Thermochemical Purification, Technical Properties, and Characterization of Ethiopian Diatomite from Adami-Tulu Deposit

  • Yonas WeldemariamEmail author
  • Dirk Enke
  • Denise Schneider
  • Esayas Alemayehu
Original Article


This article presents the purification and characterization of several application-oriented properties of Ethiopian diatomite mineral collected from Adami-Tulu deposit. The diatomite samples were purified by thermochemical method conducted in 6 M H2SO4 at 360–368 K for 48 h. The chemical, mineralogical, textural, morphological and thermal properties of the raw diatomite and treated diatomite were then characterized by X-ray diffraction, X-ray fluorescence, scanning electron microscopy, dynamic light scattering particle size analyzer, Fourier transform infrared spectroscopy, thermogravimetry analyzer, Inductively coupled plasma optical emission spectrometry, N2 sorption techniques, and Mercury intrusion porosimetry. Based on the results, the diatomite was composed of mainly amorphous silica, and a small amount of quartz, feldspar, cristobalite and montmorillonite. The diatom frustules showed cylindrical and platy morphological shapes. The treatment for 24 h decreased the impurities, and increased the silica content and Brunauer–Emmett–Teller surface area to 95 wt% and 29 m2/g, respectively. Moreover, interconnected mesopores and a significant amount of macropores were found. From the obtained results, it can be concluded that the diatomite from Adami-Tulu region is moderate in quality, and the application of a hot acid leaching was a valid approach to produce high-quality diatomite, which can be used in several industrial applications.


Characterization Diatomite Purification Technical properties 

1 Introduction

Diatomite, which is also known as diatomaceous earth is a light and chemically inert mineral that mainly consists of silicon frustules called diatoms. The diatoms have various forms and sizes, usually ranging between 5 and 200 µm in diameter. They are unicellular alga of phylum Bacillariophyta [1, 2]. The main component of diatomite is amorphous silicon hydrate with different water content (SiO2·n H2O). The content of SiO2 is mostly above 80 wt%. Of course, graduation into diatomaceous clay silt is common. Besides, diatomite minerals consist significant quantities of organic substances, quartz, pyrite, calcite, inorganic oxides, clay and volcanic ash [1]. Due to its porous structure, diatomite has numerous useful characteristics such as chemical stability, low bulk density and high absorption capacity. These features enabled diatomite to be industrial valuable as a filter aid, absorbent, thermal insulator, catalyst support and additive for numerous other purposes [3, 4, 5].

Although diatomite has unique properties; the pores of raw diatomite (RD) usually contain several types of impurities, which block some pores thus reducing its commercial applicability. Naturally occurring chemical process during their formation and environmental conditions are determinant factors for the nature and impurity content of diatomite deposit [6, 7, 8]. The choice of diatomite purification technique mainly depends on the type of impurities in it. Several approaches, such as acidification, calcination, classification and ultrasonic treatment have been used to purify RD for various applications [9, 10].

Ethiopia has substantial diatomite resources through the country, but deposits of commercial value are mainly found in the Rift Valley and the Afar depression. The Rift Valley includes the Adami-Tulu, Gade-Mota, Chefe Jilla and Abiyata deposits together amount to more than 40 million tons. The Adami-Tulu diatomite deposits are of lacustrine origin and Tertiary to Pleistocene in age deposited in or near primary volcanic structures [4, 12].

A brief report by the Geological Survey of Ethiopia regarding the chemical composition shows that Ethiopian diatomite minerals in the central Rift Valley contain water-soluble salts, carbonates, and other sandy fraction matters [11, 12]. However, the essential properties necessarily needed by the industry, such as thermal behavior, mineralogical, morphological, microstructural and textural properties of the Ethiopian diatomite have not been studied yet. Hence, the Ethiopian diatomite needs to be analyzed and modified to utilize it in industrial applications. Therefore, the objectives of this study were to purify RD mineral via a thermochemical method and investigate its full application-oriented properties using different advanced characterization techniques.

2 Materials and Methods

2.1 Materials

Raw diatomite sample (RD) was collected from Adami-Tulu, Ethiopia which is found 168 km south-east of Addis Ababa by way of the Geological Survey of Ethiopia. Since there is considerable variation in specimens of diatomite minerals even from the same deposit area, in most cases selective mining is recommended to enable a consistent product to be studied. Thus, the simple random sampling method according to the standard guide for soil sampling (ASTM D4700-91(2006)) was used to collect diatomite samples. Additionally, wax, 98% H2SO4, 40% HF, 65% HNO3, 32% HCl and KBr analytical grade reagents were obtained from ACROS ORGANICS, Steinheim, Germany. Distilled water was used in all experiments.

