Wollastonite (CaSiO3), một nguyên liệu công nghiệp quan trọng, đã được cố gắng tổng hợp từ các oxit thành phần của nó có nghĩa là CaO và SiO2. Bên cạnh tầm quan trọng công nghiệp. Tổng hợp này là mộtchomed trên tài khoản không đủ sẵn có của wollastonite trong tự nhiên chotôi trấu và đá vôi được sử dụng làm nguồn nguyên liệu cho các oxit nói. Dồi dào có sẵn vỏ rbăng đã được pyroprocessed theo các điều kiện nhiệt độ thời gian để có được tro với tỷ lệ tối đa của silica, có tro trấu nhân vật gạo vô định hình chiếm ưu thế. Với sản lượng silica cao nhất tức là 92,01%, được tinh chế bằng phương pháp lý hóa khác nhau để loại bỏ các tạp chất kim loại và carbon còn sót lại. Vì vậy, chuẩn bị silica, được 98,50% tinh khiết, đã phản ứng với CaO từ đá vôi. Hai tuyến đường đã được thông qua để tổng hợp wollastonite. Trong lần đầu tiên, CaO thu được bằng cách nung đá vôi thích hợp là liên hợp với silica, hỗn hợp bột và thủy nhiệt được điều trị trong nồi hấp dưới hơi nước pressure từ 30 đến 80 thanh thanh, trong một giờ. Kết quả là các hợp chất liên Mediate tức là hydrat canxi silicat. đã được nghiên cứu sử dụng phân tích hóa học, SEM và nhiễu xạ tia X kỹ thuật. Thủy nhiệt chạy, được thực hiện dưới áp lực 50 và 70 thanh hơi nước. dẫn đến hình thành các xonotlite và tobermorite tương ứng. Đây, là cấu trúc liên quan đến wollastonite được nung ở 950OC ba giờs, để loại bỏ các phân tử nước kết hợp hóa học. Thứ hai. chuẩn bị silica và đá vôi bột được trộn lẫn trong 1: 1 tỉ lệ mol và mineralizer có kính như thành phần được thêm vào. Tổng hợp trạng thái rắn trực tiếp đã được thực hiện bằng cách nung nóng hỗn hợp lô nguyên liệu tại Các sản phẩm thu được thông qua cả hai. thủy nhiệt và các tuyến đường trạng thái rắn. được đánh giá và so sánh lẫn nhau như chúng ta 11. Wollastonite kết quả từ quá trình thủy nhiệt cho thấy năng suất tương đối thấp và không cũng như tinh thể từ các tuyến đường sau này. Lô thiêu kết ở 1200oC và 1300oC sản xuất tỷ lệ phần trăm cao nhất của βwollastonite và αwollastonite tương ứng. Tỷ lệ phản ứng trạng thái rắn được tìm thấy tỷ lệ thuận với tăng nhiệt độ. Phản ứng gạo tro trấu silica đối với CaO cũng được coi là để so sánh nó với những thường có sẵn nhiều tức là thạch anh. Nghiên cứu động học hóa học của wollastonite sản xuất cho thấy năng lượng yêu cầu kích hoạt là 27,48 KCal.mole1. Trong khi mặt khác. giá trị này để tổng hợp tương tự, sử dụng thạch anh thay vì gạo tro trấu silica. lên tới 78 KCal.mole1. Do đó, mẫu vô định hình trấu SiO2 được chứng minh là rất nhiều lần phản ứng hơn so với nhiều tinh thể giống nhau. Điều này cũng được thể hiện bởi các kết quả liên quan đến đánh giá của CSH tổng hợp. Sử dụng wollastonite tổng hợp. trong các cơ quan sứ. cũng đã cố gắng. Cả hai, tự nhiên cũng như tổng hợp wollastonite đã được thêm vào một cách riêng biệt (55%) với các thành phần hàng loạt khác với tỷ lệ cố định và các cơ quan liệu đã được bắn trong điều kiện giống hệt nhau. Công nghệ gạch nung nhanh tiết kiệm cả thời gian và năng lượng cùng một lúc. Nó đã được quan sát trong vấn đề này là cơ quan sản xuất gạch trưởng thành trong vòng hai tiếng đồng hồ thay vì mười hai giờ. thời gian cần thiết cho gạch mà không wollastonite. Tiết kiệm đáng kể thời gian và năng lượng nhiệt có thể được thực hiện trong sản xuất gạch bằng cách sử dụng wollastonite. Tính chất vật lý tức là nghiền sức mạnh. ngang sức bền kéo đứt và hấp thụ nước vv của các cơ quan đã được xác định và tìm thấy so sánh.
