A study of the formability of Nigerian clays

Size: px

Start display at page:

Download "A study of the formability of Nigerian clays"

Arline Newman
5 years ago
Views:

Indian Jurnal f Chemical Technlgy Vl. 6, March 1999, pp. 79-84 A study f the frmability f Nigerian clays C S Nwbd Department f Materials & Metallurgical Engineering, Federal University f Technlgy, P.

1 Indian Jurnal f Chemical Technlgy Vl. 6, March 1999, pp A study f the frmability f Nigerian clays C S Nwbd Department f Materials & Metallurgical Engineering, Federal University f Technlgy, P.M.B 1526, Owerri, Im State, Nigeria Received 23 February 1998; accepted 8 December 1998 The flw prperties f a Nigerian clay has been examined. The results shw that partial deflcculatin f the clay slip fr casting can be achieved by the additin f 0.14 wt% sdium carbnate withut resrting t a mixture f sdium carbnate and sdium silicate. Weibull distributin has been used t predict the green strength f the crucibles. Cheap clay crucibles fr melting aluminium have been prduced by sintering the unfired clay crucibles at I 250 C fr ne hur. Slip-casting has been used in the fabricatin f ceramics fr mre than tw centuries. Fr a slip t prduce a suitable cast, a number f variables such as thixtrpy, viscsity and cncentratin must be cntrlled, us~ally the best cmprmise between a high slip density and a lw thixtrpy is arrived at by using a mixture f sdium carbnate and sdium silicate as deflcculant; the frmer is an excellent dispersant whilst the latter inhibits thixtrpyl. Ryan and Wrral 2 have shwn that thixtrpy may be cnsidered the mst sensitive variable fr the cntrl f industrial casting. Thixtrpy3 measurement has been fund t be f n great significance unless the slip is prepared at a state f maximum deflcculatin. Every clay has its wn flw characteristics and the prper cnditins fr a suitable casting slip can be fund nly by experiment. Althugh the deflcculatin 4-7 f clay and clay based aqueus suspensins has received detailed attentin but n infrmatin is available n the rhelgical prperties f the Nigerian clay used in this investigatin. In develping cuntries, f which Nigeria is an example, materials suitable fr peculiar cnditins are either nt available r are t expensive t be purchased. Therefre, the aim f the present study is t meassure sdme rhelgical prperties f Nigerian clay and cnsequently prduce cheap slip-cast labratry clay crucibles fr melting aluminium. Experimental Prcedure Materials The experiment was carried ut with Afikp clay frm Ebnyi State f Nigeria with A.R. grade sdium carbnate as deflcculant. The as-received clay was wet sieved using a 7S-micrn aperture sieve. The resulting slurry was dewatered n a plaster f paris bat and dried in air int clay lumps that were further grund int a pwder. A suspensin was prepared frm the Afikp clay pwder and distilled water by means f the Hamiltn Beach mechanical stirrer. The resulting suspensin was passed thrugh a ISO-micrn aperture sieve at a cncentratin f 21.S vl% slids. This cncentratin, ideally cntained the maximum amunt f slid matter cnsistent with gd flwability in the flcculated state fr this particular clay. 3S0 cm 3 f the suspensin was transferred int the Fann Viscmeter mdel n. 3SA cup. The Fann Viscmeter is perated by a tw speed, 110 V synchrnus mtr. The pwer turns a rtr sleeve 87 mm lng, with a clearance between the rtr sleeve and bb f 1.14 mm. The instrument perates at six different speeds and speed changes are made by changing gears and switching mtr speeds. The values f apparent viscsity and thixtrpic cefficient were btained at fur different rates f shear fr the flcculated clay suspensin. Sdium carbnate was added in incremental steps f O.OS g t the clay suspensin, and fr each additin, the clay suspensin was stirred befre the viscmeter readings were taken at fur different rates f shear. Apparent viscsity values were btained frm the viscmeter readings using the relatinship:

80 INDIAN J CHEM. TECHNOL., MARCH 1999 11= 300 x dial reading... (I) N where N is viscmeter speed (rpm). The shear rate was btained by multiplying the rpm reading by 1.