2.2 Purification of Raw Diatomite

The purification of diatomite samples from crude-diatomite mineral was adopted from Goren et al. [10], with certain modifications, aiming at producing quality diatomite by removing more amount of alumina and other metal impurities.

First, the RD mineral was washed, grounded by attrition alumina balls mill for 1 h, dried at 393 K for 8 h and then sieved by 45 µm apertures. The resulting fraction was subjected to hot-acid leaching and further characterization. The classified RD was treated by leaching process conducted in a 1000 ml three-neck bottom round glass container using 6 M sulphuric acid for a predetermined leaching time (1, 3, 6, 12, 24 or 48 h). The leaching process was realized under the following conditions: a diatomite to acid ratio of 1:20 (g/ml), a stirring speed of 1000 rpm (Mechanical stirrer, RW 20, Staufen, Germany) and a temperature of 360–368 K. During the treatment, a thermostat was used to hold the process at a constant temperature. After the treatment, the solid product was separated by vacuum filtration. The substantial part washed with double distilled water until the pH of the residue became neutral and dried in an oven at 393 K overnight. These samples were designated as treated diatomite (TD).

2.3 Characterization of Diatomite Samples

To investigate the fundamental properties of Ethiopian diatomite before and after thermo-chemical treatment; various advanced characterization techniques were employed. Such as X-ray fluorescence (XRF), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), mercury intrusion, thermogravimetry analyzer (TGA), scanning electron microscopy (SEM), inductively coupled plasma optical emission spectrometry (ICP-OES), N2 sorption and particle size analyzer. The detailed characterization procedures are stated below.

The FTIR spectra were measured using an FTIR spectrophotometer (Perkin-Elmer, 2000, Uberlingen, Germany) and conducted in the range of 4000–400 cm−1. The analysis was done using transmission on KBr pellets with 1% of the sample. Thermal analysis of the RD sample was performed using a TGA thermal analyzer (SDT 2960, TA instrument, USA). The sample was heated from 293 to 1373 K at a constant heating rate of 283 K min-1.

XRF analyzer (S4 Explorer, WDXRF, Bruker AXS, Germany) was used to identify the chemical composition of diatomite before and after the thermo-chemical treatment. Before the analysis, samples were prepared by mixing 0.70 g of diatomite with 0.30 g of wax; then the mixture was pressed into a 25 mm diameter disc at a force of 50 tons for 1 min in a hydraulic press (Perkin-Elmer, Germany). The content of Na2O in the RD and TD was measured using ICP-OES (GmbH, Kleve, Germany) instrument. Before the analysis, 0.1 g of solid sample was weighed and digested together with 8 ml 40% HF, 2 ml 65% HNO3 and 2 ml 32% HCI inside a microwave oven (800 W) for about 40 min, the digest was then cooled and diluted to 250 ml, samples were prepared accordingly.

XRD patterns were collected using a GE Seifert, Germany XRD machine, with Cu-Kα radiation operating at 40 kV and 30 mA. The scanning range was 2θ = (10°–80°) in the Bragg-Brentano geometry, with a step width of 0.02° 2θ and a step time of 6 s. Then, the mineralogical analysis was conducted with the help of ICDD-PDF2 database in the DIFFRACSUITE EVA-XRD software, Bruker, Rietveld-based Seifert Auto Quant software, and Match software.

An Ultra 55 (ZEISS, Germany) type of SEM working at 10 kV was used to visualize the surface morphology of the diatom frustules present in both the RD and TD samples. The textural properties of the samples were determined using an automated surface area and pore size analyzer (ASAP 2010, Micromeritics, GA, USA). Brunauer–Emmett–Teller (BET) method was used to evaluate the specific surface area [13]. For this purpose, the samples were outgassed at 523 K, first. The resulting isotherms were analyzed in the relative pressure (p/p0) range between 0.05 and 0.25.

The change in particle size of diatomite before and after the thermo-chemical treatment was measured using a particle size analyzer (CILAS 1064, Quantachrome Instruments, Florida, USA) via static laser light scattering, and the results were calculated using Fraunhofer diffraction theory. Each value is the average of three individual samples with their standard deviations. To evaluate the meso- and macropore volumes and their size distributions, mercury intrusion (Pore-Master, Quantachrome, USA) was used. The analysis was done at 484 erg cm−2 mercury surface tension and a contact angle of 141.3°.