Trang 1TE FROM THE
REECE HUSK ASH AND LIMESTONE
& THESIS PRESENTED TO
UNIVERSITY OF THE PUNJSB IN
FULFILMENT OF THE REQUIREMENTS
FOR THE
DOCTOR QF PHILOSOPHY
IN CHEMISTRY
MUHAMMAD SHARIF NIZAML
INSTITUTE OF CHEMISTRY, LINIVERSITY OF THE PUNUAS LAHGRE (PAKISTAN)
Trang 3Wollastanite material,
5
(Ca5i0x4), an important industrial hag been attempted to synth
oxides i.e CaQ and Sins this synthesis WAS Ss pertormed
plilityvy of wollastonite
Here
in natural used a5 source materials for availaoie rice husk was
perature conditions ta abtain silica, VgGhest ca yield 4 #hysico-echemical 92,017, methods ‘feSidual carban T ‘eacted hus gregared wa th Cad from limestone wrthesize wollastonite rabion oof In first, limestone ulverized
W3ad intermixed with
ADOUr pressure from 30 bars
ulting
bare
intermediate compounds i.@e
ere investigated employing chemical
iques, Hydrothermal rugs, apour pressure, carried out resukted in These calcined at peing structurally 980°C for three hours, On pyroprocessed unde ash with having dominant amorphous character, sith to @liminate ¢ eilica, Two routes
Cad obtained by proper caici-~
Trang 4namically comtaned water molecules Secondly, prenared Sylica
1©@ Powders were mixed am isi malar ratio and a ominer-
lizer having glass like composition was added, Direct solid
tate synthesis was attempted by heating the raw mixed hatches at
to 1800", The products obtained through bath, hydrather-~
ai and solid state routes, were eviuated and mutually camparced
S weil, Wollastonite resulting from hydrothermal process showed omoaratively lows yield and was mot as well crystalline as that
com the latter route Batches sintered at 12007R and TXOGĐE roduced the highest percentage at £5 -wollastonite anden® ~wollas~ onite respectively The rate of solid state reaction was found roportional toa rise in temperature Reactivity of rice husk agh ilica towards CaQ was aiso considered for its comparison with nat oof the commonly availadle variety 1.@ quartz Chemcial
inetic Study of produced wollastaonite showed that required
nergy of activation was 27.48 KCal.moleg”Ì, Mhile on the other and, this value far the same synthesis, utilizing quartz instead f rice husk ash silica, amounted to 78 KCal,male 1, Consequent-
/, amorghous form of rice husk Bids peoved to be many times more
*active than crystalline variety of the same, This was alsa sow by the results regarding evaluation of synthesized CSH
xi lizatíon of synthesized wollastonite., in ceramic tile bodies
as alsa tried Both, natural ae well as synthetic wollastonite re separately added (35%) to the other batch components with
ved praportions and rae bodies were fired under identical mditions Fast firing tile technology saves both time and f@rgy simultang@ously rt was observed in this regard that
>
Trang 5using wollagstonite
o
ime reguired for ti
316 Saving in time and heat energy
Poysical properties hae,
traneverse breaking strength and water absorp
oOo Les were determined and found comparable,
Lon
Trang 6
oO z z ia =
1 i
1 express my gincere gratitude to Prof Dr 1, Zafar qhai (Tmagha-ereimtlaz), M,5GE, (Punjab), Ph.D (Bristol), Direc- or f{astitue of Chemistry, University of the Punjab, Lahore,
akistan, for his able guidance and supervision His encourage
ant and willingness to help me throughout the caurs# Of present
tudy is really commendable
foam highly thankful to Engr Br M Khalid Farooq, -Se (Punjab), Ph.D {(Clausthal)., Chief be Scientific Officer and pad, Glass and Ceramics Rresearch Centre, Pakistan Council af
caentific and industrial Research Laboratories Complex, Lahore,
10 encouraged and practically guided me for carrying out this ark, His Grofessional experience and valuable guidance have ade this study 9 real success
Trang 7I feel mpieasure to thank Or M, A, Shah PH.D (Elief
eldi, Geatar Scientific Officer, Glass and ramic
1E re, PCSIR Laboratories Comolex, Lahore, who critically tre £ tạ @ the manuscript of the thesis and made several valuable qgestions in this regard { also appreciate the sincer: eration fram all of my collegues, during the course of this rke
Last tut mot least the services of Mr Naeem Akhter gd Mr Zafarcullah Khan for typing and computer comoosing of the
biect matter with skill and sincerity, are duly appreciated
Trang 8
sure 2uUre mre jure jure ure Pe pure (Ure Hire ure Ure HUT Ure 5 h b i 4,8: 4,17: DTA af Rice Husk TG of Rice Husk $id, Content of RHA at SOOVE for Different Time Durations XRD Data (Cu Kot) on RHA Prenared at 300°C far 8 ours XRD Data (Cu Koo) om RHA (£).2703) S50°C for 4 hours XRD Data (Cu Kog) on RRA (823.214), 6009C for 4 hours improvement in Sif Content of Acid Digested RHA with Time ARD Pattern (Cu Ken) of Hydrathermally Trested Batch HB-2 (40 Bars, 250°C) XRD Pattern Cu Ket} of Hydrothermaily Treated Batch HB-3 (50 Bars, 260°C) XRD Pattern (Cu Kee} of Hydrothermally Treated Batch HB-4 (60 Bars, 270°C) KFRD Pattern {Cu Kee} af Hydrothermally Treated Batch H8-5 (70 Bars, 280°C) ARD Pattern (Cu Koc } Hydrothermallv Teated Batch HB-& (80 Bars, 270°C)