2 80 INDIAN J CHEM. TECHNOL., MARCH = 300 x dial reading... (I) N where N is viscmeter speed (rpm). The shear rate was btained by multiplying the rpm reading by which is the viscmeter cnstant. Abslute viscsity, 11 and thixtrpic cefficient, 8 values were btained frm the flw curves using the Bingham 2 equatin: (J 1]=1] (2) D where Djs the rate f shear. The effect n apparent viscsity f percentage sdium carbnate at a shear rate f 170 S- I at 21.5 vl % slids was studied. Preparatin f casting slip J\ casting slip was prepared at a slip density f 1.72 g cm- 3 frm a mixture f 475 cm 3 f distilled water, 1000 g f clay and g f sdium carbnate. The true specific gravity f the clay pwder was btained by the evacuatin methd 8 and fund t be The slip density was determined using a pycnmeter. Fabricatin f cylindrical specimens and crucibiles Cylindrical rds f cm diameter and 100 mm lng and labratry crucibles 135 cm 3 were cast frm the casting slip in plaster f paris mulds. The slid casting methd was used fr the cylindrical rds while the drain casting methd was used fr the labratry crucibles. The casting time fr the cylindrical rds was 24 h while that fr the labratry crucibles was 3 min. The specimens and crucibles, after remval frm the mulds, were initially dried in air fr tw days and finally dried in an ven at I 10 C fr 6 h and allwed t cl in a dessicatr. Mdulus f rupture f green cylindrical specimens The mdulus f rupture was determined by a three pint bend test using the British Ceramic Research Assciatin mdulus f rupture equipment. Slid specimens f apprximately mm diameter and 100 mm lng were laded with a span f 70 mm. Five specimens were measured. The mdulus f rupture was calculated using the frmula: MOR = 8PI),.., (3) f1d where P is the breaking lad, L is the span and d is the diameter f specimen. Weibull prbability distributin 3 was used t analyse the results. The survival prbability, P, was calculated using the relatinship: P(=I_ (i-0.3)... (4) (n + 0.4) beynd the ith level f a small ppulatin n. Apparent prsity, true prsity and bulk density f i green specimens The apparent prsity, true prsity and bulk density were determined by vacuum evacuatin methd with kersene as the immersin liquid. The apparent prsity (AP), the true prsity (TP) and the bulk density (BD) were calculated using the fllwing relatinships: AP = Ws -Wd Ws-Wd TP=[I- ~J... (5)... (6) BD= Wd x D... (7) ~s -Wn where Ws is the weight f specimen saked with kersene and weighed in air, Wd is the dry weight f specimen, Wn is the weight f specimen saked with suspended in kersene, D is the density f the kersene and D t is the true density f the sample. Sintering f clay specimens The cast cylindrical rds f the clay were fired in a Wenger's electric furnace at 1000, 1150 and 1250 C. The heating-up rate was apprximately 120 C/h and the saking perid at each temperature was ne hur. The specimens, in each case, were furnace cled. Mdulus f rupture f sintered clay specimens The mdulus f rupture was determined by a three pint bend test at rm temperature. Apparent prsity and bulk density f sintered specimens The apparent prsity and bulk density f the sintered cylindrical rds were determ ined by the ht test piece, biling water (HTR W. methd 8. Sintering f labratry clay crucible:; The clay crucibles were tired at 1250 C. The heating-up rate was apprximately 120 C/h. The saking perid at temperature was ne hur. The crucibles were furnace cled.

3 NWOBODO: FORMABILITY OF NIGERIAN CLAYS 81 ~r ~ vl "I. SOlids ~) N dettccutnl (2) m ~. N.i' (3) 1 68 m ~. Na' (4)2 48 m e Na' Cs) 3 32 m e Na' (6) 4.16 m e Na' 10 :~. 6~~-- ~; h<rr rall!, s Fig. I- Variatn f the apparent viscsity f Afikp clay suspensin with increasing shear rate 8'~ ~ '" ~ vl '/, s!;ds ( I ) N dettcculnl (2) 0 84 m. Na' (3)1. 68 m c.na' (4) 2.48 me Na' (5)3 32 m. Na' (6 ) 4 16 m. ~ Na' Reciprcal sh«r reft, 5 X 10 :; 5 6 f ig. 2--Ddlccu lat in and Bingham cnstants f Afikp clay suspensin

82 INDIAN J CHEM. TECHNOL., MARCH 1999 111 & ~5 ~.~ 8~-------------------------' 21 5... 01 -I. slids.~ 40 > C.. 8. 30 ~ 2. 10 170 5-1 shr rl. O~-------O~.~, --------~, 2~------~O~3 0,. N~C03 Fig.