3 Results and Discussion

3.1 IR and Thermal Analysis of Raw Diatomite

The IR spectrum of RD is presented in Fig. 1. From the IR spectrum of the RD, two central absorption peaks were identified at about 3425 cm−1 and 1630 cm−1. The broad band at 3425 cm−1 is due to the stretching vibrations of absorbed water and the presence of adsorbed water in sheet-silicate minerals, as well as the H2O of Opal A and confirmed by the H–O–H bending vibration band observed at 1630 cm−1 [14]. However, as a result of the thermochemical purification this peak disappeared in case of treated diatomite sample (TD24). Moreover, the peak at 1637 shows slightly shifts in position and this confirms that the crystalline water is still remains in the sample even after the thermochemical treatment. The sharp peaks at 472 cm−1 and 1095 cm−1 in the RD sample represent the asymmetric stretching modes of Si-O-Si bonds, and the band at 797 cm−1 and 800 cm−1 may attribute to the stretching vibration of Al-O-Si bond present in the amorphous silica, quartz, and feldspar [14, 15]. It can also be evidenced from Table 1.
Fig. 1

FTIR spectrum of raw diatomite (RD) and treated diatomite (TD-24)

Table 1

Major IR peaks of raw diatomite (RD) and treated diatomite (TD-24)

S. no.


Functional group


3425 cm−1 and 1630 cm−1



1637 cm−1



472 cm−1 and 1095 cm−1



797 cm−1 and 800 cm−1


A typical TGA curve representing the thermal behavior of RD is given in Fig. 2. In the TG graph, a 5.6% weight loss was observed at a temperature of 583 K confirming the loss of surface humidity and bound water molecules. The later might be occurred due to the transformation of Opal A to cristobalite and quartz [8, 15].
Fig. 2

Thermogram of RD

The weight loss continued up to 1373 K, and a total of 7.9 wt% loss was registered. However, when the temperature was kept constant at 1373 K for 1 h, a 0.5 wt% loss was shown, which approves the transformation of Opal A into cristobalite thereby the formation of strong crystal bonds [16].

3.2 Chemical and Mineralogical Studies

Treatment with sulphuric acid is done aiming to remove the inorganic fractions. Table 2 shows the chemical composition of diatomite before and after purification. As expected, the RD is primarily comprised of SiO2, with some other oxides such as Al2O3, Fe2O3 and CaO, whereas all other major and the trace elements are in low concentrations. Due to the high iron content, the RD sample had a reddish color, with increasing leaching time the intensity of reddish color decreased. After 24 h treatment, the reddish color changed into slightly yellowish white. The somewhat yellowish color might be caused by the small remaining iron content in the diatomite samples after thermos-chemical purification (Table 2). During the acid treatment, the dissolution rate of the impurities varies with the change of leaching time. The percentage weight loss as a function of leaching time is depicted in Fig. 3. As can be seen, the dissolution rate is very high in the first 3 h (A–B) and nearly 13 wt% dissolution was attained. This fast decomposition might have resulted from the diminishing of the weakly bonded and soluble oxides from the surface of diatomite [17]; the dissolution continues up to 24 h (B–C). Due to the removal of montmorillonite in this region, the crystalline phase of the sample undergoes considerable change. Then after 24 h (C–D), the rate of removal of the impurities became very slow, and three crystalline phases quartz, cristobalite and feldspar are detected in the TD.
Table 2

The chemical composition of diatomite before (RD) and after treatment (TD)

Composition (wt%)


Treated diatomite/leaching time (h)






























































aNa2O detected by ICP-OES

Fig. 3

Percentage dissolution of raw diatomite to leaching time

Slightly higher values of alumina, iron and calcium shown in the RD can be attributed to the association of volcanic ash, clay and calcareous concretions present in the Adami-Tulu deposit [12]. However, after 24 h hot acidification, the impurities, particularly the metallic oxides are significantly reduced. Consequently, the silica content increased from 81.6 to 95.8 wt% (Table 2). This result confirms that purification via hot acid H2SO4 does not change the mineralogical structure (polymorphy) of the RD [17, 18]. The complete removal of montmorillonite during the purification ensued in a reduction of the Al2O3, CaO, MgO and Na2O content in the TD sample (Fig. 4, Table 2). The low residual concentration of these oxides can be assigned to the existence of feldspar species as confirmed by XRD.
Fig. 4

The XRD patterns of RD and TD24 (Q quartz, C cristobalite, Fs feldspar, M montmorillonite)

The XRD patterns of RD and TD24 (after 24 h hot acidification) are shown in Fig. 4. No characteristic peaks were observed in the 2θ range from 5° to 10°, thus data are presented over a range of 10°–80°. The Ethiopian RD collected from Adami-Tulu contains a high amount of amorphous silica, 81 wt% and can be classified as moderate in quality [4]. However, some necessary amounts of crystalline phases were also detected. Quartz and feldspar were the main crystalline phases; cristobalite and montmorillonite were the minor constituents. The amorphous silica and clay materials are represented by the wide-ranging peak 2θ = 18°–26° in the XRD patterns, while the mineral impurities are characterized by a group of peaks with lower intensity.

