c k was re-weighed w i t h i n 15 seconds o f b e i n g t aken out from the d e s i c c a t o r . B) True D e n s i t i e s The t r u e d e n s i t y was measured by the P y c n o m e t r i c method. The (9) p r o c e d u r e s f o l l o w e d t h e method o f J . I . S . R2616 ( J a p a n I n d u s t r i a l S t a n d a r d ) , s i n c e no p y c n o m e t e r b o t t l e d e s c r i b e d i n ASTM C 1 3 5 - 6 6 ^ ^ was a v a i l a b l e . The measurements were r e p e a t e d and checked to w i t h i n 0.03 g / c c . C) R e l a t i v e D e n s i t i e s The r e l a t i v e d e n s i t y (p) was c a l c u l a t e d from the r a t i o o f the b u l k d e n s i t y ( p w ) and the t r u e d e n s i t y ( p T ) . 31 3.5.3 Thermal Conductivity Measurements Fullsize bricks of 9 x 4 1/2 x 2 1/2-in. (229 x 114 x 63-mm.) were used for the measurement of the thermal conductivity. After f i r i n g at various temperatures, the densities of the f u l l size bricks were measured and confirmed that they matched the densities of the small size specimens fired at the same temperature. The faces of the brick, 9 x 4 1/2-in. (229 x 114-mm) were then ground to form smooth parallel planes and to a thickness of 2 1/2 inches (63-mm.) using a silicon-carbide brick grinder. After drying overnight in a dryer, the brick was placed in the thermal conductivity apparatus, and the heater was switched on. The brick was le f t twenty-four hours to reach thermal equilibrium after which the temperature readings were taken. To determine the thermal conductivity at a higher temperature, the current to the heater was sli g h t l y increased and again the brick was equilibrated for another twenty-four hours before a further conductivity determination was made. The thermal conductivity at o o 300 C mean temperature, i.e. approximately 500 C hot face temperature and 100 C cold face temperature, was obtained by the interpolation of the o readings taken s l i g h t l y below 300 C mean temperature and slightly above o 300 C mean temperature. 3.5.4 Strength Measurements The dimensions of the specimens for the compressive strength test were 1.5 x 1.2 x 1.0-in. (38.1 x 30.5 x 25.4-mm) and these specimens were cut from 7.5 x 1.2 x 1.0-in. (190.5 x 30.5 x 25.4-mm) small size speci-mens . 32 A loading rate of 1000 lbs/min. (454 kg/min.) was applied to the 1.2 x 1.0-in. (30.5 x 25.4-mm) faces by the Tinius Olsen Compressive Testing Machine. The average of several compressive strength tests was o used as the value of the strength, which varied + 8% (for 600 C specimens) o and + 14% (for 1050 C specimens) from the average value. 33 4.0 RESULTS AND DISCUSSION ON THE PROPERTIES OF FIRED DIATOMITE 4.1 Experimental Results on the E f f e c t of Grinding  and Compacting The shrinkage and the bulk density of diatomites versus the f i r i n g temperatures are plotted i n Figures 11 and 12. The s t a r t i n g density ( i . e . dry density) of the specimens obtained from the natural lumps was 35.58 +0.17 p.c.f. (0.570 +0.003 g/cc). The s t a r t i n g density of the specimens which were prepared by grinding and compacting was 41.71 +0.25 p.c.f. (0.668 +0.004 g/cc). From the r e s u l t s shown i n Figure 11, i t i s apparent that the shrinkage behavior was not affected by the laboratory p r a c t i c e of grinding and compacting. The laboratory compaction only increased the s t a r t i n g density but did not a f f e c t the o v e r a l l d e n s i f i c a t i o n behavior of the compacts during f i r i n g . This conclusion agrees with Calacal's findings that (1) compaction pressures normally used i n i n d u s t r i e s did not have s i g n i f i c a n t e f f e c t on the s i n t e r i n g behavior of diatomite, (2) mechanical pressure alone could not enhance mass transport to f i l l pores within diatomites and consequently, could not cause los s of surface area and (3) the mechanical compaction, therefore, only allowed the p a r t i c l e s to approach one another, r e s u l t i n g i n the lowering of the t o t a l pore volume and the midpore diameter, but never r e a l l y decreasing the number of pores 10 08 •06 0> cn S -04 C CO •02 2 • Cut specimens o Pressed specimens , - -ft--_i ,9 3 4 5 6 7 Temp, of heat treatment (°c ). 8 10 xlOO 'Figure 11 The effect of compaction on the shrinkage behaviour. •8 to c • Cut specimens °/ o Pressed specimens / / / -g-6 CD / p i <> 0 - 0 + ' I u -o q_ _ _ - / / / / / •- • -•J 1 1 I l L 1 2 3 4 5 6 7 8 9 10 x100 Temp, of heat treatment (°c) F i g u r e 12 The e f f e c t of compaction on the b u l k d e n s i t y of t h e f i r e d specimens. 36 which is an inherent property of the diatom i s e l f . Whittemore, Ayala and C a s t r o ^ ^ studied the effect of compaction on sintering of two commercial Brazilian diatomites. Their results showed that the surface area of the fired specimens was not affected by compaction pressure appplied during fabrication, and that compaction pressure alone could not change the pore population. Therefore, i t does not really matter whether the specimens are prepared by compacting the powder or not. As long as the specimens are sintered at the same temperature, the overall density change remains the same. Following these observations i t was decided to carry out further experiments on laboratory compacted specimens which were of suitable size and uniformity. 4.2 Experimental Results on Shrinkage and Density 4.2.1 Shrinkage and Density Changes of Diatomite Figure 13 shows the shrinkage of the diatomite specimens plotted against their f i r i n g temperatures. The linear change was calculated by A L / L q , where A L i s the change in length during f i r i n g and L Q i s the original length. •10 08 06 CD cn O 5 04 CO ff P 02 — o - -3 A 5 6 7 Temp, of heat treatment (°c ) 8 10 X100 F i g u r e 13 The e x p e r i m e n t a l s h r i n k a g e d a t a of pure d i a t o m i t e specimens. Co 38 Figure 14 shows the weight changes of the same specimens plotted against the f i r i n g temperatures. The weight change was calculated by AW/Wq, where AW is the change in weight of the specimen during f i r i n g and AW Q i s original dry weight. The bulk densities, calculated from the linear dimensions and the weights, are plotted against the f i r i n g temperatures in Figure 15. The calculation was done by, , , ,, , .^  weight of specimen (after firing) Fired bulk density = — 7 s — E — J—- ., J & ' volume of specimen (after firing) 4.2.2 Mineralogical Changes of Diatomite on Firing In order to determine the mineral compositions of the fired diato-mite specimens, true density measurements (by the pycnometric method) and X-ray diffraction analyses were carried out. The samples were prepared from the same batch of saw-cut specimens 2 1/2 x 1 1/2 x 3/4-in. (63 x 38 x 19-mm) in size as described in Section 3.2. The published data of true d e n s i t i e s for s i l i c a minerals a r e ( 1 2 » 1 3 » 1 4 ) quartz = 2.64 g/cc, cristobalite - 2.33 g/cc, Tridymite = 2.27g g/cc, Opal = 1.9 ~ 2.3 g/cc and raw diatomite = 1.9 ~ 2.3 g/cc. True density of raw diatomite varies from one deposit to another mainly due to the presence of impurities. 2 3 k 5 6 7 8 9 10 x100 Temp, of heat t reatment (°c ) F i g u r e 14 The e x p e r i m e n t a l weight l o s s d a t a of pure d i a t o m i t e specimens. •6 ' ' 1 1 I I I L 1 2 3 A 5 6 7 8 Temp, of heat treatment (°c ) F i g u r e 15 The e x p e r i m e n t a l b u l k d e n s i t y d a t a of pure d i a t o m i t e specimens. 41 T a b l e I I I shows the e x p e r i m e n t a l r e s u l t s o f the t r u e d e n s i t y measurements. I t appears t h a t the raw d i a t o m i t e from Q u e s n e l has the t r u e d e n s i t y o f ~ 2.15 g/cc but t h i s v a l u e changes on f i r i n g a t and above o o 950 C. The d e c r e a s e i n the t r u e d e n s i t y v a l u e between 800 and 950 C may be due to the d e c o m p o s i t i o n o f c a r b o n a t e s and o t h e r r e a c t i o n s . To c o n f i r m t h a t the t r u e d e n s i t y change o f the Quesnel d i a t o m i t e on o f i r i n g a t o r above 950 C i s due t o t h e c r y s t a l l i z a t i o n o f a m o r p h o u s s i l i c a , X - r a y d i f f r a c t i o n a n a l y s e s were conducted on the specimens d r i e d o o a t 100 C and f i r e d a t 850 and 1000 C. T a b l e s IV and V show the X- r a y d i f f r a c t i o n peaks o f the specimens and the d - s p a c i n g s o f some r e f e r e n c e m i n e r a l s . o F o r t h e d i a t o m i t e specimen d r i e d a t 100 C, the peak a t d = 3.35 A i n d i c a t e s the pres e n c e o f a - q u a r t z . A v e r y weak peak a t d = 9.6 A may a l s o suggest the presence o f some type o f c l a y m i n e r a l s , some o f which have peaks a t ~ 10 A, e.g. d e h y d r a t e d m o n t m o r i l l o n i t e and h y d r a t e d h a l l o y -s i t e . The peaks a t d = 4.26 A, a - q u a r t z (100) and d = 1.82 A, a - q u a r t z (112) a r e v e r y weak but r e c o g n i z a b l e . Other weaker peaks o f a - q u a r t z a r e no t c l e a r l y d i s c e r n a b l e . There a r e a l s o v e r y weak almost u n r e c o g n i z a b l e broad peaks a t d = 4.05 A and 3.13 A which c o r r e s p o n d to a - c r i s t o b a l i t e . T h e r e i s a l s o a g e n e r a l i n c r e a s e i n the background i n t e n s i t i e s i n t h e s e a r e a s , i n d i c a t i n g the p o s s i b l e p r e s e n c e o f v e r y f i n e p a r t i c l e s o f t h e s e m i n e r a l s . I t i s however, known t h a t u n f i r e d d i a t o m i t e i s p r i m a r i l y composed o f amorphous s i l i c a and X - r a y d i f f r a c t i o n a n a l y s i s o f u n f i r e d TABLE I I I True D e n s i t y Changes o f D i a t o m i t e on F i r i n g o F i r i n g