On mechanical activation of glauconite: Physicochemical changes, alterations in cation exchange capacity and mechanisms

Journal Pre-proof On mechanical activation of glauconite: physicochemical changes, alterations in cation exchange capacity and mechanisms Rashmi Singla, Thomas C. Alex, Rakesh Kumar PII:

S0032-5910(19)30833-2

DOI:

https://doi.org/10.1016/j.powtec.2019.10.035

Reference:

PTEC 14791

To appear in:

Powder Technology

Received Date: 5 September 2018 Revised Date:

21 August 2019

Accepted Date: 1 October 2019

Please cite this article as: R. Singla, T.C. Alex, R. Kumar, On mechanical activation of glauconite: physicochemical changes, alterations in cation exchange capacity and mechanisms, Powder Technology (2019), doi: https://doi.org/10.1016/j.powtec.2019.10.035. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

On Mechanical Activation of Glauconite: Physicochemical Changes, Alterations in Cation Exchange Capacity and Mechanisms Rashmi Singla1, Thomas C. Alex1,2, Rakesh Kumar1,2 1

Academy of Scientific and Innovative Research (AcSIR), CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India 2

CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India

Graphic abstract

On Mechanical Activation of Glauconite: Physicochemical Changes, Alterations in Cation Exchange Capacity and Mechanisms Rashmi Singla1,2, Thomas C. Alex1,2, Rakesh Kumar1,2 1

Academy of Scientific and Innovative Research (AcSIR), CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India 2

CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India

Abstract Mechanical activation (MA) of glauconite [(K,Na,Ca)(Fe3+,Al,Mg,Fe2+)2(Si,Al)4O10(OH)2] has been investigated to improve its cation exchange capacity (CEC), an impediment for its direct use as nutrients (agro-mineral), notably potassium. Glauconite is mechanically activated using planetary ball milling for varying lengths of time upto 240 min. Milling induces time dependent physicochemical changes in terms of particulate characteristics (morphology, particles size distribution, geometrical and BET specific surface area), structure (crystallite size and strain; -OH/H2O bonding) and surface charge (zeta potential). It is possible to tailor the CEC of glauconite with MA. A remarkable improvement (30-fold!) in the CEC of K+ ions is possible. The effect of milling time on the exchange behaviour is ionspecific. Plausible mechanisms of cation exchange are presented in terms of the chemical nature of the ions involved and the physicochemical changes. Lastly, MA approach is compared with reported methods of potassium recovery; its superiority as a green option is underlined.

Keywords: Agro-minerals, Glauconite, Mechanical activation, Cation-exchange capacity, Green processing

1. INTRODUCTION The increase in agriculture production to meet the growing demand of global population (estimated to be 9 billion by 2050 [1]) is having a spiralling effect on the global demand for essential soil nutrients (viz. nitrogen, phosphorus, potassium (N, P, K)). Among these, the demand for potassium, critical for plant growth and crop development [2,3] is of concern for countries dependent on the imports of soluble potassium minerals, such as sylvite (KCl) or complex K-Mg chlorides and sulphates, for K-fertilisers [4–6]. In this context, potassium bearing alumino-silicate minerals, referred to as agro-minerals [7,8], have received worldwide attention as an alternative resource for potassium [5–7,9–22]. In India, the entire potash fertiliser demand (~ 4 million tonnes (N-P-K World 2016)) is met through imports since the country is devoid of any exploitable soluble potassium reserves [4]. Consequently, increasing attention is paid towards possible exploitation of indigenous K-bearing silicate deposits of nepheline syenite [4], pyrophyllite mine waste [24], feldspar and glauconite sandstone [25–29]. High priority is given to large deposits of glauconite (~2000 million tonne) available in the states of Gujarat, Uttar Pradesh, Madhya Pradesh and Rajasthan [13,27,30–34]. Glauconite, a complex hydrous silicate mineral of iron and potassium with the chemical formula ((K,Na,Ca)(Fe3+,Al,Mg,Fe2+)2(Si,Al)4O10(OH)2 with typically 4-8 % K2O) occurs in loosely consolidated sandstone (greensand). The phyllosilicate mineral has a 2:1 layered structure, one octahedral layer sandwiched between two tetrahedral layers (T-O-T layers); the octahedral sites usually contain more Fe3+ than Al3+ and significant amounts of Mg2+ and Fe2+ [7,35–39] (see Supporting Information, Supplementary Figure S1). The mineral exhibits a relatively low cation exchange capacity for K+ ions and, the exchange occurs primarily at the limiting surfaces of the crystals [40,41]. Therefore, the explored schemes for the preparation of potash fertiliser from glauconite are based on chemical processing as summarised in Table 1; for example, high temperature volatilization as KCl [27,42], roasting/calcination followed by direct use or leaching and salt preparation [25,26,43–45] and, direct leaching under hydrothermal condition [28] or with additives [46]. The treatment by strong mineral acids (HCl, HNO3 and H2SO4) replaces exchangeable cations (K+, Na+ and Ca2+) by H+ and partially extracts Al, Fe and Mg from the 2:1 layer [39,47]. The reported processes have one or more limitations in terms of temperature, chemicals, operation, effluents, number of steps, etc (Table 1). Direct use of agro-minerals, such as glauconite has been suggested as a green

