bleaching of a nigerian kaolin by oxalic acid …dl.uctm.edu/journal/node/j2015-5/10_alafara a....

Alafara A. Baba, Mayowa A. Mosobalaje, Abdullah S. Ibrahim, Sadisu Girigisu, Omodele A. A. Eletta, Fasakin I. Aluko, Folahan A. Adekola

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Journal of Chemical Technology and Metallurgy, 50, 5, 2015, 623-630

BLEACHING OF A NIGERIAN KAOLIN BY OXALIC ACID LEACHING

Alafara A. Baba1, Mayowa A. Mosobalaje1, Abdullah S. Ibrahim2, Sadisu Girigisu3,

Omodele A. A. Eletta4, Fasakin I. Aluko5, Folahan A. Adekola1

1 Department of Industrial Chemistry, University of Ilorin, P.M.B. 1515, Ilorin-240003, Nigeria E-mail: [email protected]; [email protected] Department of Chemistry, University of Ilorin, P.M.B. 1515, Ilorin-240003, Nigeria3 Department of Science Lab. Tech., Federal Polytechnic Offa, P.M.B. 420, Offa, Nigeria4 Department of Chemical Engineering, University of Ilorin, P.M.B. 1515, Ilorin-240003, Nigeria5 Department of Mechanical Engineering, Federal Polytechnic Ado-Ekiti, P.M.B. 5315, Ado-Ekiti, Nigeria

ABSTRACT

The level of improvement of whiteness of Egbeda (Nigeria) kaolin ore by oxalic acid leaching was investigated. The effects of acid concentration, reaction temperature and particle size on the extent of the ore dissolution were examined. The results of the leaching investigation on the improvement of the ore whiteness assessment were found to increase with increasing acid concentration, reaction temperature and decreasing particle size. At optimal leaching (0.5 mol L-1 H2C2O4, 85°C, 120 minutes) with moderate stirring, the dissolution reached 79.9 %, when total iron removal was achieved as evidenced from the EDXRF and EDS analyses. The dissolution curves were analyzed and found to conform to the surface chemical reaction, and the calculated activation energy of 41.34 kJ mol-1 supported the proposed model. These results are also corroborated by the output of the Post-Hoc test by Duncan Univariate Anova Analysis using SPSS 7.1. Finally, oxalic acid proved to be effective for treating the Egbeda (Nigeria) kaolin ore for total iron impurities removal and improving the ore whiteness for possible industrial utilities.

Keywords: kaolin ore, Nigeria, leaching, oxalic acid, bleaching, iron impurities removal.

Received 15 February 2015Accepted 05 July 2015

INTRODUCTION

Kaolin, a clay mineral with chemical composition Al2Si2O5(OH)4, which was found application in paper filling and coating (45 %), refractories and ceramics (31 %), fiberglass (6 %), cement (6 %), rubber and plastic (5 %), paint (3 %) and other industries (4 %), is a layered silicate ore, formed by the linkage of SiO4 tetrahedral in an hexagonal array. The bases of the tetrahedral are approximately coplanar and their vertices are all point-

ing in one direction. Successive layers of kaolinite are superimposed so that oxygen atoms at the base are bonded to hydrogen ions at the top of its neighbor re-sulting in a single layered triclinic unit cell [1, 2]. This mineral occurs in abundance in soil being formed by the chemical weathering of feldspathic rocks in hot moist climates. In general, kaolin deposits are classified as either primary or secondary. Primary kaolins are mostly residual (in-situ) sedimentary deposits of hydrothermal alteration of feldspathic rocks, while secondary kaolins

Journal of Chemical Technology and Metallurgy, 50, 5, 2015

624

are transported and deposited pedigenic materials. The kaolins of Georgia, South Carolina in the United States and the lower Amazon basin in Brazil, are notable sedimentary kaolin deposits, while the Cornwall area of South Eastern England has extensive hydrothermally formed primary kaolin deposits [2, 3]. Kaolin deposits occur in several places in Nigeria. Notable deposits are the Ozubulu kaolin in Anambra State, Alheiri/Darazo kaolin in Bauchi State, Apkene-Obom kaolin in Cross-River State, Kankara kaolin in Katsina State and the major Porter kaolin in Plateau State and the Egbeda kaolin in Osun State [4].