Temperature, C 3 True D e n s i t y , g/cm 600 2.154 700 2.153 800 2.143 850 2.148 900 2.141 950 2.216 1000 2.345 1050 2.381 TABLE IV X-ray DiffTactometer A n a l y s e s o f D i a t o m l t e s Specimen 0 d r i e d at 100 C Specimen „ f i r e d at 850 C Specimen „ f i r e d at 1000 a - Q u a r t z ^ 1 5 ) a - C r i s t o b a l i t e ^ 1 6 ) Peaks a t d I/Io d I/Io d I / Io d I/Io d I/Io 9 A 9.60 VW 9.60 VW 4 A 4.26 4.05 VW vw 4.26 4.05 VW VW 4.26 4.05 VW VS 4.26 35 4.04 100 3 A 3.35 3.13 w vw 3.35 3.13 W VW 3.35 3.13 W VW 3.35 100 3.13 25 2 A 2.84 2.49 2.02 VW S vw 2.46 2.28 2.12 12 12 9 2.84 2.49 2.02 30 30 6 1 A 1.82 VW 1.82 VW 1.82 vw 1.82 17 1.93 1.87 1.61 10 13 10 Major Phases Amor] jhous Amorphous C r i s t o b a l i t e Minor Phases Quartz Quartz Quartz U n i d e n t i f i e d C r i s t o b a l i t e ? C l a y m i n e r a l ? C r i s t o b a l i t e ? C l a y m i n e r a l ? T A B L E 5 d - S p a c l n g s o f T r l d y m l t e a n d C l a y M i n e r a l s T r i d y m i t e ( 1 7 ) M o n t m o r l l l o n i t e ^ 1 8 ) H y d r a t e d H a l l o y a i t e * 1 8 * H a l l o y 8 l t e ( 1 8 ) K a o l l n l t e ( 1 8 ) P e a k g a t d I / I O d I / I o d I / I o d I / I o d I / I o 12 A 12.5 -15.5 10 10 A 10.1 10 7 A 7.2 -7.5 10 7.13 10 5 A 5.10 6 4.40 4.44 8 4.42 10 4.43 6 4 A 4.30 4.08 100 80 4.18 3 3 A 3.81 3.25 80 20 3.05 6 3.35 3 3.97 3.63 5 6 3.80 3.58 3.39 4 8 2 2 A 2.96 2.47 2.37 2.29 2.07 2.03 40 60 10 20 10 10 2.53 6 2.58 2.36 7 7 2.56 2.37 4 5 2.56 2.50 2.35 2.29 2.20 7 6 9 6 2 1 A 1.97 1.87 1.76 10 10 10 1.68 1.64 3 3 1.67 6 1.69 4 45 d i a t o m i t e n o r m a l l y p r o d u c e s b r o a d p a t t e r n s o f n o n - c r y s t a l l i n e (19) m a t e r i a l o F o r t h e d i a t o m i t e s p e c i m e n f i r e d a t 850 C, X - r a y d i f f r a c t i o n p a t t e r n s showed no e x t r a peaks than those o b t a i n e d from the d r i e d d i a t o m i t e specimens, i n d i c a t i n g no change i n the phases due to f i r i n g a t t h i s low t e m p e r a t u r e . o F o r t h e d i a t o m i t e specimen f i r e d a t 1000 C, a l l the peaks c o r r e s -pond to e i t h e r a - q u a r t z o r a - c r i s t o b a l i t e . T r i d y m i t e and m u l l i t e c o u l d o n o t be d e t e c t e d i n the specimen f i r e d at 1000 C a l t h o u g h C a l a c a l r e p o r t e d the f o r m a t i o n of these m i n e r a l s i n an Impure d i a t o m i t e from the Washington s t a t e . The background i n t e n s i t y was reduced f o r the specimen f i r e d a t o 1 0 0 0 C i n d i c a t i n g t h a t some c r y s t a l l i z a t i o n o f t h e amorphous s i l i c a o c c u r r e d . I t appears t h a t amorphous s i l i c a i n t h i s Q uesnel d i a t o m i t e changes o d i r e c t l y t o c r i s t o b l i t e on f i r i n g a b o v e 900 C. T h i s may be due to the e f f e c t o f the p r e s e n c e o f c e r t a i n m i n e r a l i z e r s which f a v o u r c r i s t o b a l i t e f o r m a t i o n r a t h e r than t r i d y m i t e c r y s t a l l i z a t i o n . 46 4.3 Discussion on Sintering of Diatomite 4.3.1 Sintering of Diatomite A large number of equations are available to interpret the shrink-(20 21) age data of powder compacts ' . These are primarily based on part i -cles of simple geometric shape i.e. spheres. Neck growth equations have also been developed for contact points of parabolas and cones on a f l a t plate and similarly attempts have also been made to develop shrinkage equations for compacts of complex geometric shapes. These models are, however, based on the assumptions that: (1) The mass remains constant during sintering; thus they are not applicable to sintering of decomposible compounds, and (2) There is no transformation, therefore the true density stays the same during sintering. For diatomite compacts these assumptions are not valid, as during sintering, diatomite looses weight and' also crystallographic transforma-tions occur in the system. This means that the physical property changes for diatomite on heating are very complex. The changes that occur on heating diatomite are due to sintering, mass loss and phase change. 47 These changes can be l i s t e d as f o l l o w s : (1) from L to L i n the d i m e n s i o n o (2) from W q to W i n the weight, and (3) from p^ to p^ , i n the t r u e d e n s i t y o A l t h o u g h the b u l k d e n s i t y ( i . e . weight of s o l i d / v o l u m e o f specimen) c h a n g e d f r o m p t o pg a n d t h e r e l a t i v e d e n s i t y ( i . e v o l u m e o f s o l i d / v o l u m e o f s p e c i m e n ) c h a n g e d f r o m p Q t o p, t h e s e v a l u e s c a n be c a l c u l a t e d from the p r i m a r y changes In d i m e n s i o n , mass and t r u e d e n s i t y o f compacts. The weight and the t r u e d e n s i t y changes a r e the two s p e c i a l c h a r a c t e r i s t i c s which have to be taken i n t o c o n s i d e r a t i o n w h i l e s t u d y i n g the s i n t e r i n g c h a r a c t e r i s t i c s o f d i a t o m i t e compacts. A c c o r d i n g l y , t h e n a t u r e o f these changes has to be c l a r i f i e d b e f o r e any s h r i n k a g e model can be d e v e l o p e d f o r d i a t o m i t e compacts. 4.3.2 Weight Changes o f D i a t o m i t e As mentioned p r e v i o u s l y , C a l a c a l i n h i s s t u d y on the s u r f a c e a r e a o f d i a t o m i t e compacts, e n c o u n t e r e d both pore growth and pore c r e a t i o n due to l o s s o f weight r e s u l t i n g from gas e v o l u t i o n . He r e a s o n e d t h a t the gas e v o l u t i o n was caused by (1) the escape o f water i n the o p a l i n e s t r u c t u r e o f d i a t o m i t e d u r i n g h e a t i n g , (2) the b u r n - o f f o f o r g a n i c m a t t e r i n the raw m a t e r i a l t o form CO^ and (3) the d e c o m p o s i t i o n o f c a r b o n a t e s o f Mg and Ca t o MgO, CaO and CC^ g a s . In t h i s p r e s e n t study the same ty p e s o f weight l o s s were o b s e r v e d . A c c o r d i n g l y , i t was assumed t h a t the weight l o s s was 48 due to the gas evolution and this created pores in the solid of diatomite compacts. The geometry of pores in diatomite compacts i s not as simple as most sintering models assume, and furthermore several types of pores have to be considered in the case of diatomite compacts. These are (1) pores which are formed due to mass loss, i.e. by the evaporation of gases and water, (2) pores which exist in diatom shell walls, (3) pores which form the inner cavities of diatom shells and (4) pores which are formed between diatomite particles (Figure 16). The concentration of these various pores in the compacts i s related to the weight loss and the density change of diatomite during sintering; therefore, a sintering model for diatomite compacts must represent the various types and sizes of pores in a simple geometry. 4.3.3 True Density Changes of Diatomite The true density increase of diatomite during sintering i s caused by the transformation of amorphous s i l i c a into cristobalite. It i s expected that a shrinkage of the compact would accompany the Increase in true density and the resultant decrease in solid volume. It i s assumed that this shrinkage w i l l produce equivalent dimensional reduction of test specimens, without affecting the porosity of the specimens. The va l i d i t y of this assumption stems from the experimental observations that (1) when the true density increased, the shrinkage also increased, (2) when the F i g u r e 16 Pores i n d i a t o m i t e compact. (800X) 50 t r u e d e n s i t y I n c r e a s e d , the p o r o s i t y d i d not i n c r e a s e and (3) the i n c r e a s e o f the t r u e d e n s i t y was s m a l l In comparison w i t h t h e I n c r e a s e o f the s h r i n k a g e . T h i s i m p l i e s t h a t the c o n t r i b u t i o n from the changes o f t r u e d e n s i t y to the o v e r a l l s h r i n k a g e of the compact was s m a l l . 4.4 Development o f S h r i n k a g e E q u a t i o n s W i t h r e s p e c t to the above mentioned c h a r a c t e r i s t i c s o f d i a t o m i t e compacts, the f o l l o w i n g I n f e r e n c e s can be made: (1) The weight l o s s g e n e r a t e d p o r e s . (2) T h e re were d i f f e r e n t t y p e s and s i z e s o f p o r e s i n the compacts. (3) The d e n s i t y of s o l i d changed d u r i n g f i r i n g . Any s i n t e r i n g model, t o be a p p l i c a b l e to d i a t o m i t e compacts, has to t ake a l l these i n t o account and a c c o r d i n g l y , a s i n t e r i n g model as shown In F i g u r e 17 i s p r o p o s e d . I t s h o u l d be noted t h a t the d i a t o m i t e specimens i n t h i s s t u d y o s t a r t e d s i n t e r i n g a t about 600 C as can be seen i n the p l o t s of s h r i n k a g e ( F i g u r e 13) and b u l k d e n s i t y change ( F i g u r e 15) w i t h t e m p e r a t u r e . F o r t h i s and the c o n s i d e r a t i o n t h a t the m a n u f a c t u r i n g t e m p e r a t u r e s o f commer-o c i a l d i a t o m i t e b r i c k s a r e between 800 and 1000 C, t h e d i s c u s s i o n i n t h i s s t u d y w i l l f o c u s on the p r o p e r t y changes o f d i a t o m i t e compacts f i r e d above o 600 C. A c c o r d i n g l y , t h e d a t a were a l l r e - c a l c u l a t e d u s i n g a base s e t 51 (ie. pores) (ie. so l id ) F i g u r e 17 The proposed model o f the d i a t o m i t e s h e l l w a l l . F i g u r e 18 SEM photo of d i a t o m i t e s h e l l w a l l ( f r o m C a l a c a l ) . 