Goren et al. [10] have reported the possibility of a change in chemical and phase composition of diatomite after acid leaching. As it is confirmed by the XRD analysis (Fig. 4), after 24 h leaching treatment, the organic impurities and clay materials, including montmorillonite, were utterly removed; whereas, quartz, cristobalite and feldspar are detected in the TD samples. Moreover, as confirmed from the XRF analysis (Table 2) of the TD24 sample, the amorphous silica was increased, significantly. The result agrees with the findings of a previous study by Chaiserna et al. 2004 [19] and is consistent with the XRD result in Fig. 4.

Since the TD samples have a particle size of > 2 µm (Table 3), there was not sedimentary amorphous silica (which is characterized by a particle size below 2 µm) in the sample. Accordingly, the broad peaks in the range of 2θ = 18°–24° appeared in the XRD patterns of TD24 belong to an only amorphous silica phase of the diatomite [3]. This confirms that a more amorphous phase was induced in sample. The shape of the peak of the amorphous phase became broader because during stirring for 24 h, the “hard” components in the solvent medium tended to enable the breakdown of coarse diatomite particles [17].
Table 3

Variation of BET surface area and average particle size with leaching time



Treated diatomite/leaching time (h)






BET (m2/g)







Average particle size (μm)







Comparing the XRD patterns of RD and TD, neither new peaks nor peak shift was detected; this indicates that no noticeable chemical reactions occurred and that new species were not generated. Nevertheless, the peak intensity of the TD was slightly better than RD. The change in the peak intensity could be due to the higher efficacy of impurity removal from the surface and more likely it reflects a change of Opal A to quartz thereby leading to an increase in the crystal order [15, 19]. Additionally, the feldspar in TD is expected to be formed due to the back-formation under the consumption of amorphous silica. The dissolution of a part of amorphous diatomite during the leaching also increases the amount of quartz in the treated samples [8, 19].

3.3 Morphological and Microstructural Analysis

Selected SEM images of raw Fig. 5a, b and TD are presented in Fig. 6C1–C3. The morphological analyses were done to check the ability to produce high quality diatomite particles and to preserve the porous structure integrity of diatom during the applied hot-acid treatment. It is confirmed that the original geometry of the pores and morphology were preserved after the thermos-chemical purification (Figs. 5, 6). From the SEM analysis, different diatom types and microfossils having a complex structure with various pore shapes and sizes were observed. Mainly, the Ethiopian diatomite frustules have shown a cylindrical (Figs. 5a, 6C1) and a platy morphological shape (Figs. 5b, 6C3) [20].
Fig. 5

SEM images of raw diatomite RD

Fig. 6

Magnified SEM images of the Ethiopian diatom obtained after 24 h leaching treatment (TD24)

Moreover, the diatomite exhibits small size diatom frustules with closed pores which are dispersed in clay and some other fine particles (Fig. 5a). Two types of pore shapes were identified. Round pores in the range of 30–690 nm (Fig. 6C1) and slit-shaped pores with diameters in the range between 195 and 600 nm (Fig. 6C2) [21, 22].

After the investigation of the morphology of the diatomite samples, it would be interesting to study the textural properties of the diatomite before and after treatment. Due to the impact of the naturally occurred chemical processes and environmental conditions, the physical and chemical features of diatomite varied from one to another deposit [4, 8]. Accordingly, the values of the specific surface areas of RD from Taiwan, China, USA, and Jordan deposits were found to be 4, 12, 25 and 33 m2 g−1, respectively [16].