By their different modes of formation, kaolins possess diverse compositional and physical character-istics. The dominant mineral phase is kaolinite, a 1:1 dioctahedral-structured clay mineral [5]. The other clay phases including halloysite, illite and simectite may also occur along non-clay phases, such as calcite, quartz, anatase and iron-oxides goethite and haematite [6]. The clay mineral mostly occurs as very small (< 20 µm), crystallites with large surface areas, onto which chemical species, such as hydrated iron-oxides, are commonly ad-sorbed as to impart deep reddish, yellowish or brownish colourations to the kaolin deposit. On occasions, iron in advanced weathering conditions, may also be structur-ally held in kaolinites, where it could have substituted for octahedral aluminium [7, 8]. Thus, compositional and physical property variations in kaolin affect the adop-tion of kaolins, as an industrial raw material. The deep colouration of kaolins by iron-oxide impurities renders them less suitable as a white - coloured filler, a pigment material for paint, paper, plastics, rubber, adhesives and putties [3, 4, 9 - 13]. Hence, there is the need to improve the whiteness by removal of the iron impurities.

Efforts have been made by several workers [9, 11, 12, 14 - 16] to process kaolinite ore to meet industrial standards. These authors, among others, faced with problems attributed to expensive process design, diffi-culty in the ore dewatering, etc. Consequently, leaching has been adopted for treating kaolins as industrial raw materials. For example, this technique has been found useful in the removal of iron deposition on kaolin min-eral particles. Ferric oxide dissolution from kaolin is of particular interest for producers of industrial minerals. Thus, the commercial value of kaolins as raw materials for industries must be free from iron-bearing impuri-ties for increased whiteness and refractory properties

of the manufactured products [9, 17 - 21]. Therefore, considering the vast deposits of kaolin in Nigeria versus industrial utilizations, this study was aimed to develop a low-cost purification process, and through oxalic acid leaching to improve the ore whiteness to possibly meet some industrial demands. Due to the relatively low cost, availability and diversity of its potential industrial applications, the oxalic acid leachant was chosen for bleaching experiments in this study.

EXPERIMENTAL

The kaolin ore from Egbeda area in Osogbo, Osun State, Nigeria was used for this study. The ore was grinded and sieved by the ASTM standard method to four particle sizes: 0.09 mm, 0.112 mm, 0.25 mm and > 0.25 mm. The particle size 0.09 mm (with large surface area), unless otherwise stated, was used throughout this study. De-ionised water was used for the preparation of all aqueous solutions.

Experimental procedure and methodsThe leaching experiments were performed in a 250

ml glass reactor, equipped with a magnetic stirrer. 10 g L-1 of the ore was charged to the reactor and heated to the desired temperature (55°C), using an oxalic acid (H2C2O4.2H2O, molar mass 126.07 g mol-1; density 1.653 g cm-3), at concentration ranges 0.01 - 1.0 mol L-1, at vari-ous time up to 120 minutes. The fraction of the kaolin ore reacted was followed over time. After the comple-tion of the leaching tests, leach residues were filtered, water-washed, oven-dried and weighed. The fraction of the kaolin ore reacted was calculated from the initial dif-ference in the masses of the precipitate before and after leaching, for various time [9, 22 - 24]. The calculation was done for all oxalic acid concentrations investigated. Some selected reaction residues, after optimal leaching, were examined by XRD, EDS and SEM analyses for material purity tests. The process of leaching reactions is consistent with the following stochiometry [9]:

H2C2O4(aq)→H+ + HC2O4- (1)

HC2O4- →H+ + C2O4

- (2)

Fe2O3+H+ +5HC2O4→ 2Fe(C2O4)22-+3H2O + 2CO2 (3)


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In eq. (1), oxalic acid solution - C2H2O4(aq) dissoci-ates to release bi-oxalate ion (C2HO4

-). The bi-oxalate ion formed dissociates to form oxalate ions (C2O

-4)

-2 in eq. (2). From eqs. (1) and (2), it is apparent that the bi-oxalate is responsible for the dissolution and removal of the iron impurities from the ore surface, as shown in eq. (3). Optimization of other parameters, such as reaction temperature and particle size, for better understanding of the dissolution mechanism, was done with oxalic acid concentration with high dissolution rate.