52 a r b i t r a r i l y at 6 0 0 C i. e . L Q , W q and P T q are the properties corresponding o to f i r i n g at 6 0 0 C. This should not af f e c t the test procedures as described in section 3.5 and also the shrinkage and the density data as reported in section 4.2.1. The sintering model developed to interpret the data of diatomite compacts i s based on the observations by Calacal and in this study, and can be represented by a typical microstructure as shown in Figure 18. This photomicrograph depicts small beady structures a l l throughout the solid mass with large hollow spaces. To represent this structure i t i s assumed that the diatomite shell-wall consists of an array of very small ( 22) microspheres. Photomicrographs taken by Oliver also show the minute cellular structures of the diatoms as can be seen in Figure 19. Assuming that these microspheres are solid spheres (i.e., no pores inside), a shrinkage equation of microspheres can be derived from the standard sintering equations developed for the neck growth between two spheres. It is also assumed that the mass of these microspheres does not change during f i r i n g . To deal with (1) the weight loss which results in micropore forma-tion in diatomite during f i r i n g and (2) the presence of a variety of pore sizes due to different origins of pores, a new approach i s necessary. Consider that a three dimensional array of microspheres i s arranged, extending throughout the entire diatomite compact, and some of the micro-spheres are missing from certain locations. A large pore may be equivalent to a region from which a large number of microspheres are missing. A single micropore may be simply a missing microsphere as shown 53 Figure 19 Diatom showing minute c e l l u l a r s t r u c t u r e . ( A f t e r O l i v e r ) 54 i n Figure 17. There are pores e x i s t i n g (1) between diatomite p a r t i c l e s , (2) i n the inner c a v i t i e s of diatom s h e l l s , (3) i n diatom s h e l l walls and (4) also formed by the evaporation of gases. A l l these pores may be represented by groups of missing microspheres. It i s assumed that a l l s o l i d microspheres are of i d e n t i c a l size and they are arranged i n a pattern which can represent the whole diatomite microstructure. These s o l i d microspheres are contained i n c e l l s each i d e n t i c a l i n s i z e , shape and o r i e n t a t i o n to i t s neighbours. Any one of these c e l l s can be c a l l e d a unit c e l l and a unit c e l l can be defined as a unit s i z e and shape of c e l l which contains a s o l i d sphere and open space surrounding i t . It i s also considered that missing microspheres c o n s t i t u t e vacant unit c e l l s . I t Is assumed that vacant unit c e l l s shrink i n the same manner as occupied u n i t c e l l s which contain s o l i d microspheres, since the vacant space i n powder compact i s surrounded by s o l i d microspheres. This can be v i s u a l i z e d by the shrinkage of the inner diameter of a hollow ceramic tube b u i l t with c l a y p a r t i c l e s , which may also be considered as unit c e l l s . The resem-blance of the shrinkage of a sin g l e diatom s h e l l to the shrinkage of a ceramic tube i s shown i n Figure 20, where the shrinkage of the diatom s h e l l s have the same geometric c h a r a c t e r i s t i c s as that of a ceramic tube. For further development of the shrinkage equation the problem can be approached as f o l l o w s : c o n s i d e r there are "m " vacant c e l l s and "n " v v o o occupied c e l l s per unit volume of a diatomite compact. The t o t a l number of u n i t c e l l s i n a u n i t volume i s "m + n I f N i s the f r a c t i o n of o o o occupied unit c e l l s i n a unit volume, then Raw d i a t o m i t e (800X) Diatomite a f t e r 1050*C f i r i n g (800X) Figure 20 Shrinkage of d i a t o m i t e s h e l l s on f i r i n g , m a i n t a i n i n g the o r i g i n a l geometric shape. 56 N m + n o o L e t t h i s d i a t o m i t e compact be s u b j e c t e d to s i n t e r i n g . S h r i n k a g e and weight l o s s o c c u r d u r i n g s i n t e r i n g . A f t e r s i n t e r i n g , the numbers o f v a c a n t c e l l s and o c c u p i e d c e l l s a r e "m" and "n", r e s p e c t i v e l y . The l o s s o f mass can be r e p r e s e n t e d by the d i s a p p e a r a n c e o f "q" m i c r o s p h e r e s ( i . e . "q" o c c u p i e d u n i t c e l l s t u r n i n t o "q" v a c a n t u n i t c e l l s ) . T h i s i s shown s c h e m a t i c a l l y i n F i g u r e 21. As can be seen i n the F i g u r e , m + q = m and n - q = n o M o M m + n = ( m + q ) + ( n - q ) = m + n o M o M o o C o n s e q u e n t l y , the t o t a l number o f u n i t c e l l s b e f o r e and a f t e r s i n t e r i n g i s unchanged. I f "N" i s the f r a c t i o n o f o c c u p i e d u n i t c e l l s per u n i t volume a f t e r s i n t e r i n g , then N = _n m + n N _ n/m + n N n /m +~~n o o o n n o n x weight of each m i c r o s p h e r e n Q x weight o f each m i c r o s p h e r e = weight of a u n i t volume a f t e r s i n t e r i n g weight o f a u n i t volume b e f o r e s i n t e r i n g _ weight of specimen a f t e r f i r i n g _ (W) (4 4 1) weight of specimen b e f o r e f i r i n g ( W ) Note: the mass o f m i c r o s p h e r e does not change d u r i n g f i r i n g . (1) O r i g i n a l u n i t volume. n = number of occupied u n i t c e l l s o m = number of vacant u n i t c e l l s o m + n t o t a l number of u n i t c e l l s o o (2) Subjected to s i n t e r i n g . (3) Volume shrinkage and weight l o s s occur during s i n t e r i n g . Consider the l o s s of mass can be represented by disappearance of "q" occupied u n i t c e l l s . -^,_>^  y ^ . V 1 O^ccupied ®unit cells n n = number of occupied u n i t c e l l s a f t e r s i n t e r i n g : n = no m = number of vacant u n i t c e l l s a f t e r s i n t e r i n g : m = m + o Figure 21 A schematic I l l u s t r a t i o n of the weight l o s s of di a t o m i t e . 58 The conventional spherical geometry for sintering model can now be applied. It must, however, be noted that in the case of diatomite compacts, the neck growth i s assumed to follow the sintering model shows schematically the change of geometric configuration of the unit c e l l during sintering. The symbols in this figure have been altered from Frenkel's model in order to match the ones used in this thesis. For developing the shrinkage equation, i t i s considered that the coordination number with the neighbouring spheres remains constant throughout the sintering process. proposed by Frenkel (23) for viscous flow as shown in Figure 22. Figure 23 From Figure 23, 2 _L a + y h R - Y = R - / R - 2 a h R R - / R2- a 2 R 2 1 - / 1 - (f-)2 Shrinkage = AL o h R = 1 - / 1 - (4.4.2) 59 F i g u r e 22 The geometry o f F r e n k e l ' s model f o r the i n i t i a l s t a g e o f s i n t e r i n g by v i s c o u s f l o w . Before sintering After sintering w Weight of a sphere v0 Volume of a unit cell DBO Bulk density of a unit cell F i g u r e 23 The geometry of u n i t c e l l i n c o r p o r a t i n g a m i c r o s p h e r e . 61 This equation g i v e s the r e l a t i o n between shrinkage (•=—) and o r e l a t i v e neck radius (•=-), but i n case of diatomite the r e l a t i v e neck R radius (—) in Eq. 4.2.2 is impossible to measure experimentally; therefore this equation was modified eliminating (—) as follows. 4.4.1 Development of Density Equations An occupied unit c e l l , with the weight of solid w, the length of edge Jt , the volume of unit c e l l v and the bulk density of unit c e l l D„ , b o' o J Bo' is subjected to sintering. After sintering, w i s unchanged, A q shrinks to ( 24 ) A, v decreased to v and D_ increased to D„. Rhine showed that o Bo B v a A 3 o o 3 v a JT AA A - A o - 1 -o A o DT - 1 " <^> 1/3 62 AL = AA L I o o " 1 " (V> (4 .4.3) By combining Eq's 4.4.2 and 4.4.3, the equation below can be obtained. D„ 1/3 = 7 1 " ( t ) 2 °B 1 °B R R 2 _ 3 / 2 . . . . . b — = 2-372 ° r D — = L~2 ( 4' 4' 4 ) Bo [ l - (—) ] R - a The bulk density of compact (p„) is B weight of specimen B^ volume of specimen weight of unit volume unit volume = (weight of unit volume) * (volume of total occupied unit c e l l s + volume of total vacant unit cells in unit volume) _ n x weight of each solid microsphere (m + n) x volume of each unit c e l l n m + n n x bulk density of each occupied unit c e l l D. m + n B = N D B or D B = - (4.4.5) 63 In the same way, the relative density of compact p is volume of solid in specimen volume of specimen volume of solid in unit volume unit volume volume of solid in unit volume  volume of total occupied unit cells + volume of total vacant unit c e l l s n x volume of each solid microsphere (m + n) x volume of each unit c e l l n x (relative density of each occupied unit cell) m + n = N D or D N (4.4.6) From Eq. 4.4.5 Eq. 4.4.4 i s given by, D, B 1 [1 " ( f ) 2 ] 3 / 2 1 64 pB = — 7 a 2T3/2 (*-*-7) Eq. 4.4.7 gives the relation between the bulk density of specimen and the relative neck radius. Combining equations 4.4.3 and 4.4.5 results in AL L ^ = 1 -= 1 r P B o / N o 1 / 3 L P b 7 N _ J p B o y / 3 Substituting for N / N q by Eq. 4.4.1 gives AL m pBo W Lo " " >B V 1/3 rB o o « 1 - 3 *k L o o rB o Eq. 4.4.8 gives the relation between shrinkage, bulk density and weight change, and can be tested with the data obtained in this investigation. 