Compounds in the System Cadl-Sids
Scanning Electron Microgranh of HB-3 Batch Froduct Scanning Electron Micrograph of HB-3 Batch Products at Higher Magnitficatian
Scanning Electron Micrograph of HB-S Batch Product
Seanning Electron Micrograph of HHA-S Batch Product
at Higher Magnification
Trang 9igure igure igure igure igure igure gure igure igure igure igure igure igure igure 4,19) 4,ã1¡ Pattern (Cu Ke} of Wollastonite from HR-S Product
Thermal Decomposition of Calos
P hase Enuii Libr lum Oiagram of the SiO;-CaD 5ystem 2 Solid - Golid Reaction Between CaO and Sid, under Diffusion Model
Scnematic Representation of Nuclei Growth Model Plot for Diffusion Mechanism ({(Jander'’s
Method}
Analysis
Arrhenius Plot for Ditfusion Mechanism Q@rrhenius Plat far Phase Boundry Mechanism Arrhentus Pilot for Phase Boundry Mechanism Arrhenius Plot for Nuciei Growth Mechanism Plot for Nuclei Growth Mechanism
Progress of Preaduct Formation (2!) with Time under Nucleation and Growth Process
Different Calcium Gilicates Formatian
Solid State Reaction at 1200°C through
XRD Data (Cu Keg} on Batch RB-1 (Gee) XRD Data (Cu Keg} om Batch RB-2 (Looe?c} XRD Data (Cu Kee} on Batch RB-3 (41007)
XRD Data (Cu Koc} on Batch RB~4 (1 200°C)
ARD Data (Cu ke} on Batch AB-S (1300°%C)
Trang 10CHAPTER Ls LA ¬ „1,4,1 Rice Husk Rice Husk Agh
Scope of the Study LITERATURE REVIEW RHA Preparation RHA Utilization Synthesis of Wollastonite Selid State Methods Hydrothermal Methods Special Method Utilization of Wollastonite
EXPER LTOENTAL WORK
Materials and Methods Chemicals
Equipment and Apparatus Starting Materiales
Techniques Apnolied
Powder £-Ray Diffraction Scanning Electron Micrascopy
Trang 11
bo i.dua 4.4 2.1 2.1,8 -2.4.b 2.1.0 2.2, 2.2.8 (2.2.6 -2.0 2.2.0 2.49, 2.4 2.4.8 2.4.6 2.5 2.8 2.7 2.8 2.8.a 2.8.b 2.8 2.9.8 2.9.5 2.10 Atomic Absorption Spectroscopy Wet Chemi al Analysis Methods Procedures
Preparation of REA Silica re-Treatment of Rice Husk RHA Content
Pyroprocessing of Rice Husk Purification of RHA Silica TrentmentE with an Oxidant Alkali Leaching
Digestion in Dilute Hel Dry Thermal Treatment
RHA Quality Evaluation
Hydrothermal Synthesis of Calcium Silicate Hydrates Reaetion in Autoclave Product Processing Investigations on Synthesized CSH Preperation of Wollastonite from Synthesized Intermediates Preparation of Mineralizer for Solid State Synthesis Solid State Synthesis Study Batch Preparation Dry Thermal Treatment in Electric Furnace Product Identification
Quantitative Chemical Analysis Mineral Phase Analysis
Characterization of Synthetic #ollastonite
an
Trang 12Z.10,8 Colaur and Lustre 2.10.b Hardness
2.40.0 Gpeoific Gravity 2,.19.d Melting Point
2.10.e@ Reaction with Hydrochloric Acid
2.11 Wtiligation of Wollastonite in Tile Bocies 2.it.a Tile Bodies Using Synthetic Wollastonite Z.il.b Tile Bodies Using Natural Wollastenite
2.42 Physical Proserties of the Tile Bodies 2.12.8 Drying and Firing Shrinkage 2.12.5 Warpage 2.42.0 Apparent Porosity Z2.12.d ¥Water Absorption 2.12.e Bulk Density and Apparent Specific Gravity 2.12.£ Compressive Strength 2.12.@ Modulus of Rupture
APTER 4: RESULTS AND DISCUSSION 73
1 Pyroprocessing of Rice Husk 1,1 RHA Content
1.2 Rice Husk Ash Silica
Trang 13tr œ we »10,2 „ii „xà1,Ÿ} k4 đua „11, sổ K-Ray Điffraction lnvestigationes Comparative Study Morphology of CSH
Comparison of XRD and SEM Results
Mechanism of Hydrothermal Reaction
Chemically Combined Water Cantent of CSH Tobermorite xonotilite Woblastonite fram CSH Thermal Decomposition af CaCO, In-Situ Reaction Potential £ , Temperature = 4H Effect Phase Rule Consideratian Field Strength Kinetic Model of Solid State Reaction Diffusian Model
Nuclei Growth Model Phase Boundry Madel
Mechanism of Solid State Reaction
Trang 14pL 128 Physieo-Chem Hollastonite ical Characteristics of Synthetic 172 12.1 Colour 172 (12.2 Hardness 175 1.12.3 Chemical Composition 175 4.12.4 Meiting Point Le 4.12.5 Action of Hydrochioric Acid 180 4.12.8 Specific Gravity 384
4.13 Physieal Properties of Prepared Tile Bodies 181
Trang 15CHAPTER 1:
DRE RODUCTION
Wollastonite is a naturally coccuring ealeium metasilicate and in it’s pure form is denoted by the chemical formila, Ca5iGg(i} It is one of the single chain calcium silicates which falls under the clinopyroxene group of minerals and is petrofraphically represented as CaQ.Si02(2) Wollastonite is often formed in nature through chemical reaction between quartz and limestone C3) The reaction occurs under varying conditions of temperature and pressure at their contact zones, therefore, it’s different modifications e.8 e€-CaSiOg and B -CaSidy ete (4) are Found