3.4 Particle Size Distribution and Pore Structure Analysis

Figure 7 shows the nitrogen sorption isotherms of the RD and TD24 samples. Based on the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, the RD shows a type II isotherm with a type H3 hysteresis loop which is resulted from the occurrence of macroporous materials in the RD sample. Whereas the treated ones show a type IV isotherm with a closed hysteresis loop [23]. Since there is no plateau at high relative pressures (p/p0), the small hysteresis loop observed in the RD didn’t indicate a type IV isotherm [24].
Fig. 7

Nitrogen sorption isotherms of RD and TD24

The nitrogen adsorption isotherm of TD24 has shown a small lap at a lower relative pressure p/p0 = 0.03. Compared with the RD, the TD24 sample is characterized by a slightly modified shape of the isotherms, and an increased nitrogen adsorption. This indicates the improvement of the mesoporous structure of diatom, which is also confirmed by the mercury intrusion result [24, 25]. After thermo-chemical treatment, the BET specific surface area of the diatomite increased as the function of leaching time, and a specific surface area of 29 m2 g−1 was achieved after 24 h treatment. The surface increment could be associated with the breakdown and intensive loss of the larger particles, opening of closed pores, and the presence of the large concentration of amorphous phase diatomite in the TD24 sample [17, 24].

The particle size distributions for RD and the treated samples were investigated by DLS particle size analyzer. Table 3 summarizes the results of particle size analysis of RD and TD samples. In fact, the acid treatment was applied to remove impurities in the crude diatomite. However, the treatment alters the particle size distribution of the samples.

When comparing the particle size distribution between the RD and TD samples, it is observed that TD possesses the lower value in the average particle diameter. The average particle size of diatomite decreased with increasing leaching time and an average particle size of 17 µm was obtained at the most extended leaching time (48 h). This can be explained by a breakdown of large particles during the leaching process [17].

The pore size and volume distributions of Ethiopian diatomite before and after treatment determined by mercury intrusion are depicted in Fig. 8 and Table 4. Mercury intrusion can be used to identify meso/macropores inside the particles as well as the interparticle volume [24].
Fig. 8

Pore volume distribution of raw diatomite (RD) and after 24 h thermochemical treatment (TD24) determined by mercury intrusion

Table 4

Textural properties of diatomite samples before and after treatment obtained from mercury intrusion


dmeso (nm)

Vmeso (cm3/g)

dmacro (nm)

Vmacro (cm3/g)

£total (%)

Vtotal (cm3/g)















After thermo-chemical treatment, a significant modification of the textural properties was induced which was manifested by a wide range of pore size in all diatomite skeletons covering meso- and macropores between 30 and 690 nm. The mesopore diameter increased while the total cumulative pore volume and interparticle diameter decreased due to the reduction of the particle size (Table 4). The results can also be supported by SEM images depicted in Fig. 6.

4 Conclusion

A first complete characterization of Ethiopian diatomite collected from Adami-Tulu was presented in this study. An optimized thermo-chemical method was used for the purification of RD. The most important features, such as chemical, mineralogical, thermal behavior, morphological and textural properties of Ethiopian natural diatomite minerals were studied in detail. Additionally, the impact of treatment on these features has been studied in detail using different characterization techniques including XRF, XRD, SEM, FTIR, TGA, particle size analyzer, mercury intrusion and N2 sorption.

It was shown by chemical and mineralogical analysis that the mineralogy of Ethiopian diatomite from Adami-Tulu region is dominated by the presence of amorphous silica up to 80 wt%. Besides, mineral impurities (quartz, feldspars, cristobalite, and montmorillonite) are significant components in the samples. After a thermochemical treatment, the purified diatomite showed a SiO2 content of 96 wt%. The high amount of silica is due to the presence of quartz, feldspar and amorphous diatomite in the final product. SEM investigations confirmed the enlargement of pore structure and preservation of the original morphology of diatomite after thermo-chemical treatment. Moreover, two morphological shapes of the frustules have been identified, namely, cylindrical and platy. It is confirmed by textural analysis (nitrogen sorption and mercury intrusion) that the specific surface area, mesopore diameter and mesopore volume (by N2 sorption) were increased by thermo-chemical treatment. The existence of a regular meso- and macropore structure was also confirmed. In summary, the application of a hot acid leaching is a useful approach for the purification and modification of the textural properties of low-quality diatomite minerals. As a result, the RD was refined to a material with higher silica content and specific surface area. This significantly improves the application potential.



The authors acknowledge with sincere gratefulness to the University of Leipzig, Institute of Chemical Technology for technical and materials supports. Special thanks to Aksum University and Jimma University for financial support.


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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Yonas Weldemariam
    • 1
    • 2
    Email author
  • Dirk Enke
    • 3
  • Denise Schneider
    • 3
  • Esayas Alemayehu
    • 1
  1. 1.Institute of TechnologyJimma UniversityJimmaEthiopia
  2. 2.College of Natural and Computational SciencesAksum UniversityAksumEthiopia
  3. 3.Institute of Chemical TechnologyLeipzig UniversityLeipzigGermany

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