RESULTS AND DISCUSSION

Ore characterizationThe composition of kaolinite ore by the EDXRF

MINIPAL model is summarized in Table 1.The elemental composition of the ore by EDS is

depicted in Fig. 1.From Fig. 1, the major elements expressed in per-

centage give oxygen (46.63 %), aluminium (14.58 %), silicon (15.98 %), iron (3.08 %), carbon (19.76 %). Trace amounts of sulphur and titanium (≤ 1 %) were also detected. The mineralogical analysis, examined by the EMPEREAN XRD (Fig. 2) according to [5, 25], 1997 standards, revealed the presence of the following intense peaks: impure kaolinite (Al2O3.2SiO.2H2O) and α-quartz (SiO2).

Also, the FEI Nova SEM 230 analyser with an emission gun showed that kaolin ore is made up of microscopic structure with typical flat–like shaped particles. This particle exists as agglomerated layer of flat sheets (Fig. 3).

Leaching resultsInfluence of acid concentrationThe influence of oxalic acid concentration on the

fraction of kaolin ore was studied within the acid range 0.01 - 1.0 mol L-1 at 55oC. The amounts of ore reacted at different oxalic acid concentrations and various leaching time are shown in Fig. 4.

The results, depicted in Fig. 4, show that the fraction of the reacted ore increases linearly with increasing acid

Table 1. Chemical analysis of kaolinite ore by EDXRF.

Compound Al2O3 SiO2 K2O CaO TiO2 Fe2O3 CuO ZnO Ag2O

RKao conc. (%) 40.90 50.90 0.14 0.13 2.79

3.09 0.0072 0.0021 1.46

PKao conc. (%) 43.93 11.07 Nd nd nd 0.00 nd Nd Nd

Fig. 1. EDS analysis of raw kaolinite ore.

Fig. 2. XRD analysis of the raw kaolinite ore: (1) quartz (SiO2); (2) kaolinite (Al2O32SiO.2H2O), [29].


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concentration. A marginal increase in the amount of the ore reacted with leaching time were observed within the acid concentration ranges (0.01 - 0.5 mol L-1), where the dissolution increased from 10.5 % to 53.7 % within 120 minutes, respectively. However, further increase of the acid to 1.0 mol L-1, reduces the amount of the ore reacted to 49.7 % at 120 minutes contact. The possible reason for this observation could be attributed to a precipitation phenomenon [23, 26]. Hence, 0.5 mol L-1 oxalic acid concentration with high value of dissolution was selected for further investigations in this study.

Influence of reaction temperature The experiments were carried out in the 25 - 85°C

temperature range with a 0.5 mol L-1 C2H2O4 solution. The results obtained are presented in Fig. 5.

From Fig. 5, it is evident that the fraction of kaolin reacted, increases with increasing temperature. The dis-solution curves appear to be parabolic as temperature increases from 40 to 85°C. The possible reason, as earlier observed [24], could be due to the retardation of the rate of kaolin oxidation at elevated temperatures. Hence, the higher the temperature, the higher is the percentage of iron impurities removal [9]. At 85°C, for example, the dissolution reached 79.9 % within 120 minutes. At optimal leaching (0.5 mol L-1 C2H2O4, 85°C, 120 min-utes), the residual product analyzed by EDXRF (Table 1) affirmed the total iron impurities removal to virtually 0.0 % as compared to the raw kaolin sample. However, an increase of the Al2O3 content in the purified product, was also observed. The EDS spectra (Fig. 6) after op-timal leaching, also corroborate the total iron removal, as indicated by the EDXRF data.