Though deposits of natural wollastonite have been reported in the U.S.4&., Japan, Australia, India, China, Canada, Romania, Kazikistan, Kenya and Mexico ete., yet its ever ineressing demand in the vorld market is not being fulfilled (5) It is due to wide utilization of this raw material ain «a good number of industrial products such as ceramic wall tiles and glazes etc Wellastonite especially hes received much attention during the last decade and this Situation has tempted a number of research groups to produce
Trang 16
g._Hugk :
Rice husk (RH}, LÝ properlV burnt yields about 20% (average) agh which consists of 80-8âk 5i02 (7,9,8) RH silica is originally amorphous and therefore expected ta be chemically more resctive than ordinary erystailine form i.e
quartz (C10)
Agro-industrial wastes are abundant in Pakistan among which RH occupy important position Secondly it’s silica content is highest among all of them C11), Pakistan produces sbout 4 million tons of paddy every year From which 8.8 million tons of RH are separated as by-product Sueh huge amounts are not utilized properly and common practice is to burn out the rice husk which is wasteful exereise (12), Heap burning of BH causes pollution hazards for surrounding population and has disposal problems also
Present problematic situation, in this regard, is net moth different in other rice-growing countries €13, 14)
1.2 Rice Husk Ash:
Chemical composition of rice husk ash CRHAD depends upon a number of factors; type of soil Por growing rice plants, the fertilizing practices, environment, temper-
Trang 175 both, the pereentage o øilica and it mineralomical The heap burning method pr
duces poor guality RHA (ios Tt consists of large amount of unburnt carbon which lowers the sllica content Secondly, the original amorphous nature (16) of silica is destroyed and resulting ash is largely of erystalline character Consequentiv, it becomes compars~ tively less reactive regarding the anticipated chemical combination C17)
& Seape of the Study:
The present studies, in the sbove perspective, were carried cut mainly due ta the commercial significance
m wollastonite Consequently, the first and foremost aim igs to synthesize the said mineral by making anticipated use of silica prepared from RH In fact, it is an attempt to improve upon the involved hydrothermal and solid state (S5! reactions which hed been frequently performed by using
common source of 510 Ì.e, Quartz ete
Trang 18eercentage and highest silica content Similarly optimun lor burning temperature and avoiding unnecessary prolonged
heat treatment protects the amorphous character of silica All this requires the judicious selection of RH pyropro- oessng parameters and their mathodical impismantation under controlled conditions
The utilization of RHA for various purposes de- pends on the amorphous or crystalline nature of silica present in it Temperature, environment and time play vital role to obtain specific nature af silica in RHA The chemi- cal composition may also depend on the source of RH and set of experimental conditions Generally, a considerable rumber of impurities, in addition to residual carbon, accon- pany the buik amount of RHA silica Commonly, these are aluminium, iron, titanium, phosphorus, calcium, magnesiun,
sodium and potassium (18> If residual (retained) carbon
and these impurities are eliminated, the silica content will
automatically increase thereby improving the quality of RHA However, such purifileation procedures should be adopted which must not affect the reactive nature of upgraded sili- ce Hence obtaining purified RHA silica with unaffected original amorphous nature is another important cbject of the present study
Trang 19
native limestone (for CAO) aid properly † (3
subsequently purified RHA (for SiO>s} were selected san ray materials for the planned synthesis Jb may be affected through tro routes: hydrothermal and solid state Some calcium silicate hydrates (CSH) e.g tobermorite ete, are structurally related to wollastonite (18) end can be
prepared from their constitutent materials i.e, Ca0 and 5i0 5, under hydrothermal conditions Such hydrothermally synthesized CSH using RHA silica are expected to sect as intermediates Por the Final product Similarly solid state reaction is another potentisl process Optimization of conditions i.e time, temperature and pressure ete for efficient reactions (hydrothermal and $$) is the most sig-
nificant aim of these studies
As target product has industrial and commercial importance, the time and energy saving aspect should be especially considered Consequently, this requirement set the objective to perform syntheses at lowest possible tem- perature and pressure and within possible minimum time