4 82 INDIAN J CHEM. TECHNOL., MARCH & ~5 ~.~ 8~ ' I. slids.~ 40 > C ~ shr rl. O~ O~.~, ~, 2~------~O~3 0,. N~C03 Fig. 3-The relatinship between the apparent viscsity f Afikp clay suspensin with increasing additins f sdium carbnate. Results and Discussin Clay characterisatin The variatin f the apparent viscsity f the clay suspensin with increasing shear rate and increasing electrlyte cncentratin is shwn in Fig. I. It can be seen that the viscsity decreased as the shear rate increases. This is typical f clay suspensins. Curve (1) represents the clay suspensin in the flcculated state which favurs the frmatin f particle-particle links, leading t high yield values and thixtrpic structures, and hence a high viscsity. The flcculatec state are usually favured by flcculating catins sucl as OH r. Ca 2 + and Mg2+. If the flcculating catins are replaced by a Na+, K+, Lt r NH/, which favur a high zeta-ptential the interparticle frces f attractin are utweighed by the electrstatic frces f repulsin and a deflcculated system results, giving rise t a suspensin with a small r negligible yield value and a lw viscsity. Curves 2-6 shw hw the viscsity decreases crrespndingly as the electrlyte cncentratin increases frm 0.84 m.e. Na+ thrugh t m.e. Na +" per 100 g clay, with the electrlyte cncentratin. C lay suspensins bey Bingham's law apprximately and researchers have shwn that the Table I--- ffect f electrlyte cncentratin n thixtrpic cefficient f Afikp clay suspensin Electrlyte Cncentratin 'Thixtrpic % m.e. Na+ per cefficient 100 g clay mpa Table 2-Bulk density, prsity and mdulus f rupture data n green specimens Sample MOR Apparent True prs- Bulk density Identifica- MPa prsity ity V l. gcm- J tin Vl. fractin fractin II I. I I thixtrpic cefficient, fj, diminishes rapidly as a suspensin is deflcculated, becming very small r zer near the ptimum. The abslute viscsity, 77, is apparently little affected by deflcculatin since it is in effect the limiting viscsity at the pint where all internal structure is brken dwn by shear. The effect f electrlyte cncentratin n thixtrpic cefficient is shwn quantitatively in Table I. The thixtrpic cefficient has reduced frm mpa at zer electrlyte (i.e. flcculated state) t 264 mpa at 5.00 m.e. Na+ per 100 g clay. Subsequently, the thixtrpic cefficient increases t 437 mpa and remains cnstant irrespective f the increasse in electrlyte cncentratin up t 9.32 m.e. Na+ per 100 g clay. Fig. 2 shws. Bingham plts f the same clay suspensin but at different states f deflcculatin. A series f lines is btained, all f differing slpe but tending almst t the same intercept n the apparent viscsity axis. It can be seen that as the mil liequivalents f sdium is increased, the system becmes prgressively mre deflcculated until the ptimum amunt is reached, when the thixtrpic cefficient has decreased frm I 1286 t 80 I mpa. Fig. 3 shws the effect f percentage sdium carbnate n the apparent viscsity at a shear rate f 170 S- I. It is bserved that minimum viscsity is