Fig. 3. Morphological structure of the raw Egbeda kaolin ore.

Fig. 4. Fraction of kaolin ore reacted at different concentra-tions of oxalic acid.

Fig. 5. Fraction of kaolin reacted against leaching time at different temperatures.

Fig. 6. EDS spectra after optimal leaching showing total iron removal from the product.


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The morphological structure of the purified product, examined by SEM (Fig. 7), gave stacked layers which exist as clump, thinly sheets with smooth surface, unlike the raw ore, characterized with a disintegrated stacks layered structure (Fig. 3).

Influence of particle sizeExperiments were performed with three particle

sizes, obtained by sieving for the sample preparation when evaluating the amount of kaolin, reacted with 0.5 mol L-1 C2H2O4 at 85°C at different leaching times. The results of this investigation are summarized in Fig. 8.

From Fig. 8, it is evident that the kaolin ore with the smallest particle size, was the most leached. Apparently, the greater the particle size, the lesser the fraction, that is being dissolved owing to the increasing surface area, exhibited by small powdered particle size. For example, with 0.09 mm, 0.112 mm and 0.25 mm, the kaolinite ore reacted at 85°C with 0.5 mol L-1 C2H2O4 solution within 120 minutes is 77.9 %, 49.2 % and 32.2 %, respectively.

However, the un-leached product was analyzed by XRD and found to contain α-quartz (SiO2 : 79-1906).

Dissolution kinetics analysisThe leaching of kaolinite ore in oxalic acid solution

involves a heterogeneous reaction represented by eqs. (1) - (3). The following shrinking core models, as proposed by several investigators, were tested to describe the dis-solution kinetics of the leaching process. Assuming the kaolinite ore particles have a spherical geometry, and the chemical reaction is the rate-controlling step, the following expression is applicable:

1- (1-α)1/3 = krt (4)

Similarly, when the diffusion of the reagent through the product layer is the rate-controlling step, the follow-ing expression of the shrinking core model can be used to describe the kinetics for the ore dissolution process:

1 - 2/3α - (1-α)2/3 = kdt (5)

α is the fraction of the kaolin ore reacted; t - the leaching time (min), kr and kd are the reaction rate constants, for the chemical and diffusion controlled reactions, respectively [22, 24, 26 - 28]. The dissolution data in Figs. 4, 5 and 8 were appropriately fitted to the shrinking core model of eqs. (4) and (5). It is important to note that the dissolu-tion data fitted perfectly with model eq. (4), where an average correlation, R2 value of 0.976 was obtained, as compared to the R2 value of 0.529 obtained with eq. (5). Hence, all dissolution data in this study were treated with the chemical control model expression. For the reaction order determination, the dissolution data in Fig. 4 were linearized with model eq. (4) as seen in Fig. 9.

Fig. 7. SEM image of the leached kaolin ore at optimal leaching.

Fig. 8. Fraction of kaolin reacted against leaching time for different particle sizes.

Fig. 9. Variation of 1-(1-α)1/3 against leaching time for various oxalic acid concentrations.


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c. Duncan Analysis for Influence of reaction temperature.Tempt. Time

N Subset 1 2 3 4 1

.0000 5 .000000 5.0000 5 .119400 10.0000 5 .191800 30.0000 5 .337400 60.0000 5 .467600 120.0000

5 .527200

Sig. 1.000 .220 1.000 .310 Means for groups in homogeneous subsets are displayed,

Based on Type III Sum of SquaresThe error term is Mean Square(Error) = .008.a Uses Harmonic Mean Sample Size = 5.000.b Alpha = .05.

d. Duncan Analysis for Influence of Particle size.Particle size Time

N Subset 1 2 3 4 1

.0000 3 .000000 5.0000 3 .136667 .136667 10.0000 3 .185333 .185333 30.0000 3 .297333 .297333 60.0000 3 .428667 .428667 120.0000

3 .531000

Sig. .065 .103 .156 .259 Means for groups in homogeneous subsets are displayed.Based on

Type III Sum of Squares The error term is Mean Square(Error) = .011.