duration
Present study, im addition to said syntheses, includes the possible utilization of synthetie product Ceramic wall tiles, among several applications of wollasto-
a
nite ere its common and mejor use (68, 20), Such tiles with appropriate percentage of this raw material exhibit desira-
Trang 20prac-Eionl1 utility of a newly produoesd taw materisl to compare Í1 with a good quslity natkuralìy available variety Therefore, this study was also aimed at including both of the wollastno- nite varieties in ceramic tile bodies and to evaluate these
Trang 21CHAPTER 2:
LITERATURE REVIEW
2.4 RHA Preparation
The relative proportions of different silica polymorphs in RHA i.e quartz, tridymite ete depend not only upon the combustion temperature but also on the dura-~ tion for which RH has been burnt Mehta (21) obtained totally amorphous silica by keeping the temperature below 500% for prolonged periods and maintaining oxidizing condi- tions While, Yeoh et el.(22) on the other hand observed that ash remained still] amorphous at 800°C when this temper- ature was maintained for a period less than one hour He also noted that ash became crystalline when heated at 1ooo% For a time more than five minutes,
Trang 22The time-temperature relationsh in to degree of crystallinity, simultaneously influences the specifie surface i.e the surface area Con) occupied by one gram of a Solid converted to fine particles It i8 alse a parameter which closely relates to chemical resctivity of the ash Ankra (24) showed that the burning environment equally affects the surface erea, therefore it must also be considered for efficlent RH pyroprocessing Besides, he Studied the effect of chemical treatment and grinding of &H before preparing ach It vas proposed that cellulose and other combustibles should be burnt out without damaging the pore structure of silica-rich skeleton He further showed that if pyroprocessing oocurs in the range 450°C ~ 550°C the residual carbon, though amorphous in nature, could not be
removed on later thermal treatment
Ikram et al (253 prepared RHA containing 87% &
$105, tóc produce polverystselline silioon from it Acid
leached RH was heat treated at 300°C to 41200°C for four
hours and subjected to extensive XRD studies The results revealed that ash was amorphous below 800°C while its con- version to crystalline form commenced at S002C, Tridymite and © -quartz co-existed in comparable quantities at this
Trang 23Pitt (28.275 while developing a process for large seale burning of RH design
fs furnace inte whieh RH was sucked under the negative pressure maintained by an exhaust fan The hot gases from furnace, being mixed with ash, were taken to a boiler and finally separated by s3 muiticone pe
separator In addition, this process had the provision tos recover the heat produced by combustion of the husk
e: Similariv, Shah et al (28> conducted studies at
Ñ
pilot plant level and Fabricated a low-cost incinerator to produce RHA for cement manufacture Faetor of burning atmosphere was duly considered while designing and the RH combustion environment was controlled by varying the air Flow through @ central tube While on the other hand, Yeoh and co-workers (22) used @ modified Yamamoto paddy drier and carried out field studies to produce samorphous ash Howev- ex, their studies were centered round the feasibility of Mehtea-Pitt system The Rice Growers Co-operative Society (28) also developed a fluidized bed furnace or combustor in which two tons of RH/hour could be burnt to produce amor- phous RHA The heat generated during combustion was used to dry the citrus pulp
Trang 24HNCq at room temperature for three months The resulting white residue, after proper washing was heated at 400°C
six hours
2.2 RHA Utilization:
Basu et al (4L} obtained RHA by proper burning of husk and chlorinated its silica content to silicon tetra- ohloride, a raw material for silicon Theoretically, the direct reduction of 8105), with carbon to yield Si, could be
possible but temperature must be more than 2000°C
h
Acharya and co-vorkers (323, with above perSpe©~ tive, eonsidered the energy saving fsctor and easy avail- ability of reducing agent thoroughly and evolved @ process Consequentiy, direct reduction with magnesium powder was affected at 605 ~- 6 50°C While on the other hand, Amick
(33) concentrated his work on leaching RHA and established techniques to set optimum carbon : silica ratio, for direct veduction Bose et al (343, while following the process of Acharys, made several improvements and claimed to prepare
silicon with purity level 99.50%
Trang 25uti-lized RHA for extracting solar grade gilieon Depending upan the silica content of prepared RHA, 18:27 (Ng@:SiG.)