option to overcome these limitations and, in addition, reap the beneficial effect on soil texture and longer retention [13,14]. The direct use of glauconite is limited by low concentration of the mineral in greensand and, slow release of potassium during soil application. Physical upgradation of glauconite sands has been achieved using mineral processing operations, namely, size reduction, sieving, magnetic separation and flotation [20,21,31,48]. Mechanical activation, the change in reactivity of solids due to physicochemical changes induced during milling, is emerging as a frontier area for the development of novel materials and greener processes [49–58]. The application of mechanical activation is reported for phosphate minerals and proposed for silicate agro-minerals [13,14]. Mechanical activation of various clay minerals has been extensively investigated [8,9,15,58–77]. Among these, few studies [58,71,73,75,76] have dealt with effect of mechanical activation on cation exchange capacity (CEC); it is a measure of the total negative charges within the mineral/soils that adsorb plant nutrient cations such as potassium (K+), sodium (Na+), calcium (Ca2+) and magnesium (Mg2+) [78,79]). Kuanysheva et al. [15] have investigated the effect of co-milling of glauconite sand and sodium dihydrophosphate NaH2PO4 mixtures on degrees of sorption for ions of manganese, copper, nickel and zinc; the scope of characterisation has been limited to XRD and thermal analysis alone and no attempt has been made to study the CEC. Literature review reveals that for mechanically activated glauconite: (a) there is no comprehensive report on physicochemical characterisation of the activated samples; (b) total and ion-specific cation-exchange capacity of the activated samples have not been investigated; and (c) there is no attempt to correlate the exchange capacity with physicochemical characteristics of mechanically activated samples. This paper is an attempt to bridge these gaps. In the paper, CEC is correlated with physicochemical properties of the samples and, a plausible explanation for exchange mechanisms is presented. Further, the significance of mechanical activation induced alteration in cation exchange capacity (notably for potassium ions) is underlined for wide spread direct application of glauconite and as a greener option vis-à-vis hitherto explored chemical processes.