65 4.4.2 Testing of shrinkage and Density Equations A plot of the shrinkage of the compacts versus the f i r i n g temperature is shown in Figure 24. In the derivation of the shrinkage equation 4.4.8 i t i s assumed that R ~ R q . This approximation is only v a l i d i f RQ does not change significantly and the boundary condition is (25) that a/R should be less than ~ 0.4 (as shown by Kakar ). In order to test this point, the values of a/R were calculated using equation 4.4.2 and the shrinkage (AL /L ) data. These calculated values of (•§•) are shown O R i n Table VI. The largest value of is less than 0.40 which satisfies R the above boundary condition that a/R < 0.4 and R ~ R Q . Equation 4.4.8 can also be used to calculate the shrinkage of the compact from the experimental data of weight loss (W/WQ) and bulk density ( p B / p B o ) . Then the calculated values can be compared with experimentally measured shrinkage values. This is shown in Figure 25 to confirm the va l i d i t y of Eq. 4.4.8. Good agreement between the calculated and the experimental values suggests that the proposed sintering model may be operative in the case of diatomite compacts. 4.5 Results and Discussion on Thermal Conductivity One of the most important properties of diatomite insulating bricks i s the thermal conductivity since these bricks are used as heat insulating material. Another important property beside thermal conductivity i s the strength of the brick since they are also used as construction material. 66 to 02 h F i g u r e 24 The experimental shrdrtkage data of pure diatomite specimens, normalized to the 600°C f i r i n g . TABLE VI The Calculated Value of — with Heat Treated Specimens Temperatures of heat treatment Experimental Results of AL/L o Calculated — K from AL/L 0 600°C 0.0000 0.0000 700 0.0022 0.0663 800 0.0090 0.1338 850 0.0147 0.1709 900 0.0254 0.2241 950 0.0439 0.2931 1000 0.0696 0.3667 1050 0.0782 0.3877 Figure 25 The t h e o r e t i c a l shrinkage p l o t as a f u n c t i o n of r e l a t i v e d ensity and weight. Experimental data are a l s o i n c l u d e d . 69 The changes of the thermal conductivity value of diatomite on f i r i n g are discussed below while the changes of the strength of diatomite on f i r i n g w i l l be dealt with in the next section. 4.5.1 Experimental Results of Thermal Conductivity The thermal conductivity of diatomite specimens was determined after f i r i n g the specimens at elevated temperatures. Thermal conductivity o measurements were carried out at 300 C test temperature. Details of the experiments are given in Secion 3.3.5 (Equipment) and Section 3.5.3 (Test Procedures) in this thesis. The experimental results of thermal conductivity measurements of fired specimens are plotted in Figure 26 as a function of the f i r i n g temperature of the specimens. The thermal conductivity values were f i r s t o decreased, on f i r i n g up to 600 C, i n the same way as the bulk density dropped on f i r i n g (Figure 15). These curves, however, are not similar to the shrinkage curve which continued to increase over the whole test temp-erature range (Figure 13). 4.5.2 Development of Thermal Conductivity Equations Numerous thermal conductivity equations for porous compacts are available in literature but most of them have been found to be unsuitable to test the present data, because of the peculiar sintering behaviour of diatomite, i n which mass loss and mineral transformations occur 3 4 5 - 6 7 8 9 10x100 Temp of heat treatment (°c) Figure 26 The experimental thermal c o n d u c t i v i t y data of pure d i a t o m i t e specimens. 71 concurrently with sintering. Of these models the one, which Ramanan derived to relate neck radius after sintering to the el e c t r i c a l conductiv-i t y of ceramic compacts, appears to be a suitable model for modification for the testing of the thermal conductivity data determined on diatomite compacts. By combining Ramanan's method and the density equation 4.4.7, and then by applying the vacant unit c e l l concept developed in a previous section, i t i s possible to derive an equation relating the thermal conduc-t i v i t y and density of the compacts. The derivation of the equation i s as follows. Figure 27 shows the basic geometry of two spheres in contact and the equivalent e l e c t r i c a l circuit as developed by Ramanan. Figure 28 shows spheres in a two dimensional cubic array and the equivalent e l e c t r i c a l network. In this present case the electrical current i s replaced by heat flow and the e l e c t r i c a l conductivity by thermal conductivity. The analogy between the flow of el e c t r i c a l current and heat i s well established and has been used by many authors previously. From Ramanan's treatment, the conductivity of the simple cubic c e l l , k y (Figure 29) i s given by: , area of neck perpendicular to current , ._ c -k = ? T -n—5 x conductivity of particle u area of unit c e l l perpendicular to current A A s u 2 ira Geometry of two spheres Equivalent electrical network Figure 27 The geometry of two spheres i n contact and the equivalent e l e c t r i c a l network ( a f t e r Ramanan) Two dimensional Equivalent electrical array of spheres network Figure 28 Spheres i n a two-dimensional cubic array and an equivalent e l e c t r i c a l network ( a f t e r Ramanan) F i g u r e 29 Heat f l o w through a sphere i n a s i m p l e c u b i c model. 74 where k g = C o n d u c t i v i t y o f s o l i d ( p a r t i c l e ) A = A r e a o f u n i t c e l l u A = A r e a o f heat f l o w , i . e . a r e a o f neck The g e n e r a l form o f t h i s e q u a t i o n f o r d i f f e r e n t t y p e s of p a c k i n g i s k 2 k" = a l 2 2 ( A ' 5 - 1 ) s R - a where k = c o n d u c t i v i t y of u n i t c e l l u " a " i s a g e o m e t r i c c o n s t a n t , whose v a l u e i s dependent on t y p e s o f p a c k i n g . The v a l u e o f t h i s g e o m e t r i c c o n s t a n t (a) w i l l be d i s c u s s e d l a t e r . Thermal c o n d u c t i v i t y of a u n i t volume (K) i s K = N k (4.5.2) u where N i s the f r a c t i o n o f the number of s o l i d c e l l s i n a u n i t volume. By combi n i n g 4.5.1 and 4.5.2, K = N a k 8 R 2 - a 2 = N a k s D2 2 R - a = N a k s 1 (-) 2 - 1 75 0 when — = 0 i.e . before sintering started, K = K i.e. thermal conductivity R O o value before heat treatment (in this case at 600 C). K = N a k s 1 (V VR' - 1 + K or K = N a k o s „2 R - a 2 + K o (4.5.3) Eq . 4.5.3 can be r e w r i t t e n as below assuming that N, a and k are s constants. K - K = N a k 1 o s 1 _ ^  (-) 2 " = (const) - 1  (V " 1 a = (const) when a « R - (const) (|-)2 (4.5.4) Eq. 4.5.4 shows that the thermal conductivity increases with almost the square of the neck radius (assuming "R" remains constant). Eq. 4.5.3 and 4.5.4, however, are not practical to apply to the experimental data of thermal conductivity because the value of a/R in diatomite compacts is not known. The equations of thermal conductivity and density can however be combined to eliminate (—) as follows. 76 Eq. 4.4.7 I s m o d i f i e d to - No P B , R 2 r 1 = V R 2 - a 2 r B o R - a 2 a 2 2 R - a and then s u b s t i t u t e d i n 4.5.3 r e s u l t i n g i n N n PR 2 / 3 K = N a k ( (rf—-) - 1 ) + K s N p_ ' c r B o N W s i n c e ^ — = — ( Eq. 4 . 4 . 1 ) ( 4 . 5 . 5 ) W PT, 2/3 K = N a k f(-°-—5.) - 1 ) + K ( 4 . 5 . 6 ) s W p_ ; o v rBo Eq. 4.5.6 g i v e s the r e l a t i o n between t h e r m a l c o n d u c t i v i t y , b u l k d e n s i t y and w e i g h t l o s s o f a d i a t o m i t e specimen. The term ( k g ) i n Eq. 4.5.6 i s the t h e r m a l c o n d u c t i v i t y of s o l i d d i a t o m i t e . 77 A.5.2.1 C o r r e c t i o n f o r M i n e r a l T r a n s f o r m a t i o n I n r e a l d i a t o m i t e compacts the v a l u e of k changes on f i r i n g due to the m i n e r a l o g i c a l t r a n s f o r m a t i o n from amorphous s i l i c a to a m i x t u r e of c r i s t o b a l i t e and g l a s s m a t r i x , as observed i n the X-ray a n a l y s e s and t r u e d e n s i t y measurements. I t i s not p o s s i b l e to know the t h e r m a l c o n d u c t i v i t y o f the s o l i d component i n d i a t o m i t e a f t e r heat t r e a t m e n t , but by assuming t h a t k c h a n g e s p r o p o r t i o n a t e l y w i t h f r a c t i o n o f amorphous phase which i s c r y s t a l l i z e d and t h a t t h i s I s r e f l e c t e d i n the t r u e d e n s i t y change, i t i s p o s s i b l e t o e s t i m a t e t h e v a l u e o f k g w i t h the e x t e n t o f c r y s t a l l i z a -P T " P T g t i o n . F r a c t i o n of c r y s t a l l i z a t i o n = — P T c " P T g t r u e d e n s i t y of t e s t specimen t r u e d e n s i t y o f g l a s s , i n t h i s case the t r u e d e n s i t y o f the o specimen f i r e d a t 600 C where d i a t o m i t e Is amorphous t r u e d e n s i t y o f c r y s t a l , i . e . t r u e d e n s i t y o f s i n t e r e d d i a t o m i t e , i n t h i s case the t r u e d e n s i t y o f the specimen o f i r e d a t 1050 C ( a f t e r c r y s t a l l i z a t i o n ) . Thermal c o n d u c t i v i t y change c o r r e s p o n d i n g to the amount of c r y s t a l l i z a t i o n i s , where p T P T g = P T c 78 where k c = thermal conductivity of crystal k = thermal conductivity of glass when Therefore the thermal conductivity of a zero porosity specimen can be written as, k = k + (k - k ) — =£- (4.5.7) s g c g p T c - p T g Eq. 4.5.7 gives the thermal conductivity value of solid diatomite at different stages of sintering. The value of "a" In Eq. 4.5.6 is a constant, but is dependent on the type of packing. The theoretical values of a calculated by Ramanan for various packings are listed in Table VII. 4.5.3 Testing of Thermal Conductivity Equations Using the experimental data of thermal conductivity reported in the previous chapter, the thermal conductivity equation (Eq. 4.5.6) can be tested as follows. « « 2 / 3 P B K = Nctk ((=?