found as the most suitable ratio for reduetion Resulting silicon powder was leached with different acids and evaluat-
ed as 98.85% pure
Patel and co-vorkers (37) treated RH at tempera- tures up to 10009, For different time durations and attempt~ ed to retain the amount of RH carbon, necessary for carbo-~ thermic reduetion of accompanying Si0a Husk was also treated with varicus acids and bases separately This experimentation resulted in 99% pure silicon which was to be
utilised in solar cell production technology
Ordinary Portland cement (OPC) manufacture is energy intensive process RHA in this concern has alse been utilized to produce an alternate binding material When mixed with lime in certain ratic Ccammonly 1:2, lime : RHA), it finds wide application to produce conerete blocks, well- yings, Plooring and road sub-bases (38)
Trang 26
to construct floors of food and ° chemical process industries C27, 40% The desirable blacui
look in architectural coneretes is commonly produced by adding different pigments which are not durable as they faint on longer exposure to environment The use of BHA, instead of these pigments, has been reported to impart permanent blackish shade te such coneretes and safeguards sagainsh the adverse weather effects (C431)
Massive conerete structures containing reactive aggregates experience dangerous delayed expansion RRA, being a highly active pozzolanic material i.e showing ever increasing binding strength on coming in contact with water has been applied to control this unwanted situation It has been reported that 10% addition of RHA to OPC controlled delayed expansion upto 80% While, 25% of conventional |
pozzolana (burnt shale) was reguired to achieve the same results C21)
Trang 27resulting ash pulverized to specifie particle size and then employed for the said purpose
Finely ground amorphous silica and ‘carbon black’ are conventional rubber Fillers Raxo et al (44) ignited the hydrocarbon constituents of RH in such a gay that re- suiting agh contained minor amount of carbon in free state Thi W material, termed ss ‘black silica’ was compared with the cenventional fillers and found superior to amorphous
si]
Silica and eguivalent to carbon black
James and Rao (S$, 45) recentiy investigated the chemical reaction between CalOH 4 and RHA The used ash vas ebtained by heating washed and dried RK in muffle furnsce at 500, 990, 700, 960 and SOOOG for one to thirty hours RHA and lime were reacted For ome te seventy hours and BMG the products evaluated by various experimental techniquesine lud- ing chemical analysis and XRD It was concluded that reac- tivity of BHA towards lime entirely depended on ash preparae- tion temperature i.e low temperatures shoved higher reac- tivity index The surface ares of the ash exhibited ea similar trend,
2.3 Synthesis of Wollastonite:
Wallastonite is generally formed in nature under the condition of high temperature thermal metamorphism
the
Trang 28reorystalligation of previously formed rock materials with~- cut melting (46) Actually, when guartz and calcite come in conteset in presence of underground sigh pressure the former is saitered into stishovite Co-ordination number of silicon changes from 4 to 6 as a result and an open structure which is more reactive towards calcite, becomes available (47)
According ta Laney (483), if recks with ealecite ard quartz co-exist under 265600 bars pressure, the reaction would take place at 800°C and new mineral i.e wollastcnite, would be produced Similarly Goldschmidt (47) while ex- plaining the conditions regarding chemical reaction 2.1,
reported that temperature in a closed system (pressure, about 7FOOO bars) will ineresse from normal, thereby produc~-
ing wollastonite
CaCOg + 510g - › Ca5i0a + COs (2,1)
Trang 29Deer er al ¢50) indicated the possibility of wollastonite Formation in contact calcareous
silicon was metasomatically introduced aceording to the resetion 2.3, If, po la) aduced COp escaped through fissures thereby lowering the reaction pressure, the mineral might be formed at relatively lower temperature
During his work regarding the study of CaO-5105 system, Filippov (51) noted the existence of calcium sili- cate (Cs0.5109) among 2CaQ.5109 and 3CaQ0.5i10, phases Tn fact specific calcium silicate phases co-existed even with
unreacted raw materials i.e CaQ and $10
considerable amount was shown to depend upon CaQ:5105 ratic and temperature e.g f -CaSids ange ~Gasids alongwith tridy-
mite were Found around 1200°C and CaO:SiOa ratio, 48:52,
2.3.4 Solid State Methods:
Cristopher et a4 pet (32> obtained wollastonite intermixed with other minerals They used 3109 AlnO, 3Cad as araw material and got final product consisting of the required mineral and garnet In an other process,
Trang 30Foshagite is a erxleium silicate hydrate closely resembling to 8 ~vollastonite Sotually the structure resemblance of the both and elinz ination of water molecules
chemically combined with CaQ.Si09 units, resuited in the
: ae 2
said product Hamedov and Belov (33) converted it tof -
wollastonite at 750°C
Hikheilov (34) reported theoretical possibility of
getting CaSi0g at 152320, provided $i05 was present in
sufficient amount and effluent gas totally condensed He hydrolysed Caks by steam in the presence of 3102 €guartz) and produced Cal, CaSids, £ -Caos SiƯa, -Gaa S10 and Cđa 5 Ôn alongwith F and HF, in gaseous state
Muster C523 adopted solar methed to synthesize wollastonite He claimed that it was an excellent process for heating refractory oxides at elevated temperature with- out contamination Appropriate quantities of CaO and 5105 were mixed and Fired in solar furnace Chemical analysis and XRD technigues were employed for identification