2. MATERIALS AND METHODS 2.1 Materials

Glauconite sample from Guneri, Kachchh district of Gujarat (23.7847° N, 68.8481° E), India was used in this study. The as received sample was in the form of lumps (Fig. 1(a)). The lumps consisted of quartz particles embedded in a soft and friable clayey matrix (Fig. 1(b)). The friable nature of the sample and presence of easily liberated quartz particles led to the adoption of a simple beneficiation scheme involving progressive size reduction and removal of the coarse (harder) quartz particles by screening at each stage of grinding. The beneficiated, -72 mesh (i.e. -200 µm) sample (Fig. 1(c)) was used as a feed for mechanical activation experiments. The ammonium acetate (CH3COONH4) used in the study was obtained from Merck (EMPARTA grade; purity ≥97%, ≤ 2.5% H2O, residue on ignition (as sulphate) ≤ 0.02%). Distilled and deionized water was used for the preparation of solutions and zeta potential measurements. 2.2. Methods 2.2.1 Mechanical activation of glauconite Mechanical activation was carried out in a batch type planetary mill (Pulverisette 6, Fritsch GmbH, Germany) under following conditions: rotational speed – 400 min-1; milling media – stainless steel balls, 10 mm diameter; vial – stainless steel; ball to powder ratio – 10:1. During milling, the direction of rotation was reversed every 5 min. To ensure uniform grinding and to minimise heating up of vial and the material, milling was stopped every 15 min for about 10 min. The material sticking to the walls of the vial was scrapped off, mixed thoroughly and milling was resumed. The duration of milling (tMA) was varied up to 240 min. The milled samples were stored in closed plastic bottles; there has been no moisture pick up from the surrounding environment as evidenced in the particle size analysis data and TG/DTG of the stored samples (not shown here for brevity).

2.2.2 Characterization Chemical analysis For chemical analysis, the sample was fused with Na2CO3 and borax (Na2B4O7) at 950 °C, dissolved in 30% (v/v) HCl, and filtered [80]. The extract was used for the analysis. Total iron, silicon, aluminium, magnesium, titanium, calcium and phosphorus were determined using Inductively Coupled Plasma Spectrometer (Thermo Scientific iCAP 7000). Standard potassium dichromate titration method [81] was used for ferrous iron determination. The concentrations of potassium and sodium were measured by atomic absorption spectrometer (Thermo Scientific iCE3000). The selection of the chosen techniques above is made based upon the detection limit of the instrument for the element(s) concerned; due to lower spectral interference, detection limits of AAS are superior to ICP-OES for K and Na while for the other elements relevant to this study such as Si, Al, Ca, Mg, etc ICP-OES has better detection limits, and hence more accurate analysis [82].

Particulate characterization The particulate characterisation of non-activated and mechanically activated glauconite samples involved: morphology (by scanning electron microscopy with X-ray microanalysis (SEM-EDS)), particle size analysis (by laser diffraction technique), BET specific surface area and porosity (by N2 adsorption) and, surface charge and zeta potential (by light scattering). A field emission gun scanning electron microscope (Model: FEI Nova nano 430) fitted with an EDX detector (Ametek, Inc) was used to examine morphological features and record X-ray microanalysis. The samples were coated with silver to avoid charging during the examination [83,84]. The particle size analysis was carried out using Malvern laser diffraction particle size analyser (Model: MASTERSIZER S, Malvern Instruments, UK). During analysis, proper dispersion was ensured using sodium hexa-metaphosphate as a dispersant and the application of ultrasonic vibration for 15 s [85,86]. Mie theory option was selected in the software to obtain particle size distribution from the diffraction data. BET surface area and pore characteristics were determined using surface area analyser (ASAP2020, Micromeritics, USA). High purity N2 gas (Excel grade, Linde Gas) was used for adsorption desorption experiments at 77 K. N2 adsorption in the relative pressure (p/p0) range up to 0.3 was used for the estimation of SSABET [87–89]. Surface charge was measured as a function of pH in the range 1 to 6 using a light scattering based equipment, Zetasizer (Model: Nano-ZS, Malvern