-—-) - l ) + k (4.5.6) s v W p _ • ' o rBo TABLE V I I Geometric Parameters o f V a r i o u s P a c k i n g s ( a f t e r Ramanan) Type o f P a c k i n g a R e l a t i v e D e n s i t y , D J ' o Simple Cubic n/4 0.5236 Orthorhombic n/2/3~ 0.6046 Rhombohedral i x / / 2 ~ 0.7403 b . c . c . H/V374 0.6802 80 I n t h i s e q u a t i o n — (the weight l o s s o f specimen), ( t h e b u l k d e n s i t y W PBo o f s p e c i m e n ) and k Q ( t h e t h e r m a l c o n d u c t i v i t y b e f o r e s i n t e r i n g , i n t h i s o c a s e t h e t h e r m a l c o n d u c t i v i t y o f the d i a t o m i t e specimen f i r e d a t 600 C) were dete r m i n e d e x p e r i m e n t a l l y . But N (the f r a c t i o n o f the number o f s o l i d c e l l s i n the u n i t volume) and a (a c o n s t a n t , whose v a l u e i s dependent on the types of p a c k i n g ) a r e unknown. S i n c e b o t h N and a a r e a f u n c t i o n o f the type of p a c k i n g , t h e s e f i g u r e s can be c a l c u l a t e d t h e o r e t i -c a l l y f o r a v a r i e t y o f p a c k i n g g e o m e t r i e s as f o l l o w s . From Eq. 4.4.1 and 4.4.6, N W A « P ° — — = - — and N = — N W o D o o o T h e r e f o r e N = D W o o p Q was e x p e r i m e n t a l l y determined and found to be e q u a l to 0.3044 a t 600 C. D i s t h e r e l a t i v e d e n s i t y o f t h e u n i t c e l l . T h i s was c a l c u l a t e d o t h e o r e t i c a l l y by Ramanan and i s shown i n T a b l e V I I . The weight change o f W ° d i a t o m i t e specimens (^—) between 600 and 1050 C was l e s s t h a n 1.7%, t h e r e -w o w f o r e y— = 1. The v a l u e s o f "N" c a l c u l a t e d u s i n g the above v a l u e s a r e o shown i n T a b l e V I I I . TABLE V I I I The C a l c u l a t e d V a l u e s o f N ( N = r T h w h e r e W~* ^ o o o Types o f P a c k i n g V a l u e s o f N Simple Cubic 0.5813 Orthorhombic 0.5035 b .c .c 0.4475 82 As mentioned before, Eq. 4.5.7 gives the thermal conductivity value of solid diatomite, which i s also necessary to calculate the thermal conductivity of heat treated diatomite compacts. The values of k g are calculated as follows. P T " P T e k = k + (k - k ) — i s - (4.5.7) s g c g p - p In this equation p,^ i s 2.1543 g/cc (true density of amorphous diatomite, in this case the o true density of the specimen fired at 600 C). p^,c i s 2.3807 g/cc (true density of crystallized diatomite, in this case o the true density of the specimen fired at 1050 C). _3 k i s 2.3 x 10 cal/cm.s.C (0.96 w/m K) (thermal conductivity of solid amorphous diatomite, in this case the typical value of crown glass was used) . _3 k c i s 4.6 x 10 cal/cm.s.C (1.92 w/mK) (thermal conductivity of solid crystallized diatomite, in this case the typical value of siliceous porcelain was used). p^ , i s true density of test specimen which is shown in Table III. The calculated values of k are shown in Table IX. s TABLE IX The Calculated Values of k g by Eq. 4.5.7 Temperature of Heat Treatment Values of k s 600( C) 0.96 (w/mK) 700 0.96 800 0.92 850 0.92 900 0.92 950 1.21 1000 1.76 1050 1.92 84 It i s now possible to calculate the value of K from the known and P B w o estimated values of N, a, k , , r r — and k for specimens heat treated at ' s' p_ ' W o r rBo different temperatures using Eq. 4.5.6. Three different values of N and a were used in the calculation corresponding to three different packings. The theoretical lines and the experimental data are shown in Figures 30 to 32. It appears that the predicted curve for the simple cubic packing fitt e d best with the experimental values of K, indicating that the overall packing geometry of microspheres in these diatomite specimens may be represented by the simple cubic packing. As can be seen in these figures, simple cubic packing is in better agreement with the experimental results o o for k values of 0.96 w/mk up to 900 C and 1.92 w/mk above 1000 C. s From this set of data i t appears that the thermal conductivity of diatomite compacts increased with the f i r i n g temperatures. The increase is l i k e l y due to (1) the growth of contact areas between particles and (2) the crystallization of amorphous s i l i c a , which is the main structural element in diatomite shells. This increase of thermal conductivity on (27) f i r i n g i s similar to that reported by Ruh and Renkey who showed that the thermal conductivity of castable refractories increased as ceramic bonds were formed In the refractory concrete. 4.5.4 Porosity Effect on Thermal Conductivity In the past extensive studies have been made to explain the thermal (28 29 30) conductivity of porous bodies ' ' . In these studies, consideration has been given not only to the volume fraction of pores but also to such F i g u r e 30 The e x p e r i m e n t a l d a t a and the t h e o r e t i c a l ( s i m p l e c u b i c p a c k i n g ) t h e r m a l c o n d u c t i v i t y p l o t . 00 F i g u r e 31 The e x p e r i m e n t a l d a t a and the t h e o r e t i c a l ( o r t h o r h o m b i c p a c k i n g ) t h e r m a l c o n d u c t i v i t y p l o t . 00 •35 F i g u r e 32 The e x p e r i m e n t a l d a t a and the t h e o r e t i c a l ( b . c c . p a c k i n g ) t h e r m a l c o n d u c t i v i t y p l o t . 00 88 factors as crystalline nature, impurities, lattice imperfection, chemical composition and the size, shape, orientation and emissivity of the pores. Numerous equations are thus available to approximate the thermal conduc-t i v i t y of porous compacts. Some of the well known thermal conductivity equations are shown in Table X and two representative plots are shown In Figures 33 and 34. These equations indicate that the thermal conductivity of a given porous compact can be approximated to a linear function of the porosity of the compact. For instance, Loeb's equation shows that the thermal conductivity changes linearly with porosity, i f the geometric effect of the pore is ignored. Loeb's equation i s included in Figure 35 along with the thermal conductivity results of pure diatomite specimens and a commercial brand of diatomite bricks. These commercial bricks were used in the latter part of this study. In Figure 35 i t can be seen that the thermal conductivities of diatomaceous compacts (i.e. both pure diatomite specimens and commercial diatomite bricks) increased rapidly with l i t t l e decrease in porosity; the fractional porosity of pure diatomite specimens decreased from 0.696 (for o the specimen heat treated to 600 C) to 0.654 (for the specimen heat o treated to 1050 C). This is a 6% decrease in fractional porosity. On the other hand, the thermal conductivity of these specimens increased from 0.15 w/mK to 0.30 w/mK, respectively, i.e. 100% increase in the thermal conductivity value. This increase in thermal conductivity i s apparently caused by a combination of the increase in contact areas and the crystallization of diatomite particles. This type of increase in thermal (33 ) c o n d u c t i v i t y of powder compacts was noted by Kingery , that the TABLE X E q u a t i o n s f o r the Thermal C o n d u c t i v i t y o f a Porous Body (29) 1 + 2P - — ^ j ^ " E ucken : k = k 2 < ^ "t ^ P s i - P 1 " Q 2Q + 1 R u s s e l l ( 3 0 ) : k - k Q d " ffi p S p 2 / 3 - P + Q ( i - P 2 / 3 + P ) p L o e b ( 3 1 ) : k p - k s (1 - P c ) + j-^ £ AoervT ' m ^ = c o n d u c t i v i t y o f porous sample k g = c o n d u c t i v i t y of s o l i d sample P = volume pore f r a c t i o n Q = k /k ^ s a k g = c o n d u c t i v i t y o f a i r P = c r o s s s e c t i o n a l pore f r a c t i o n c P = l o n g i t u d i n a l pore f r a c t i o n Li a = r a d i a t i o n c o n s t a n t Y = g e o m e t r i c a l pore f r a c t i o n e = e m i s s i v i t y d = d i m e n s i o n of pore T = mean a b s o l u t e temperature m 90 I E >-* </> 0.0\0, U J IT < EC ~ T i 1 1 y T T 7 1 1 • f i i 1 ' • --* • • -/* -• -I i i t I . . . . 1 , i i i i i 0.150 0.200 0.2 50 POROSITY 0.300 F i g u r e 33 The e f f e c t o f p o r o s i t y on t h e r m a l r e s i s t i v i t y t * • - i i ' ( 3 1 ) ^ ( a f t e r Wagner ) • 0.2 0.3 0. FRACTIONAL POROSITY F i g u r e 34 D i m e n s i o n l e s s t h e r m a l c o n d u c t i v i t y v s . f r a c t i o n a l p o r o s i t y f o r p o l y c r y s t a l l i n e g r a p h i t e ( a f t e r R h e e ^ 2 )) 91 •4 -5 -6 7 -8 Fractional porosity F i g u r e 35 The e x p e r i m e n t a l d a t a of t h e r m a l c o n d u c t i v i t y v s . p o r o s i t y . 92 crystalline nature and structure of ceramic materials strongly affect the thermal conductivity of the compact but that these relationships are d i f f i c u l t to assess quantitatively because of the wide divergence in values reported for pure materials. A detailed curve of the thermal conductivity of diatomite specimens was plotted against the relative density of the specimens in Figure 36. In this figure, the thermal conductivity curve appears to satisfy the 2/3 r e l a t i o n s h i p : (thermal conductivity) a (density) for specimens heat o treated below 900 C. This r e l a t i o n s h i p can e a s i l y be assumed by Eq. 4.5.6. On the other hand, the thermal conductivity value of specimens o heat treated above 900 C increased d r a s t i c a l l y with l i t t l e change in r e l a t i v e density. This behaviour of rapid increase i n thermal conductivity appears to coincide with the sudden formation of cristobalites in specimens heat treated in this temperature range. From these observations of Figures 30, 35 and 36 i t can be Inferred that two factors are affecting the thermal conductivity value of diatomite compacts. The f i r s t - the amount of pore volume in the compact, which results in lower thermal conductivity value for a lighter brick and a higher thermal conductivity value for a heavier brick. The second - the effect of f i r i n g , which influences the thermal conductivity value by changing the size of contact areas and by transforming amorphous s i l i c a Into cristobalites in the diatomite compact. In this mechanism, thermal conductivity values change with respect to the degree of sintering and the amount of crystals formed in the compact. 