From Fig. 9, the rate constants, k were determined from the slopes of each acid concentration and were used to construct the Arrhenius plot ln k versus ln [C2H2O4]. The slope of the resulting plot (Fig. 10) gave 0.47 and could be approximated as one-half order with respect to C2H2O4 concentration ranges: 0.1 mol L-1 and 0.5 mol L-1.

The dissolution data in Fig. 5 were also linearized by means of eq. (4) to obtain the plots, shown in Fig. 11.

The rate constants were evaluated as slopes of the straight lines. By using these values, the Arrhenius rela-tion in Fig. 12 was drawn, to obtain activation energy of 41.34 kJ mol-1. This value apparently supports the

Fig. 10. Plot of ln k against ln[C2H2O4].

Fig. 11. Plot of 1-(1-α)1/3 versus leaching time at different temperatures.

Fig. 12. Arrhenius relation ln k versus 1/T (K-1).

Table 2. Post-Hoc test by Duncan Univariate Anova Analy-sis using SPSS 7.1.a. Duncan Analysis for Influence of acid concentration.

Conc. mol L-1

N Subset 1 2 3 4 5 1

.0100 6 .041067

.0200 6 .082767 .082767

.0500 6 .153833 .153833

.1000 6 .207333 .207333

.2000 6 .243167 .243167 1.0000 6 .250333 .250333 .5000 6 .311833 Sig. .245 .052 .139 .258 .074

Means for groups in homogeneous subsets are displayed, Based on Type III Sum of Squares The error term is Mean Square(Error) = .004. a Uses Harmonic Mean Sample Size = 6.000. b Alpha = .05.

Leaching time, min

N Subset 1 2 3 4 1

.0000 7 .000000 5.0000 7 .111186 10.0000 7 .148571 30.0000 7 .224771 60.0000 7 .279757 .279757 120.0000 7 .341714 Sig. 1.000 .260 .102 .067

b. Duncan Analysis for Leaching time variation.

Means for groups in homogeneous subsets are displayed,Based on Type III Sum of SquaresThe error term is Mean Square(Error) = .004.


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proposed surface chemical reaction mechanism for the leaching process. Finally, the kinetic data accuracy exam-ined by the Univariate Anova Analysis, using SPSS 7.1, corroborates the results obtained in this study (Table 2 -d).

CONCLUSIONS

Based on the results obtained in this study, the fol-lowing conclusions can be drawn:

The chemical and mineralogical studies showed that the Egbeda kaolin ore contain primarily impure kaolinite (Al2O3.2SiO.2H2O) and quartz (SiO2).

The effects of oxalic acid concentration, reaction temperature and particle size on the improvement of the ore whiteness were evaluated, and the rate of the ore dissolution was found to increase with increasing acid concentration, reaction temperature and decreasing particle size.

At optimal leaching (0.5 mol L-1 C2H2O4, 85°C, 120 minutes), the dissolution reached 79.9 % when total iron removal was achieved as evidenced by EDXRF and EDS results. The un-leached product, as evidenced from XRD analysis, contains α-quartz (SiO2 : 79-1906).

The dissolution data for iron removal monitoring were analyzed by shrinking core models to follow the surface chemical reactions and the calculated activation energy of 41.34 kJ mol-1 supported the proposed model for the dissolution process. Also, the dissolution kinetic data accuracy using Post-Hoc test by Duncan Analysis (Univariate Anova Analysis – SPSS 7.1) corroborates the results of this study.

Finally, the oxalic acid leachant is effective in the treatment and removal of iron impurities from the kaolin ore of Nigeria origin.

AcknowledgementsThe authors wish to thank Miranda Waldron of the

Centre for Imaging & Analysis, University of Cape Town, South Africa, for assisting in SEM and EDS analyses.

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