A Route to the Synthesis of Aluminium Sulphate from . Local Raw Materials

The annual requirement of aluminium sulphate in Sri Lanka is about 15,000 metric tons and this is totally imported. Since bauxite is not available in Sri Lanka, Dediyawela ball clay containing about 36% Al 2 O 3 and lateritic clays containing about 20% Al 2 O 3 , were used as sources of alumina for the synthesis of aluminium sulphate. Treatment of calcined ball clay with H 2 SO 4 results in the formation of aluminium sulphate. The recyclic process of treatment reduced the free acid content in the product. By recrystallisation the excess iron was removed.


Introduction
Aluminium sulphate is available in three different forms as anhydrous Al,(SO,),, Al,(SO,),.14H,O and Al,(SO,),.18H20.iCommercially available aluminium sulphate is Al,(SO,),.18H,O and it is mainly used in water purification and in the paper industry.
Commercial grade aluminium sulphate is available in two forms.1 (a) Iron Free (less than 0.OQ3% Fe,O,) (b) Commercial Grade (0,4% to 0.5% Fe,O,) The annual requirement of aluminium sulphate in Sri Lanka is 15,000 metric tons and this is totally imported.~a h x i t e is a mineral mainly consisting of A1,0,.2H,O.By treatment of bauxite or China Clay with conc, H,SO, aluminium sulphate is p r o d ~e d .i + ~ Mineral bauxite is not found in Sri Lanka.2Investigations were carried out at the Minerals Technology Section of the CISIR to find out the possibility of using Dediyawela ball clay, kaolin and laterite as a source of alumina (Altoa) for the production of aluminium sulphate.
Dediyawela ball clay is an alumino-silicate type of clay and it contains about 36% A1,0,.
Laterite is a weathered clay and it contains about 20% of AI,O,.

Chemical Analysis of Ball Clay and Laterite
Determinations of SiO,, A120, and Fe20, of ball clay and laterite were carried out according to standard methods.3SiO, content was determined by HF treatment.
Al+++ and Fe+++ contents were determined by spectrophotometry method and the wave lengths of 370 nm a'nd 700 nm, respectively.

Calcination of Ball Clay and Laterite
Calcination was done using a muffle furnace at different temperatures and periods.The temperature of furnace was measured to an accuracy of f 5°K.

Grinding of Laterite and Ball Clay
A laboratory ball grinder was used for grinding laterite to pass 80 mesh sieve.Powdered ball clay was obtained from Dediyawela ball clay refining plant of the Ceylon Ceramics Corporation.

Acid Treatment
98 % analytical grade sulphuric acid diluted to different strengths was used for treatment of ball clay and laterite.Slurry obtained from 2.4 was leached at different temperatures for varying periods, using a steam bath and a magnetic stirrer.

Concentration and Crystallization
Slurry obtained after leaching was filtered under vacuum and the filtrate was concentrated using a steam bath and crystallized by cooling.

X-ray Analysis of Samples
XRD patterns of the samples were taken by using "JEOL JDX-85" type X-ray Powder Diffractometer.

Dediyawela Ball Clay as a Source of Alumina
Dediyawela ball clay is a mixture of mineral kaolinite (A1,0,.2Si0,.2H20)gibbsite (AI20,.3H.,O) and boehmite (AI,O,.H,O).The X-ray diffraction pattern of the Dediyawela Ball Clay (Figure I ) shows the presence of small amount of silica ( a quartz) as impurity.The total alumina (A1,0,) content in Dediyawela ball clay is 36:; and the chemical composition of the Dediyawela ball clay is given in Table I .This experiment clearly shows that the AI,O, present in ball clay is in bound form and hence the solubility in conc.H,SO, is very low.On calcination, formation of meta-kaolin which i,s a metastable phase enhances the solubility of A120, in conc.H,SO,.

Eflect of calcination temperature of ball clay on the solubility of Also, in conc. H2S0,
Known quantities (50 g) of ball clay calcinated at different temperatures for a duration of 2 hours were digested with known quantities (40 ml) of 98 % H,SO, diluted to 400 ml with distilled water for a duration of 2 hours at a constant temperature of 338" K.The filtrate was concentrated and crystallized.
The yield of aluminium sulphate obtained from ball clay calcined at different temperatures was determirged and variation of yield with temperature is shown in Figure 3, The results clearly show that.the most suitable temperature for calcination of ball clay to yield soluble alumina is 1173' K.

Effect of duration o f . calcination of ball clay on the formation of aluminium sulphate
Known quantities (50g) of ball clay calcined at 1173' K for different durations were digested with 40 ml of 98% H,SO, for a duration of 2 hours.The aluminium sulphate formed was crystallized from the filtrate and the weight was determined.
The results are shown in Figure 4. Maximum yield of aluminium sulphate was obtained from.thesample calcined for a period of 2 hours.

Minimum quaniiry of conc. H,SO, necessary for complete leaching of alumina from calcined ball clay
Dediyawela ball clay calcined at different temperatures was used in this investigation.Durations of calcination and leaching were kept constant throughout this investigation. .The results obtained are pIotted in Figure 5.In all the experiments the maximum yield was obtained when ball clay calcined at 1 173" K was used.It was also revealed that the quantity of H,SO, used for leaching is also important in getting the maximum yield of aluminium sulphate.

. Fflect of temperature and duration of leaching on the yield of aluminium sulphate
Dediyawela ball clay calcined at 1173' K was used in this experiment.Leaching of alumina was carried out at two temperature ranges between 338" K and 373" K using 40 ml conc.H,SO, diluted to 400 ml with distilled water for different durations.
The results are shown in Figure 6.Yield of aluminium sulphate obtained increases with temperature of leaching.Complete leaching of alumina was obtained with durations of 2 hours.

