STUDIES ON THE SYNTHESIS OF WOLLASTONITE FROM RICE HUSK ASH AND LIMESTONE (Full Thesis) NGHIÊN CỨU VỀ TỔNG HỢP wollastonite từ vỏ trấu ASH và đá vôi

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.

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)

L993

Wollastanite 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

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

and hydrothermally treated in autoclave

the said

stone

oMides, Abundantlyv varying time

were adopted Le

silica, the mixture

under water far ane hour, The re-

namically 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

>

oduced Cile bodies matured within two hours instead mf fuatue

using wollagstonite

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

e

oO z z ia =

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

ioam also highly indebted to Dr Khadim Hussain, Assa- late Professor, Centre for Solid State Physics, University of

se Punjab, Lahore, for extending the facility of ERD and many cuitful discussions while evaluating the experimental products

7 thanks are also due to Dr S A Siddiaui, wha helsed much to arry out SEM investigations on produced calcium silicate hy-

“ates

I 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

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

MUHAMMAD SHARIF NIZAM)

improvement in Sif Content of Acid Digested RHA with Time

ARD Pattern (Cu Ken) of Hydrathermally Trested

at Higher Magnitficatian Scanning Electron Micrograph of HB-S Batch Product

Seanning Electron Micrograph of HHA-S Batch Product

at Higher Magnification KRDO Pattern {Cu Kel) of Wollastonite from HAa-~3 Batch 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

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)

Reactian Progress at Bitferent Temperatures (°C;

Utilization of Wollastonite

EXPER LTOENTAL WORK

Materials and Methods Chemicals

Equipment and Apparatus Starting Materiales

Techniques Apnolied Powder £-Ray Diffraction Scanning Electron Micrascopy

ig 72

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

43

Z.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

1 Pyroprocessing of Rice Husk

1,1 RHA Content

1.2 Rice Husk Ash Silica

1.3 Amorphous Phase Content of RHA

2 Purificaiton of RHA Silica

Comparison of XRD and SEM Results

Mechanism of Hydrothermal Reaction

Chemically Combined Water Cantent of CSH

Nuclei Growth Model

Phase Boundry Madel

Mechanism of Solid State Reaction

Quantitative Kinetic Analysis of Solid State

16?

169

4.13 Physieal Properties of Prepared Tile Bodies 181

CHAPTER 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

it synthetically (5,8).

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)

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- ature and duration of burning These factors influence

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

The possibility of utiliging cheaper and easily available source material i.e RH, yielding $105 demands a number of special processing considerations The burning environment, temperature and time olay very important role for optimum yield of reactive silica in RHA and must be taken care of if better quality product is desired Ample supply of fresh air helps in oxidizing the BH carbon to carbon dioxide This results in lowest residual carbon

eercentage 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

Components of wollastonite, as mentioned earlier, are calcium oxide and silica Therefore, high quality

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- ble properties It is advisable for establishing the prac-

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 for mutual comparison.

Chopra at e1.(23) burnt RH at 7O0°C, evaluated ash through X-ray diffraction (XRD) technique and found that it WAS amorphous, Further heating of this ash at the same temperature transformed some of it into crystalline state This showed the direct effeot of time (keeping the tempera- ture constant) on the nature of prepar ed RHA.

The 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 temperature and proportion of the latter silica phase

creased whith rise in temperature

Pitt (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

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

A detailed study for the preparation of RHA, free from Cationic impurities was carried out by James and Rao (30) RH was Suspended in 1:1 mixture of HaS0a¿ and

HNCq at room temperature for three months The resulting white residue, after proper washing was heated at 400°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%

Ikram and Akhter (35), in another work prepared RHA at 620°C, reduced it by metallo-thermic process and the silicon thus obtained was purified subsequently The total impurity content, after purification, was found to be leas than S00 ppm Similarly, Hussain and co-workers (36) uti-

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-

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)