Trang 31at the highest temperature of the £g)ven range Obtained products were jdentified by XRD and Formation of /-?2a0.5i02 alongwith wollastonite was observed in case af
CaO:Si0 above 1.2
Kakitani (57) attempted to sffect 5S reaction by bringing the reacting oxides together in the form of layers Thus a thin layer of SiQ was covered with another layer of CaO and both heated at 1590°C For three hours The resultE- ing preduct lsver was & mm thick and consisted of M-Cad Sid, -oristobalite and /5-quartz
Twamoto and Sudo (58) heated various clay minerals with CaCOa and NHạ€1, at differant temperatures, for one neor and identified the reaction products lt was observed that clay minerals, rich in Al yielded wollastonite contami- nated with larnite (Cao5iG,) It was also found that reac- tion eccured at higher temperature when BHyCl was exeluded
from the raw batches
Fahrenberger and Harkort (88) synthesized wollas- tonite using sililoious and calcareous residues of 81909 extraction from clays This reaction was affected with and without mineralizers and formation conditions were studied
Trang 32alkali-alominnsiti-cate with Ga(OH)2 at 80-SOSC Fe half an hour Res product, on dehyd
sbove L1C0°C, yielded p ~wollosto-
nite
Wuhrer C61) concentrated on SS synthesis and calcined 68-25% excessive silicic acid with lime i.e L:b.i?, CaQ:Si0s ratio, The vaw mixed batches, composed of 612% limestone and 39% quarte poxder, were heated in a rotary kiin at 1420°%C for i.8 hours Caleination of the compo- nents, in the hottest gone of the kiln was continued For 30 minutes The product was evaluated as 95% “X -woliasEonite
with 5% erigtcbalite i.e a polymorph of quarte
Fumico and Akira (62) developed a process for rather larger scale production by including a flux to lower the sintering temperature Consequentiy, & mixture composed of: Limestone, 0.449 parts; slaked lime, O.718 parts; silica 1.000 part and PbO.Al904.5105, 0.072 parts, was intermixed with water containing 0.5% dispersed wheat powder The raw
batch was granulated first and then Fired at 1370 obtained clinker was mainly cé€-wollastenite
Trang 33caw materials were fed inte the rotary Riln HIEHOUE compael-
ing end using no flux Kolisstonite with 4.5% free Si0Q> was produced by Sintering the sh j4edec, Its chemical compna- sition was: CaO, 45.12% and 810- $3.78%,
Wuhrer (64), , in another work, repleeced guick line by caustic lime and manufactured the
under consideration
rotary kibn by maintaining all of the above described conditions However, resulting wollastonite vas reported to show high ehiteness It was also found free of dicalcium silicate which is first formed and then converted eompletely or partially, into the metacalcium silicate
Saltivskaya et al (85) attempted the said synthe- sis by utilizing rae materials like chalk, marble, opoks, guarta sand and volesnic ash one by one under optimal forma- tion conditions Maximum yield (80%) was obtained from marble and opoka,
Joseob C66) centered his work on preparing wollas- tonite From some newer silica bearing raw materisis He, therefore, made the optional use of opal, kiesulguhr and @ther Forms of silica slongwith natural chalk Although quartz was used but in very small quantities Mineralizers (0.00-6.50%) Like; LiCl, LiF, LigCOg, NaF, NaCl or Na jCOg were also added to the starting materials The reported range of batch compositions was: caleite, S8-BO0X; opal,
Trang 34were caleined nE LIO0ÔG to Leak Sc for one to three hours and o -wollastonite eontaining 1o-154 impurities ng
Nippon Sheet Glass Co developed a process (87) for producing wollastonite from soda-lime-silica glass Mixed raw materials containing: 3195, 49,8; CaO, 39.7, 50a, 2.0; Bbie®, 8.5: Ala0s, 4.0; NaaO, 1.0 and R90, 3.0%, were melted in a platimun crucible at 1450°C for 4.45 hours and eooled slowly from 600°C downward Glass was heated at
(rate, 9.5% /min.3 900°C For three hours, cooled and pow-
dered Its one gram was soaked in 400 mL IMNaGH for 24 hours and pure wollastonite (84.8% yield) fibres were ob-
fained
Balkeyiech and co-workers (68) produced wollasto- nite from naturally occuring silicious limestone sithout treating by any other material A maximum content of wol- lastonite (85%), in the reaction products, was obtained by e@leining the said limestone at L10G°C High reactivity of limestone was asSigned to the high dispersivity and honege- nous distribution of CaO and Si0 9 within raw material and to
the amorphous natere of Sid) 2 Content of the low tempera~
3
ture wardety ¢ f) ~CaSid,) of obtained product was observed to decrease above 1100° This was noted due to its transi-
Trang 35Gal'’pernia et al (898)
in $105 : Cad molar ratio 4:41.77 and studied the preduct /8-dicnleium silicate was the phase detected first, however it was mot stable thermodyvnamically under the normal condi- tions Suggested mechanism was that: reaetion started at the surface of tripoli globules in presence of considerable excessive seamount of CaO Net result was the formation of fr-wollastonite Chang et al (70), who fired the similar
composition at 1450°C, reported that the grains of fj ~dival- Cium silicate were surrounded by an amorphous phase Final- ly, the /6-Pelymorph of the mineral under current considere-
tion wes obteined from this intermediate
The progress of wa S resetion between lime and Silica was also studied by Kurgeyk and Wuhrer (71) The appearance of fi -dicaleciom silicate, as initial phase, in a wide range of compositions, was explained in terms of dif- fernt bulk densities of the reactants The observation of Kurzeyvk and Wuhrer, regarding the initial formation of dicalicium silicate was duly supported by Hlavac (48), TẾ wae veported in this regard that 20a0.5i05 is first formed when CaO in silica refractory production batches, reacts with 810s of the quartzite However, this modification