5 NWOBODO: FORMABILITY OF NIGERIAN CLAYS 83 Table 3---Effect f sintering temperature n apparent prsity bulk density, water absrptin and mdulus f rupture ftest specimens N. f Firing Apparent Bulk Water MOR specimens temperature prsity density absrptin MPa Vl. fractin gcm- 3 % C \ \, , , )0 >- :::;20 ~ e 10 n. ~ 5 > 5 ) III 2 0:5 0 ) 0 2 \ \ 0 1 '-- L---'-----''---_ -'--L.LL--'----'-_ --'--- '-_ -' ,)) 0'44 0 ' ,11' 18 1-) Mdulus f rup ture, MfI::J Fig. 4---Survival prbability versus mdulus f rupture f green specimens achieved at abut 0.20% sdium carbnate. This suggests a pint f maximum deflcculatin. Hwever, experi ence has shwn that casting slip shuld be partially deflcculated hence the casting slip prepared fr the crucibles and cylindrical rds cntinues wt% sdium carbnate n dry clay basis. Thi s electrlyte cncentratin has given gd and reprducible results. Table 2 shws the bulk density, prsity and mdulus f rupture data n the green (unfired) samples. The effect f prsity n strength 10 is given by the Ryshkewitch-Duckwrth equatin: S=S exp ( bp ), where S ts the strength at prsity, p; S is the strength at zer prsity and b is a cnstant fr the materi al. The equatin has been used t analyse strength data fr ht-pressed brn subxide having prsities ranging frm less than I t 21 % and htpressed brn having prsities ranging frm less than 1 't 30%. The data fr the unfired clay specimens d nt fit the strength versus prsity equatin. This is t be expected because the range f prsity f the cast specimens in the green state is negligible. The specimens were cast under similar cnditins, hence the bulk density and prsity values are fairly cnsistent, with very little variatin. Their strengths vary because f inherent flaws which differ in size, in shape, in their psitins within the specimens and in their rientatin relative t the directin f maximum tensile stress. The effect f such flaws IS very prnunced in unfired clay specimens. The strength data have t be treated statistically. Weibull distributin f the mdulus f rupture data, n the green specimens, is presented in Fig. 4. The Weibull mdulus is Since the vlume f test speci men stressed is apprximately 7.46 cm 3, and the average mdulus f rupture is MPa, it fll ws that the average mdulus f rupture fr the clay crucible f vlume 135 cm 3 will be 0.62 MPa. This is cnfi rmed by the adequate green strength pssessed by the cast clay crucibles. Table 3 shws the effect f sintering temperature n apparent prsity, bulk density, water absrptin and mdulus f rupture f test specimens. As expected, specimens fired at 1250 C have the least apparent prsity and water absrpti n and the hi ghest bulk density and mdulus f rupture. At a sintering temperature f 1000 C the apparent prsity has increased beynd the va lue fr the green specimens, suggesting sme thermal decmpsitin and recnstitutin f the clay. Cnclusin The rhe lgical data btained suggest that the Nigerian clay used has gd flw prperties that gi ve rise t reprduc ible results. Prductin f cheap

6 84 INDIAN J CHEM. TECHNOL., MARCH 1999 crucibles frm the Nigerian clay by slip casting technique can be achieved by the use f sdium carbnate alne as the deflcculant withut resrting t a mixture f sdium carbnate and sdium silicate. Weibull statistics has been reasnably used t predict the green strength f the crucibles frm the strength data n the cylindrical specimens. The crucibles prduced at 1250 C are relatively cheap and suitable fr melting aluminium but nt suitable fr melting brass. Acknwledgement The authr is indebted t the persnnel f the Prject Develpment Institute, Enugu, Nigeria fr their cperatin in sintering the samples and detennining the apparent prsity, bulk density, water absrptin and mdulus f rupture f the samples. References I Grimshaw R W, The chemistry and physics f clays (Ernest Benn Limited, Lndn), 1971, Ryan W & Wrral W E, Trans Br Ceram Sc, 60 (1961) Mulry B, Ceramics, December (1972) Mre F, Institute f Ceramics textbk series (1965) I. 5 Wrrall W E, Clays & ceramic raw materials (Applied Science Publishers Ltd, Lndn), 1975, Phelps G W & Mclarea M G, Particle size distributin and slip prperties, ceramic prcessing befre firing (Jhn Wiley & Sns, New Yrk), 1978, Cughanur L W & Nrtn F H, J Am Ceram Sc, 32(4) (1949) Chesters J H, Refractinies: Prductin & prperties (The. Irn & Steel Institute Lndn), 1973, Davidge R W, Mechanical behaviur f ceramics,(cambridge University Press), 1980, Petrak D R, Rbert Ruh & Atkins G R, Am Ceram Sc Bull, 50 (8) ( 1974) 569.

Of Processing Parameters In SLS Of Metal...Polymer Powders

Of Processing Parameters In SLS Of Metal...Polymer Powders Of Prcessing Parameters In SLS Of Metal...Plymer Pwders B.Badrinarayan and J.W.Barlw Dept. fchemical Engineering, The University ftexas at Austin, Austin, Texas- 78712,U.S.A. Abstract paper describes the