Laterite type of clay as the starting material
Laterites are extensively developed in the South-West Region of the island.They are mainly kaolinite clay materials with gibbsite, geothite and silica, Most of the laterites are developed in siiu.In addition there are secondary lateritezc.
Laterites obtained from ~i r i b e d d a and Homagama were used in this investigation.The chemical composition of laterites used are given in Table 2.  Quantities of alumina leached using cone.H,SO, were very low compared to the total available alumina in laterite.Therefore calcination of laterite at high temperature is necessary lo convert the alumina to a n acid soluble form.

Eflbct of' temperature of calcination otl t1w formution o f crl~~miniurn .s~rlp/zcrtc~
Known weights (1000 g) of laterites calcined at different temperatures were leached using conc.H,SO; and the durations of calcination and leaching were kept constant at 2 hours and 2 hours respectively throughout the experiment.The temperature of leaching was 343" K and the volume of H,SO, added was 40 ml diluted t o 400 ml with distilled water.Maximum yield of aluminium sulphate was obtained from the sample of laterite calcined at 1173" K.The results are given in Figure 7.
1:igure 7. kcffect of temperature of calcination on the formation of aluminium sulphate.

Dzrration of calcination of laterite on the yield of aluminium su@hate
Laterite calcined at 1173' K for durations varying from 112 hr to 05 hrs were used in this investigation.The volume of H,SO; added was 40 ml diluted to 400 ml with distilled water.
The results are given in Figure 8. Increase of duration of calcination beyond 2 hours did not increase the yield of aluminium sulphate appreciably.
Minimum duration of calcination required for complete conversion of alumina to acid soluble form.

Temperature and duration required for complete leaclling of alumina
Laterite calcined at 1173" K was used in this investigation.Temperature of treatment was studied at two ranges between 338" K and 373'K.Duration of treatment was varied between I and 10 hours The results are given in Figure 9.At temperature of 3730 K in a duration of 7 hours, maximum yield of aluminium sulphate was obtained.

Potential of ball clay as the starting material
The experiments carried out clearly show that to get a maximum yield of aluminium sulphate it is necessary to calcine ball clay at 1173°K.Durationof calcination, concentration of H,SO, and temperature and duration of leaching are also important factors which affect the final yield of aluminium sulphate.It is also observed that ball clay calcined at 1173" K for a duration of 2hrs when treated with conc.
H,S04 for 2hrs resulted in a conversion of about 73 :/, of alumina present in ball clay to aluminium sulphate.
The X-ray diffraction powder patterns of the commercially available aluminium sulphate and the aluminium sulphate prepared are given in Figure 10.
. The iron content of the aluminium sulphate prepared (Al,(S04),.18H,0) by the process developed is very low (less than 0.4%).
Recycle process of treatment was also tried in order to reduce the free acid in the crystallized aluminium sulphate.The use of excess water in the process of leaching with H,SO, resulted in slow crystallization of aluminium sulphate.

Potential of laterite as the starting material
The chemical analysis shows the presence of high percentage of Fe,O, which affect the final quality of the product.
To get a maximum yield it is necessary to calcine laterite at 11 73" K for a duration of 2 hours.
Leaching also has to be carried .-outunder controlled conditions.About 83 % of the alumina present in laterite can be converted into aluminium sulphate.The main impurity observed was iron and by recrystallization the major fraction of iron present in aluminium sulphate was removed as filtrate.

Conclusion
Ball clay is one of the main raw materials3 used in ceramic and rubber industry.The reserves are limited and it is not advisable to use it as the starting material for manufacture of aluminium sulphate, whereas laterites are hardly used as a raw material at present.The disadvantages in using laterites are the low alumina content and high iron content.By carefully selecting laterites of high alumina and low iron content, it may be possible to develop a method for the manufacture of aluminium sulphate.It is necessary to carry out pilot plant scale trials before commencing any large scale manufacturing of aluminium sulphate using laterites as the starting material.

.I. M. 3 .
Jayatileke and M. G. M. U Ismail A Route to the synthesis o f Aluminium SulphateJrrom Local Raw Materials Calcination sf ball clay at temperatures above 723'K shows the formation of meta-kaolin.The Differential Thermal Analysis curve af ball clay used in this investigation is given in Figure 2. At temperatures above 1198°K formation sf silicon spinel (2Al2?,.3Si0,) is favoured.The sequence of formation of diflerent types of alumino silicate is given below.6723" -823°K A12Si,0, + 2H20 Al,Si,O,(UH), t meta-kaolin 2(Al,O,.SiO,) + SiO, 1673°K + 3Al20,.2Si0,+ SiO, mull ite, cristobalite I 1198°K 1373% c 2A1 ,O,.SiO, + Si02 silicon spinel Digestion of ball clay with conc.H,SO, at a temperature of 353°K resulted in only a very low yield of aluminium sulphate.Treatment of calcinated ball clay with the same quantity of conc.H,SO, at 353°K gave a better yield of aluminium sulphate.

Figure 3 .
Figure 3.Effect of calcination temperature on the yield of aluminium sulphate.

Figure 4 .
Figure 4. Effect of duration of calcination on the formation of aluminium sulphate.

,Figure 5 .
Effect of quantity of Conc H2S04 acid on the yield of aluminium $&hate.

Figure 6 .
Figure 6.Effect of duration and temperature of treatment on the formation of aluminium sulvhate.
9. Temperature and Iluration required for complete lcaching of alumina.

Table 2 .
Chemical composition of laterites