93 F i g u r e 36 The e x p e r i m e n t a l r e l a t i o n s h i p between r e l a t i v e d e n s i t y (Ap/p) change and t h e r m a l c o n d u c t i v i t y (AK/K q) change. 94 4.6 Results and Discussion on Strength 4.6.1 Experimental Results of Strength Strength is one of the important properties of diatomite Insulating bricks because these bricks are routinely used in structural and load-bearing walls. The measured compressive strength of diatomite specimens were plotted against their f i r i n g temperatures as shown in Figure 37. The strength of diatomite was reduced slightly on heating between 200 and o o 400 C but an increase was observed at about 600 C and especially above o 800 C. 4.6.2 Development of Strength Equations In the previous section, equation of shrinkage, bulk density and thermal conductivity were related to the ratio of neck radius (a) to p a r t i c l e radius (R) , Similarly i t has been found to be convenient to R r e l a t e the change of — on heating with the increase in strength of R diatomite. Figure 38 shows a simple schematic model of the breaking down of contact areas during compressive strength tests. Consider a unit volume of specimen. It is assumed that particles of diatomite are bonded to one another by necks formed at contact points of particles during the heat treatment. 3 4 5 6 7 8 Temp of heat treatment Cc) 10x100 F i g u r e 37 The e x p e r i m e n t a l compressive s t r e n g t h d a t a of pure d i a t o m i t e specimens. 96 F i g u r e 38 A model f o r the breakage of necks d u r i n g the compressive s t r e n g t h t e s t . 97 When the d i a t o m i t e specimen f r a c t u r e s under a c o m p r e s s i v e l o a d , the c o n t a c t a r e a s o f the p a r t i c l e s f r a c t u r e . T h e r e f o r e , where Q = f r a c t u r e l o a d on a u n i t volume of specimen = t o t a l f r a c t u r e d neck a r e a on f r a c t u r e p l a n e / u n i t volume = f r a c t u r e s t r e s s a t neck a r e a Q = a f A f The t o t a l f r a c t u r e d neck a r e a on f r a c t u r e p l a n e / u n i t volume ( i . e . A^) i s a l s o e q u a l to ( t h e f r a c t u r e d neck a r e a per p a r t i c l e ) x (number o f p a r t i c l e s on the f r a c t u r e p l a n e ) . f . n 2/3 . Ap . n 2/3 A = C A . n P 2/3 where A f r a c t u r e d neck a r e a / p a r t i c l e A P t o t a l neck a r e a / p a r t i c l e n number of p a r t i c l e s / u n i t volume C A ,/A i . e . f r a c t i o n o f f r a c t u r e d neck a r e a to t o t a l neck pf p a r e a / p a r t i c l e on f r a c t u r e p l a n e . 98 .*. Q = a f A f - ac C A n 2 / 3 (4.6.1) f P "n" is the number of solid (occupied) particles in a unit volume of diatomite compacts. This was defined in Section 4.3. The fracture strength (S) of a unit volume of specimen i s , S = unit area 2/3 a, C A n f , P (4.6.2) unit area A unit area is equal to (the area of one facet of a unit c e l l ) x (number of unit cells on a unit area). Therefore, 1 / 1 2 2/3 A Unit Area = (8 1 y) x (m + n) / J Q2/3,D2 2.. . .2/3 = B (R - a )(m + n) where 8 is a geometric constant. This was defined and calculated by Kakar f o r various types of packing. The values of 8 are l i s t e d i n Table XI. Eq. 4.6.2 thus becomes, 2/3 ac C A n ' c _ f P  ~ „2/3, 2 2 ^ T7I B (R - a )(m + n) a, C A 2/3 - f P f n t ,2/3 .2 27 lm + n ; B ' (R - a ) a, C A TABLE XI Geometric Parameter (B) for Various Packings (after K a k a r ) ( 2 5 ) Types of Packing Values of 8 Simple Cubic 8 Orthorhombic Rhombohedral 4/2" b.c.c. 32/3/9 100 "A " i n Eq. 4 .6.3 can be c a l c u l a t e d as f o l l o w s : P A = t o t a l neck a r e a o f one p a r t i c l e P where Z = c o o r d i n a t i o n number A = a r e a o f one neck a = r a d i u s o f one neck S u b s t i t u t i n g t h i s i n Eq. 4 . 6 . 3 , S = ac C A f P 2/3 2 2~~ 6 ' (R - a Z ) N 2/3 a f C N 2/3 Z it a P 2/3 2 2 R - a 2/3 a{ C N ' Z u 2 B 273" T , 2 2 R - a o r 2/3 a f C N Z TI 2/3 2 B VR^ - 1 or (4 .6 .4) 101 0 f C Z n where C'= 5-7^— (4.6.5) 2 6 ' C ' is a constant because, o N = a constant when weight loss on f i r i n g is very small (above 600 C) = fracture stress of neck area = a constant C, Z and B = constants In Eq. 4.6.4 S = S when a = o M o .-. S = C" —= - + S (4.6.6) o (-) 2 - 1 The strength equation 4.6.6 i s very similar to the thermal conductivity equation 4.5.3 as shown in Section 4.5.2; thus, the relation between the strength and the bulk density of compacts can also be derived in the same way as the thermal conductivity and the bulk density equation. The f i n a l form of the strength and the bulk density equation is given by p W 2 / 3 S = (:'((-*-£) -1) + S (4.6.7) pBo 102 4.6.3 Testing of Strength Equations 4.6.3.1 Experimental Value of "C " The values of the constant "C " in Eq. 4.6.7 were calculated from the experimental data of strength, bulk density and weight loss. The results of this calculation are shown in Table XII. 5 2 The value of C is 4.6 x 10 g/cm and i t should be constant as a l l the factors in Eq. 4.6.5 are constant. For example, N (the fraction of the number of occupied unit cells to total unit cells) was found to vary experimentally less than 1.7% In the temperature range of testing (600 to o 1050 C ) . Z and 8 are functions of the type of packing and, i n the development of the sintering theory, these are assumed to be constants, o^ - the stress necessary to fracture the neck region should also be a constant ( i . e . stress per unit area). If however, i t i s considered that there is a reduction in the value of C (Table XII) with temperature of o heat treatment above 1000 C, then this reduction can be attributed to the following: (1) The transformation of amorphous glass to crystal phase may reduce the strength, i f the crystallization i s followed by heterogeneous nucleation, (2) The concentration of Impure glassy phase into the neck region, and TABLE XII The Calculated Values of C Temperatures of heat treatment Values of C 700°C 2 445 kg/cm 800 522 850 535 900 510 950 487 1000 351 1050 378 Av. Value of C 461 104 (3) the creation of microcracks at neck areas, which is well known for recrystallization of amorphous s i l i c a with significant grain-.(34,35) growth ' Specifically, when cristobalite crystals are formed in diatomite compacts, these crystals would tend to invert from the high temperature 8-phase to the low temperature a-phase in the temperature range 200 to o (36 ^ 300 C. This inversion i s associated with about a 3 volume % change which may disrupt the structure, resulting in microcrack formation. Specimens which were cooled rapidly In this temperature range might have poor compressive strength due to this type of crack formation. 4.6.3.2 Evaluation of the Results with Strength Equation The average value of experimentally determined C in Table XII i s used to calculate the strength of the compact using Equation 4.6.7 and other data on bulk density and weight loss. The calculated strengths are used to draw a line in Figure 39, in which experimental points are also shown. It can be seen that the measured strength of specimens heat o treated above 1000 C are significantly lower than the predicted values of strength. This temperature coincided with the massive formation of cristobalite in the specimen (see Table IV); thus, this may be an indication that the formation of cristobalite may have caused microcrack formation and that this reduced the strength of the diatomite compacts. Figure 39 The experimental data and the predicted strength. 106 It can be noted that the strength as well as the thermal conductivity of diatomite compacts increased with respect to the square of a 2 the r e l a t i v e neck radius, (—) (see eqs. 4.6.6, p. 101 and 4.5.3, p. 75). Since both the strength and the thermal conductivity have the same form of equation, i t appears that for a given diatomite specimen, the higher the strength, the higher w i l l be the thermal conductivity. In addition to bulk density and weight changes, both of these equations also contain several different factors some of which are func-tions of the packing of the particles. Consequently, the change of packing of particles should affect both the thermal conductivity and the strength of diatomite specimen. Besides, i t might yet be possible to obtain a specimen with high strength with low thermal conductivity by c a r e f u l l y manipulating the other factors in these equations, such as k g against 0 - and k against S . For example, some kind of mineralizers ° f o o (37) such as a mixture of lime and nepheline syenite may r e s t r i c t the formation of cristobalites, resulting in a diatomite compact with high and low k g. Alternatively the use of compounds of colloidal s i l i c a such as sodium s i l i c a t e s o l u t i o n may increase S q without s i g n i f i c a n t l y increasing K . 