Instruments, UK) having recommended particle size range of 0.3 nm to 10 µm. The measurements were carried out in water (conductivity 0.055 µS) and 0.1 M HCl and 0.1 M NaOH were employed for pH adjustment. Structural characterization X-ray powder diffraction techniques were employed for phase analysis and the study of structural changes, including degree of amorphisation, microcrystallite size and microstrain. The XRD patterns were recorded on a X-ray diffractometer (Model: D8, Discovery, Bruker, US) in the 2θ range of 7-80° at the scan rate 1°/min using CuKα radiation. The most intense peak of glauconite (i.e. (131)) was used for the determination of degree of amorphisation (Am) and, also microcrystallite dimension (MCD) and microstrain (ε). Degree of amorphisation (Am) in the milled samples vis-à-vis un-milled sample was determined using integrated intensity ratio method [89–91]. Single line analysis method as reported by Langford et al [92] was employed for the evaluation of MCD and ε. Full width at half maxima (FWHM) and integral breadth values of XRD peaks were extracted from the XRD pattern using WinPLOTR software. Fourier Transform Infrared (FTIR) spectra were recorded in the wavenumber range of 4000-400 cm-1 on a Nicolet 5700 spectrometer using KBr method. 2.2.3 Estimation of Cation-Exchange Capacity Cation-Exchange Capacity (CEC) was measured using ammonium acetate method [93]. The non-activated and mechanically activated samples were treated with 1M ammonium acetate (CH3COONH4) solution buffered at pH 7. The treatment time with the extracting solution was 5 min, unless stated otherwise. Shaking was carried out at 200 rpm while maintaining an initial solid to liquid ratio of 1:10. After the treatment, the concentration of exchangeable ions (K+, Na+, Ca2+ and Mg2+) were determined in the filtrate. The total CEC was obtained by summing up of the exchangeable capacity of individual ions and reported in terms of meq/100 g of glauconite.

3. RESULTS The main focus here is on physicochemical characterisation (particulate and structural) and, cation-exchange properties of the non-activated and mechanically activated glauconite

samples. The characterisation take into account parameters affecting potassium release, namely particle size, specific surface area and porosity, structural attributes (e.g. crystallinity/amorphisation, chemical bonding) and, surface charge [29,94]. Insight into the nature of glauconite is given first as a prelude to the presented results. 3.1 Nature of Glauconite The chemical composition of beneficiated (-200 µm) glauconite sample used was found to be as follows (in mass %): SiO2 – 48.97, Al2O3 – 8.19, Fe (total) – 15.29, FeO – 0.52, TiO2 – 0.58, MgO – 2.82, CaO – 0.59, K2O – 5.6, Na2O – 0.82, P2O5 – 0.26, Loss on Ignition (LOI) – 10.46 (see Supporting Information, Supplementary Table T1 for comparison with as received sample). Based on XRD analysis, it was established that the sample contained glauconite (JCPDS file #09-0439) and quartz (JCPDS file #46-1045) as major phases along with a potassium-aluminium-silicate mineral (JCPDS file #12-0134) as a minor phase (Supplementary Figure S2). Rietveld analysis using MAUD software [95] revealed that glauconite and quartz together account for 98.1% of the sample (Glauconite – 85.2% and Quartz – 12.9%). Glauconite particles typically exhibited a porous structure (Supplementary Figure S3). Further, X-ray microanalysis of the particles showed that: (a) the average K content of the particles (9.8±0.46 mass % K2O) was typical of glauconite as reported in literature [7], and (b) the particles were chemically heterogeneous, especially in terms of Fe and Al (an increase in Al was associated with a decrease in Fe (r = -0.81)) (Supplementary Table T2). 3.2 Particulate Characterisation 3.2.1 Particle size distribution The effect of milling on particle size distribution (PSD) is depicted in Fig. 2 for selected milling time (tMA = 0, 15, 30, 60, 120, 180 and 240 min). The distribution is expressed in terms of cumulative undersize (Fig. 2(a)) and frequency distribution (Fig. 2(b)). During 0 to 15 min, the distribution shifts towards lower particle size indicating breakage of particles. With further progress of milling, a reverse trend is observed indicating an increase in size. The frequency distribution plots show a multi-modal character. The position of the mode at the submicron size is not appreciably affected by the milling process though the population of particles shows an increase. The dominant maxima appearing at 137 µm in the un-milled sample shift to a lower size, 7 µm after 15 min of milling. With further milling, the maxima