It has been reported that OPC on hydration, (setting) contains about 24% CacOH jo which is responsible for poor acid resistance of the built structures (939) Hixing of appropriate amount of RHA with OPC results in acid proof mortars which are even superior to commonly seld acid resistant cements Such blends have been successfully used

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)

Lot of heat energy can be saved by proper insula- tion of the industrial furnaces RHA prepared in sioping kiln was osed by Faroog et al (42) to produce lightweight refractory bricks by adding combustible fillers before tiring These bricks are applied as furnace cladding and insulate heat due to numerous pores within the occupied volume The same group of workers (43) patented s process about using RHA as Filter aid RH were calcined properly,

resulting 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,

reorystalligation 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)

Tt has also been observed that CaO, present as mineralizger ain raw batehes for producing silica refracto- ries, reacts with 8105 Such interaction of CaQ and Si05 starts even at 600-700 °C, however, caleium silicate phase produced at this stage is not wollastonite Subsequent rise

im temperature, with the passage of time, converts this initial silicate modification into pseudo-wollastomite (49)

Deer 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,

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, 38105.Fe 0, Cad was utilized which produced wollastonite but

im agseciation with erystalline anorthite (CaAl 253 2Ö g3

Foshagite 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

said product Hamedov and Belov (33) converted it tof -

wollastonite at 750°C

Hikheilov (34) reported theoretical possibility of

sufficient amount and effluent gas totally condensed He hydrolysed Caks by steam in the presence of 3102 €guartz) and produced Cal, CaSids, £ -Caos SiÖa,oó -Gaa S10 and Cña 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

Lawrence (56) first prepared CSH with Cad : 5102 ratios; 1,0 ~ 1.5 and 1.0 - 1.2 The intermediate compound

je BH, with the former raw mix ratio, was dehydrated at 550-770° which yielded mìmost amorphous material Then CS5H with Ca0:Sity, 1.0 - 1.2 were given similar heat treatment This material was converted to wollastonite on dehydration

at 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

Stefanov et al ¢€680) adopted an indirect procedure fer producing wollastonite First of all, caleium metasili- cate hydrate was prepared by reacting an alkali-alominnsiti-

cate with Ga(OH)2 at 80-SOSC Fe half an hour Res

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

Wuhrer (63) while upscaling and commercializing the previously carried out synthesis, altered some of the conditions and substituted new source materiais for Caf and Gia, Consequently, he formalated batches consisting of 46.9% quick lime and 53.1% guartz sand Finely pulverized

caw 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

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, 27-31%; quartz, 2%, impurities, @-14% Mixed raw batches

were 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

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- tion into higher temperature modification (€~CaSids d

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

Kotsis and Balogh (72) performed reaction between limestone and silica gel for the productian of fp =woillosto~

nite through S5 synthesis Thev investigated the expented intermediate pha

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- ing the obtained vields from three parallel reactions i.e diatomite - chalk, quarte-chalk and silica gel-chalk The effects of small percentage of glass, added as mineralizer and that of soda Feldspar were also observed All of the batches were based on i:l molar ratio of Cad, 5105 and reac~ Lion progress was continuously followed by KRD The forma- tion of & ~dicalcium silicate as first phase, was also confirmed during these investigations

HydxoEharmalHetheds-

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

at & temperature higher than 700°C Heat stable and ex- tremely white A -fWollastonite was obtained as S 8 final product

Hanyhyr et al ¢€775 synthesized wollastonite using Waste raw materials from silicon industry Hydrothermal reaction between Ca(OH)»5 and S105 was carried out under normal pressvure It’s decisive step depanded upon the initial molar ratio i.e CaG:5i05 less than one Suose-

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-

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- ing products were CSH showing thereby the possibility of farming such potential intermediate compounds for preparing

They 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

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

fy, purity, inertness, brilliance, fiberous particle shape and ease of grinding upto fine particle sigs These promi-

nent oharacteristics have fPrequen

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