107 5.0 VERIFICATION OF EQUATION WITH DATA OF  COMMERCIAL DIATOMITE BRICKS In the objective of this research project, It was stated that the purpose of this programme would be to develop an understanding of the factors affecting the properties of diatomite during f i r i n g so that the knowledge from this research project could be applied to benefit Clayburn Refractories Ltd. In order to extend this work to commercial diatomite products, attempts were made to test some of the equations developed in previous sections with the data obtained from a commercial brand of insulating refractories, made from the same diatomite materials. This particular brand of insulating bricks i s the heaviest of a l l diatomite product lines manufactured in the Clayburn plant. The bulk density of these bricks i s approximately 1.3 g/cc which i s about twice as dense as the diatomite secimens made in the laboratory and tested in this project. Whereas the laboratory specimens were made from one ingredient, "diatomite", the commercial brick i s made from mixtures of diatomite, sawdust and fireclay. Table XIII shows the physical properties and the chemical analysis of this refractory insulating brick. A large number of unfired bricks of the size 9 x 4 1/2 x 2 1/2-in (229 x 114 x 64-mm) were collected from a production run. After being dried in a laboratory oven, the bricks were stored in a dry place. TABLE XIII Properties of Commercial Diatomite Bricks (from published technical data of Clayburn Ref. Ltd.) Bulk Density 1.20-1.35 g/cm3 Cold Crushing Strength 176-282 kg/cm2 Thermal Conductivity o Mean Temp. (120 C) 0.37 W/mK (260°C) 0.41 (540 C) 0.51 (820°C) 0.60 Chemical Analysis Si0 2 67.1% A1 20 3 20.7 Fe 20 3 6.8 Ti0 2 1.2 CaO 1.1 MgO 1.1 L.O.I. 0.2 109 The dry densities of the bricks were found to be 85.2 +0.7 p.c.f. (1.365 + 0.011 g/cc). These bricks were then fired in a laboratory electric o furnace at the same heating rate (5 C/min) and over the same temperature o range (between 200 and 1050 C) as done before with the laboratory diatomite specimens. In this case however, when the f i r i n g temperature 0 o was higher than 650 C, the furnace temperature was held at 650 C overnight (16 hours) to oxidize the organic materials in these bricks. After f i r i n g , the following properties were determined: (1) shrinkage, (2) bulk density, (3) thermal conductivity and (4) strength. 5.1 Results and Discussion on Shrinkage and Density of  Commercial Diatomite Bricks 5.1.1 Experimental R e s u l t s of Shrinkage and D e n s i t y on Commercial Diatomite Bricks The results of the shrinkage and the bulk density are plotted against their f i r i n g temperatures in Figures 40 & 41. As shown in Figure o 41, the bulk density dropped significantly between 200 and 500 C. This was mainly due to the loss of moisture and hydrocarbons from sawdust and diatomite. •06 •OA CD cn O p / / 55 / / / / / — / 9 / / 1 _ / P / f — — " <v — _ — — V o - ~" ~~ 1 1 1 1 1 1 1 1 1 1 2 3 4 5 6 7 8 9 10 *100 Temp, of heat tretment (°c ) F i g u r e 40 The e x p e r i m e n t a l s h r i n k a g e d a t a of commercial d i a t o m i t e b r i c k s . 2 3 4 5 6 7 8 9 10 *100 Temp, of heat treatment (°c) F i g u r e 41 The e x p e r i m e n t a l b u l k d e n s i t y d a t a of commercial d i a t o m i t e b r i c k s . 112 5.1.2 Testing of shrinkage and Density Equations with Commercial Diatomite Bricks The experimental data on shrinkage, weight loss and bulk density can be tested with Eq. 4.4.8 by p l o t t i n g AL/L versus [ l - (p., Ip-) O BO B ( W / W q)]. This i s done in Figure 42. According to the model, the slope of the line should be 1/3. Experimentally i t has been found to be 0.334 confirming the validity of the equation. 5.2 Results and Discussion on Thermal Conductivity of Commercial Diatomite Bricks 5.2.1 Experimental Results of Thermal Conductivity on Commercial Diatomite Bricks The thermal conductivity data of the commercial diatomite bricks are plotted against their f i r i n g temperatures in Figure 43. The drop in o the thermal conductivity value up to 600 C on heating corresponds to the drop of the bulk density in the same temperature range due to the mass loss from sawdust and diatomite in the raw material mixtures. 5.2.2 Testing of the Thermal Conductivity Equations with Commercial Diatomite Bricks The thermal conductivity equation 4.5.6 as derived previously Is given by: 113 ( «]_ i°B0W PB WO F i g u r e 42 E v a l u a t i o n o f the s h r i n k a g e e q u a t i o n u s i n g the e x p e r i m e n t a l d a t a of commercial d i a t o m i t e b r i c k s . o - . 0 5 -4—• > / / / / §•041- / o o ^ / o Jc -03- P 2 3 4 . 5 6 7 8 9 10 x100 Temp, of heat tretment (°c ) F i g u r e 43 The e x p e r i m e n t a l thermal c o n d u c t i v i t y d a t a of commercial d i a t o m i t e b r i c k s . 115 2/3 K = N a k ( ( — s v p (A.5.6) In order to plot this equation, the value of k g and the type of packing (which gives the values of N and a) must be known. It is possible to assume that the value of k g used previously for laboratory diatomite specimens may also be applied in the case of commercial diatomite bricks, as chemically and mineralogically they are very similar. On the other hand, i t i s not possible to determine the nature of the packing for commercial diatomite bricks, from which N and a can be calculated, because the packing of particles in this system must be quite complex consisting of a heterogeneous mixture of different materials with different sizes. For these reasons, no attempt has been made to calculate the thermal conductivity values using Equation A.5.6. The conductivity data can however, s t i l l be tested with Equation A.5.6 using the experimental bulk density and weight loss data, assuming a l l factors outside the bracket are constant. The experimental values are plotted in Figure AA. As can be seen in Figure AA, the distribution of the experimental results of commercial diatomite bricks i s almost indentical to that of laboratory diatomite specimens. Since N and a are almost constant for the temperature range of the tests, the increase in the slope for the specimen fired at 1050 C must be due to the increase of the k value because of the s formation of cristobalite. It appears that the identical behavior of o •OA > B 0 W J F i g u r e 44 E v a l u a t i o n of the t h e r m a l c o n d u c t i v i t y e q u a t i o n u s i n g t h e e x p e r i m e n t a l d a t a of commercial d i a t o m i t e b r i c k s . 117 thermal conductivity on f i r i n g , for both the pure diatomite specimens and the commercial diatomite bricks, suggests that the sintering behaviour, the crystallization and the heat transfer mechanism for both systems are controlled mainly by the diatomite phase i t s e l f . 5.3 Results and Discussion on Strength of Commercial  Diatomite Bricks 5.3.1 Experimental Results of Strength on Commercial The Compressive strengths of commercial diatomite bricks after f i r i n g at elevated temperatures were determined by the procedures described in previous sections. The results of the strength of these bricks are plotted in Figure 45. Similar to the strength of the laboratory diatomite specimens, the strength of the commercial diatomite b r i c k s reduced s l i g h t l y on heating between 200 and 500 C but then o o increased at about 600 C and especially above 800 C. 5.3.2 Testing of Strength Equations with Commercial Diatomite Bricks The strength and the density equation has been correlated by the following equation for pure diatomite specimens. Diatomite Bricks o S = C i ( PBo W 2/3 ) - 1) + S (4.6.7) o 300L E 200 gioo ID 3 4 5 6 7 Temp, of heat treatment ( c ) 8 10 *100 oo F i g u r e 45 The e x p e r i m e n t a l s t r e n g t h d a t a o f commercial d i a t o m i t e b r i c k s , 119 To test the applicability of this equation to the strength data of the commercial diatomite bricks, the strength was plotted as a function of "X" in Figure 46, where Pp W . PBo - 1 The values of C and S q in Eq. 4.6.7 can be determined respectively from the slope, and the intercept of the line in Figure 46. The values were 3 2 2 calculated to be 1.95 x 10 kg/cm and 87 kg/cm respectively, with a correlation coefficient of 0.99. The corresponding values for pure 2 2 2 diatomite specimens are 4.65 x 10 kg/cm and 23 kg/cm respectively, reflecting that the strengths of the commercial diatomite bricks are higher than those of the pure diatomite specimens. This was expected as there was no extra binder phase present in the laboratory prepared diato-mite specimens. This study shows that the properties of commercial diatomite bricks are primarily controlled by the diatomite phase in the system. Normally, addition of a binder phase to the brick to increase the strength of brick w i l l also increase the thermal conductivity. However, i t appears that there may be s t i l l some possibility to modify the raw material composi-tions so that the thermal conductivity may not Increase In direct proportion to the Increase in strength. 300 o ^ - 1050 CM §200 - " 1000 Strength (kg o 700 800 r ' 600 °^  900 '850 ^ o - 950 1 1 1 1 I X = •04 / PB WO i 1 ft»W J •08 •12 F i g u r e 46 E v a l u a t i o n of the s t r e n g t h e q u a t i o n u s i n g t h e e x p e r i m e n t a l d a t a of commercial d i a t o m i t e b r i c k s . 