later on is converted to wollastonite
Trang 36nite through S5 synthesis Thev investigated the expented intermediate pha
and while differing from several others, elaimed that reaction st i200° onecured without the Porma-
2
tion o we) -dicalcium silicate
i và
Hivan and Benedicto (73) prepared high purity om -wollastonite by mixing lime or limestone, silics
combustible solid Provision for recyeling the sintered mass with unreacted components was also made Freshiy mixed batches were transferred to & granolator where water was
admixed and obtained mass heat treated
Tbneg and his co-workers (74) followed the same reaction route as by Gal,pernia et al (69) but used three sets of reacting raw materials for producing / ~#o11asto-
nite, Major target of this study was to get required product by uliliging abundantly available diatomites Crystalline guartz and silics gel were the other source materials of S105 These were used for comparison with diatomites regerd-
Trang 37HydxoEharmalHetheds-
A&A procegs based on energy congervation point of view was patented in U.K No matter common batch materials (quatz and Guicklime) were used but heat evolved during slaking Lime was managed te initiate the reaction Steam and water were introduced to a closed autoclave conhaining the raw batehes A sludge-like mass of hydrated Casio, Wag collected dried and fired in a rotary kiln at 1200-1400°C it was then roasted for one to two hours to yield o¢ ~Casibs
C78)
Fumuda (76) attempted to utilize $105 ootained from waste gas of the Furnace for above mentioned synthesis
pe
Silica was mixed with CacOR) > in i:i ratio and hydrother - mally treated in an autoclave under 7-10 atmospheres and at 170-LBO°% for twenty hours The resulting stuff, being very Fine precipitate of Casicy was Separated and then calcined at & temperature higher than 700°C Heat stable and ex- tremely white A -fWollastonite was obtained as S 8 final product
Trang 38
gu cB nation of the intermed compound at 180Đ0- 11DĐĐ0 resEted in ©ervsE of ff -wallastonite,
Henvivy et al (785 synthesised wOllestonite through hydrothermal route by using spongolite Industrially produced CacQHu do was another starting material Molar ratio of CaQ and Si0y was kept constant i.e 1:1.1 Tha interns
ate resetion products i.e, C8O.$105,.Ho0, on opioining at 10ao - 1150°C for one hour, resulted in CaSiOg
Nellenthin eb al (79), prepared high qualltw £P ~Hollastonite from technical grade tobermorite group with
CaO:$i0, molar ratio: 0.87-
OO and AlgGg:3i09, 0.1-0.3
Raw materials were leached by 1-5% HC1, having solid to liquid ratio » 1:3 and the resulting residue dried at 60-
300°C It was then calcined in air at 850-1000°C for 6.25 to số
4 hours yielding the product contaminated with traces of gehlenite and larnite Thus £3 -wollssonite suitable for fast
firing ceramics was produced in bulk amounts
RHA, as mentioned earlier, hss been considerably used for manufacturing cements Chemical resection between Silica of RBA and added lime playsbasic role in this regerd
While studying this reaction in detail, Cook and Suwanvitaya (80) examined its progress under moisture, for prolonged périods i.e 3,7,28 and $O days They observed that result-
Trang 39They tried several ratios and investigated the morphoiogy of
ing products The strueture of CSH From high lime mixes consisted of poorly defined crystalline form However, well-recognized hexago-
nal plates of Cac OH do Were not observed
2.3.3 Special Method:
Feres et al (81), evolved a unique process for manufacturing the under review mineral Biomass was in- volved while carrying out the experimental work Natural siblicious limestone was ground and then roasted to affect reaetion between its CaO and 5iQ5 contents Reaction
clency was improved by treating the ground material with an agueous suspension of 10 -~ 2000% 104 eelis/gream of s live culture of tỉ ey a W iligate bacteria (Baccilus mucitaginosus) ‘strain 4’, at 10 - 35% The amount of added water was 12 ~ 50% and treatment prior to roasting was continued for 3 to
2.4 ULilization of |
ilastonite:
Trang 40promi-nent oharacteristics have fPrequen
3 For
manufacturing large number cf industrial products from it 5 g
Nen-cracking quịch hardening moulds (82), rein- Forced thermoplastic resins with improved strength (83) permesole ceramic moulds (84) end welding fluxes for st ee ~_
(85) have been produced utilizing wollastonite in one way or i the other, Lt is alse included as an important ingredient in the receipes of electric insulators for lamp sockets s
(88) aggregates for high strength mortar C97), fluxes for rapid hardening cements (38) and opague glazes (88) Similarly, coatings for glass on incandescent bulbs (84) and those on welding electrodes for steel at high temperature (GL), Fillers for epoxy resin moulding compounds (92) and pavement patching compositions (83) have also been reported ag outcome Ÿ the woellastonite utilization Refractory monlds and binders (84}, glazed tiles with improved quality
h
(953, polishing powders for glasses containing rare eart oxides (88), fast Firing wall tiles (87), thermal insulators
(98), fillers for fire proofing compositions (88}, binding materials regarding fireproofing and thermal insulating eoatings (i003 mould mixtores for low melting metals (1614), and light-weight non-flamable products (102), have been prepared using wollastonite alongwith other batch compo- nents
In sddition to above, wollastenite has been