shift to 18 and 32 µm in 30 and 60 min milled samples and attain a steady value of ~ 34 µm in 120-240 min milled samples. In the sample milled for 120, 180 and 240 min, new maxima appear above 100 µm. The results for samples milled from 30 to 240 min clearly points towards the increase in size. The increase is more clearly depicted in Fig. 3 which shows the variation of median size (d50) with milling time. Median size values for 3, 5 and 10 min are also included in Fig. 3 to elucidate effect of milling during initial stages. Median size decreases sharply and after 3 min itself attains a value of 3.4 µm from the initial value 66.3 µm. After 3 min, there is no further decrease. Instead, the median size increases to a maximum value of about 14 µm after 60 min of milling; the size got stabilized or show a slow decrease during further milling (d50 = 11 µm for tMA = 240 min). 3.2.2 Morphological changes Typical SEM micrographs in Fig. 4 elucidate changes in morphological features of un-milled sample (tMA = 0 min) after different duration of milling (tMA = 15, 60 and 240 min). The micrographs support the results of particle size analysis (Fig. 2 and 3). Breakage of coarse particles in the un-milled sample (Fig. 4(a)) is quite evident in the 15 min milled sample (Fig. 4(b)) which shows dominant presence of smaller isolated particles mostly free from agglomeration. Micrograph in Fig. 4(c) reveals that as the milling time is increased to 60 min, the particles show greater signs of fusing with one another, leading to agglomeration and rounding of shape. The 240 min milled glauconite consists mostly of fused particles of round shape indicating higher degree of agglomeration and possible compaction (Fig. 4(d)). 3.2.3 Specific surface area and nature of porosity N2 adsorption-desorption isotherms, plots of N2 adsorbed at different relative pressure (p/po) (Fig. 5) were used to determine BET specific surface area (SSABET) and understand the porous nature of the samples at different milling times. The adsorption capacity of glauconite sand increases with milling. However, this trend is reversed in the samples milled for 60 min and beyond; the adsorption isotherm for 240 min milled sample lies even below that of the un-milled sample. Based on N2 adsorption isotherms, the estimated values of SSABET were 50.1, 74.9, 64.2 and 28.3 for 0, 15, 60 and 240 min milled samples, respectively. Large difference in SSABET and geometrical surface area SSAgeo (typically, SSAgeo was 1 to 6 % of SSABET, see Supplementary Table T3) indicates porous nature of the samples, and it was probed further by analyzing the adsorption isotherms in Fig 5.

Hysteresis in the N2 adsorption-desorption isotherms (Fig. 5) is indicative of the presence of mesopores (pore size from 2-50 nm as per IUPAC classification [87]), along with macropores (pores with pore size > 50 nm) as inferred from the Type IIB nature of the isotherms [96,97]. The hysteresis loops resemble H3 type, which is indicative of slit-shaped pores originating from aggregates of plate-like particles [88]. Using adsorption-desorption data, BJH analysis [98] was carried out to obtain pore size distribution. Variations of pore area and cumulative pore area as a function of average pore diameter are given in Fig. 6 for the samples milled for different duration. The pore distribution in Fig. 6 confirms the absence of microporosity (pores less than 2 nm) as inferred from the Type IIB nature of the isotherms [96,97]. The major contribution to the surface area results from mesopores (pores in the size range of 2 to 50 nm). Increase in specific surface area of 15 min milled glauconite is mainly due to increase in the area of mesopores (Fig. 6). In the 60 min milled sample, decreased contribution from 2-3 nm and > 10 nm pores points towards onset of pore annihilation and/or coalescence which results in a net decrease in surface area vis-à-vis 15 min milled sample even though overall surface area is still larger than the un-milled sample. Significant decrease in surface area for the 240 min milled sample results from pore annihilation and/or coalescence of pores (leading to bigger pores) as indicated by a dramatic decrease in contribution from 0-10 nm pores and a possible increase in contribution from > 10 nm pores. Average pore width was estimated from total pore volume and BET specific surface area and the results are presented in Fig. 7. The pore width shows an increase with milling time; initially a sharp increase is observed which gradually slows down (Fig. 7). 3.2.4 Surface charge and Zeta potential The nature and magnitude of the surface charges govern the behaviour of the particles when they are in a liquid suspension. Since the particle size of un-milled sample was greater than the upper limit of Zetasizer, the 5 min milled sample was used for comparison. Variation of zeta (ζ) potential with pH for glauconite sand samples milled for 5, 15 and 240 min is presented in Fig. 8. For the 5 and 15 min samples, the zeta (ζ) potential values are negative in the entire pH range and no iso-electric point (IEP) is detected. The IEP is observed in the 240 min milled sample at pH ~ 2.3 (similar value for 60 min milled sample). In general, for any given pH, the ζ-potential value is more for sample that has been milled for longer duration; however, the potential values for the 15 and 240 min samples are nearly identical negative values (~ - 25 mV) at pH ~ 4.5 and above.