121 SUMMARY AND CONCLUSIONS The linear dimensional change, density, thermal conductivity and strength of diatomite compacts were determined after heating specimens in o the temperature range 200 to 1050 C. The mineral constituents in these heat treated specimens were also determined by the X-ray diffraction method, and the true density measurements were carried out using the pycnometric method. The data from these experiments was analysed using a model of sintering which was developed on the basis of observations of microstructural change in diatomite particles on heating. The following conclusions were made: (1) The experiments on diatomite compacts were not affected by the laboratory treatments of grinding and compacting. The mechanical compaction of diatomite only increased the starting density and did not affect the rate of sintering. (2) Quesnel diatomite transforms from amorphous s i l i c a to cristobalite o above 950 C. This was determined by X-ray diffraction. (3) The linear dimensional change and the bulk density of diatomite specimens remained almost constant during the heat treatment of the o o specimen, up to 600 C, but increased slightly between 600 and 800 C and o more d r a s t i c a l l y above 800 C. To interpret the data of the change in the linear dimension and bulk density of diatomite compacts, a shrinkage equation has been developed using a sintering model, in which i t i s 122 assumed that after decomposition ( i . e . weight loss) each diatomite particle is composed of a large number of microspheres. It has been shown from this model that the shrinkage and bulk density of diatomite compacts can be related by: AL = 1_ r pBo W^ L 3 1 p n W ' o KB o The vali d i t y of this equation was tested with experimental data and i t has been shown that the shrinkage can be predicted with reasonable accuracy from the weight loss and bulk density data. (4) In the case of thermal conductivity, the conductivity value f i r s t o decreased s l i g h t l y on f i r i n g up to 600 C and then increased above this temperature. The thermal conductivity of diatomite compacts can be correlated to the bulk density and weight loss by the sintering equation, K = N a k [ ( a °) - 1 ] + K s L p_ W J o KBo This has been developed by modifying Ramanan's equation for the e l e c t r i c a l conductivity of powder compacts. This equation shows good agreement with the experimental data when the value of N and a correspond to the single c u b i c packing. The value of k g appears to change on heating due to cristobalite formation from amorphous s i l i c a in the diatomite, specially o above 950 C. 123 (5) The strength of diatomite compacts reduced slightly after heating o o specimens to between 200 and 400 C, but increased above 600 C. The strength of diatomite compacts also can be correlated to the bulk density and weight loss by an equation similar to the conductivity equation. W 2 / 3 s = c [(-^ -4) - 1] + s P B o w' This equation also showed good agreement with the experimental data for o specimens heat treated below 950 C. The experimental values of strength were significantly lower than the predicted values for specimens heat o treated above 1000 C. This may be due to the microcrack formation during the crystallization of amorphous s i l i c a . (6) Finally to test the general applicability of the equations developed in this research programme, attempts were made to apply these equations to the data obtained from a commercial brand of diatomite insulating refractories. The linear dimensional change, bulk density, thermal conductivity and strength of these bricks, determined after heat-o ing bricks i n the temperature range 200 to 1050 C, were tested with the equations developed for pure diatomite specimens. Good agreement was obtained between the strength data and equation, because most probably the added clay phase prevented crystallization of the amorphous s i l i c a phase Into cristobalite. In the case of~"the thermal conductivity data, crystal-l i z a t i o n of the clay fraction into mullite most probably caused the o disagreement between the predicted value and experimental data at 1050 C. 124 REFERENCES 1. J.H. Chesters, Steelplant Refractories, the United Steel Co., Sheffield, P. 279, (1957). 2. Taikabutsu Techo (Refractories Hand Book), ed. A. Wakabayashi, et a l . , The Technical Association of Refractories, Japan, P. 66, (1976). 3. Industrial Minerals and Rocks, ed. J.L. Gillson et a l . , The American Institute of Mining, Metallurgical and Petroleum Engineers, PP. 305-306, (1960). 4. L.R. Barrett, "Heat Transfer in Refractory Insulating Materials", Trans. B r i t . Ceram. Soc, 48_, P. 235, (1949). 5. E.L. Calacal, "Sintering Characteristics of Diatomite", Ph.D. Thesis, Univ. of Washington, Seattle, (1980). 6. G.C. Kuczynski, "Sintering and Related Phenomena", Material Science  Research, Vol. 6, P. 217, (1973). 7. T.H. Blakeley and J.W. Cobb, J. Soc. Chem. Ind., 51_, 83T-89T, (1931). 8. E. G r i f f i t h and A.H. Davis, "The Transmission of Heat by Radiation and Convection", Special Report No. 9, Food Investigation Board, British Dept. of Sci. and Ind. Res., (1922). 9. "Test Method for Porosity and Density of Insulating Refractory Brick", Japan Industrial Standard, JIS R2614, (1976). 10. "Standard Test Method for True Specific Gravity of Refractory Materials by Water Immersion", ASTM C135-66, 1980 Annual Book of ASTM Standard, Part 17, (1980). 11. O.J. Whittemore Jr., C. Ayala and J.H. Castro, "Characterization of Cellular Ceramic Materials", Ceramica, 2_5, P. 116 (1979). 12. Taikabutsu Techo (Refractories Hand Book), ed. A. Wakabayashi, et a l . , The Technical Association of Refractories, Japan, P. 10 (1976). 13. Industrial Minerals and Rocks, ed. J.L. Gillson et a l . , The American Institute of Mining, Metallurgical and Petroleum Engineers, P. 389, (1960). 14. Y. Shiraki, Ceramics Gairon Vol. 2, P. 175, (1963). 15. "Alpha Quartz", JCPDS diffraction data card, 5-0490. 16. T. Tokuda, "The X-ray Powder Patterns and the Lattice Constants of Natural Cristobalites", Mineral J. 3, P. 3, (1960). 125 17. "Tridymite (Low Form)", JCPDS diffraction data card, 3-0227. 18. T. Sudo, Nendo Kobutsu (Clay~ Minerals), 5th ed., PP. 3A-39, (1959). 19. E.L. Calacal, "Sintering Characteristics of Diatomite", Ph.D. Thesis, Univ. of Washington, Seattle, P. 71, (1980). 20. D.L. Johnson and I.B. Cutler, " I n i t i a l Stage Sintering Models and Their Application to Shrinkage of Powder Compacts", J. Am. Ceram. Soc. 46, P. 541, (1963). 21. E.M.H. Sallam and A.CD. Chaklader, "Sintering Characteristics of Porcelain", Ceramurgia International, 4_, P. 151, (1978). 22. H. Oliver and J.S. Rigby, Trans. Brit. Ceram. Soc, 40_, P. 335, (1941). 23. J. Frenkel, "Viscous Flow of Crystalline Bodies Under the Action of Surface Tension", J. of Physics (U.S.S.R.), 9_y P. 387, (1945), (in English). 24. R.T. DeHoff, R.A. Rummel and F.N. Rhines, "The Role of Interparticle Contact in Sintering", Powder Metallurgy, ed. W. Leszynski, P. 31, (1961). 25. A.K. Kakar, Ph.D. Thesis, Univ. of B.C., (1969). 26. T. Ramanan, M.A.Sc. Thesis, Univ. of B.C., (1970). 27. E. Ruh and A.L. Renkey, "Thermal Conductivity of Refractory Cast-ables", J. Am. Ceram. Soc, 46_, P. 89, (1963). 28. A. Eucken, "Thermal Conductivity of Ceramic Refractory Materials", Farsch GEbiete Ingenieurw., B 3_, Forschungshef t No. 353, P. 16, (1932). 29. H.W. Russell, "Principle of Heat Flow in Porous Insulators", J. Am. Ceram. Soc, 18_, P. 1, (1935). 30. A.L. Loeb, "A Theory of Thermal Conductivity of Porous Materials", J. Am. Ceram. Soc, 37_, P. 96, (1954). 31. P. Wagner, J.A. O'Rourke and P.E. Armstrong, "Porosity Effects in Polycrystalline Graphite", J. Am. Ceram. Soc. 55, P. 214, (1972). 32. S.K. Rhee, "Discussion of Porosity Effects in Polycrystalline Graphite", J. Am. Ceram. Soc, 55_, P. 580, (1972). 33. W.D. Kingery and M.C McQuarrie, "Thermal Conductivity:I", J. Am. Ceram. Soc, 3_7_, P. 67, (1954). 34. A.CD. Chaklader and A.L. Roberts, "Effect of the Devitrifying of Sili c a Glass", Trans. B r i t . Ceram. Soc, 56_, P. 331, (1957). 126 35. F.E. Wagstaff, S.D. Brown and I.B. Cutler, "The Influence of H20 and O2 Atmospheres on the C r y s t a l l i z a t i o n of Vitreous S i l i c a " , Physics and Chemistry of Glasses, Vol. 5, P. 76, (1964). 36. A.E. Dodd, "Cristobalite", Dictionary of Ceramics, George Newnes Ltd. London, P. 73, (1964). 37. Y. Shiraki, Ceramics Gairon Vol. 2, P. 176, (1963). 127 APPENDIX The Results of the Experiments Carried out by E.L. Calacal 128 TABLE 1 Sintering Stages of Qulncy I Diatomite Firs t Stage: Pores Coalesce: The surface area by Hg porosimetry increases, pore diameter and total pore volume increase or may remain constant. Temp. (°C) Time (hrs) Surface Area Hg BET (m2/g) Mid Pore Diameter (urn) Total Pore Volume (cc/g) Raw - 9.9 21.4 0.23 0.384 700 2 10.1 19.7 0.24 0.420 - 4 12.7 18.5 0.24 0.421 900 0.25 10.6 12.0 0.25 0.410 - 0.50 11.7 11.9 0.27 0.443 1000 0.25 11.6 11.6 0.24 0.427 1100 0.5 5.4 9.7 0.37 0.374 1.0 6.1 8.8 0.40 0.423 129 TABLE i i Sintering Stages of Quincy I Diatomite Second Stage: The surface area increases. Total pore volume and mid pore diameter decrease. Temp. <°C) Time (hrs) Surface Area Hg BET (m2/g) Mid Pore Diameter (um) Total Pore Volume (cc/g) 700 4 12.7 18.5 0.24 0.421 - 8 13.2 17.8 0.21 0.419 900 0.5 11.7 11.9 0.27 0.443 - 1 13.0 11.6 0.24 0.418 1000 0.5 6.3 10.1 0.34 0.394 - 1 6.4 9.3 0.34 0.394 1100 2 6.5 8.0 0.35 0.401 130 TABLE 111 Sintering Stages of Qulncy I Diatomite Third Stage: The Surface area and total pore volume decrease, mid pore diameter increases. Temp. C O Time (hrs) Surface Area Hg BET (m2/g) Mid Pore Diameter (^ m) Total Pore Volume (cc/g) 700 8 13.2 17.8 0.21 0.419 - 16 12.3 16.0 0.24 0.417 900 1 13.0 11.6 0.24 0.418 - 2 10.0 11.1 0.26 0.416 - 4 9.7 11.0 0.27 0.407 - 8 9.1 10.5 0.27 0.402 - 16 7.4 8.9 0.28 0.398 1000 2 6.9 8.4 0.27 0.394 - 4 6.7 7.8 0.27 0.390 8 6.2 7.1 0.31 0.371 131 TABLE iv Sintering Stages of Quincy I Diatomite Fourth Stage: The surface area and total pore volume decrease, mid pore diameter increases. Temp. (°C) Time (hrs) Surface Area Hg BET (m2/g) Mid Pore Diameter (lam) Total Pore Volume (cc/g) 1000 16 4.8 6.0 0.33 0.363 1100 8 5.1 5.2 0.40 3.385 1300 2 1.28 3.8 1.81 0.430 - 4 0.72 3.5 2.20 0.426 - 8 0.56 3.0 2.75 0.387 - 16 0.40 2.7 3.61 0.382 132 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 1 2 0 0 1 3 0 0 Temperature, °C F i g , i : SURFACE AREA BY Hg POROSIMETRY VS TEMPERATURE IN KEATING QUINCY I DIATOMITE AT VARIOUS TIME SCHEDULES. 133 o o 0) o > CD u o p - l i-H CD 4-> O ' 0 . 4 5 - 0 . 4 0 - 0 . 3 5 0 it _L 7 0 0 8 0 0 1 0 0 0 o , 1 1 0 0 1 2 0 0 9 0 0 Temperature, °C F i g . i i : PORE VOLUME VS TEMPERATURE IN HEATING QUINCY I DIATOMITE AT VARIOUS TIME SCHEDULES 134 Temperature, °C F i g . i i i : MIDPORE DIAMETER VS TIME IN SINTERING QUINCY I DIATOMITE 

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