3.3 Structural Characterisation 3.3.1 X-Ray diffraction studies X-ray diffraction patterns of un-milled sample and the samples after different duration of planetary milling up to 240 min are compared in Fig. 9 to understand the nature of changes occurring during milling. Absence of any new diffraction peak in the milled samples indicates that no new phase formation is induced by milling. However, the nature of patterns changes continuously in terms of intensities and widths of peaks, especially for the glauconite phase. For quartz, the peaks remain practically unchanged, whereas the glauconite peaks progressively broaden and decrease in intensity with the increase in milling time. The greater changes in the intensity of glauconite peaks indicate that it is more susceptible to structural degradation vis-à-vis quartz. The most intense peaks of glauconite, i.e. (001), (020) and (131) are marked in Fig. 9(a) to highlight the decrease in their intensity; the peaks nearly vanish in the 240 min milled sample. The changes in the most intense (131) peak of glauconite which was used to quantify structural changes are amplified in Fig. 9(b). The change in degree of amorphisation (Am) and microcrystallite dimension and strain (MCD and ε) with milling time is depicted in Fig. 10(a) and 10(b), respectively. Am increases progressively with milling time and highest value of ~ 70% is observed in the 240 min milled sample. MCD is reduced with milling time, whereas a reverse trend is observed for ε; MCD value decreased from 19.2 nm to 8.1 nm and ε value increased from 4x10-3 to 13x10-3 after 240 min of milling. MCD and ε are all indicative of increase in grain boundary area and defects induced by plastic deformation, respectively [58,99]. 3.3.2 FTIR studies FTIR spectra of un-milled and milled glauconite samples are presented in Fig. 11 in the mid infrared region (4000-400 cm-1). Table 2 gives a summary of observed bands and their assignment to different bonds. The spectrum of the un-milled sample resembles that of glauconite mineral as reported in literature [34,36,41,100–104]. Even though basic character of spectrum remains similar after milling, the bonds get affected in terms of splitting, sharpening or shift in peak position, thus pointing towards milling induced structural changes. The bands at 3419 and 3539 cm-1 which correspond to molecular H2O and R–OH (R = octahedrally coordinated ions, Al3+, Mg2+, Fe2+, Fe3+), respectively [103–106] and appear as a single merged band in the un-milled sample split into separate bands (e.g. at tMA = 15 and 30 min) and, merge again and appear as a single diffused band on prolonging the milling.

Similarly, presence of interlayer water (not water of hydration [102,103]) indicated by the band at 1637 cm-1 is affected after milling. Thus, the IR results indicate that milling leads to redistribution of interlayer water and –OH bonding. The principal broad band which is centered around 1000 cm-1 is due to Si-O-Si/Si-O-Al lattice vibration; the asymmetry in the peak occurs due to isomorphous substitution of Si4+ ions by Al3+ ions in the structural network of tetrahedral sheet [102,103]. After milling, the peak observed at 1028 cm-1 in the un-milled sample shifts towards a lower value and the peak appearance changes. The bands below 500 cm-1 are sensitive to octahedrally coordinated ions in the structure, particularly Fe3+. Significant changes are observed in the IR spectra with increasing milling time. The disappearance of two bending bands, Si-O-Fe3+ (492 cm-1) [102] and Si-O-Mg (434cm-1) [16] is observed and they seem to merge with the stronger Si-O-Si bending band at 458 cm-1 [34]. Band positions at 808 cm-1 and 675 cm-1 which are ascribed to Fe23+ OH/ Fe2+Fe3+OH bending [38] and Si-O/R-OH bending [100,101] are also found to shift with milling. 3.4 Cation-Exchange Capacity (CEC) The total cation-exchange capacity (CEC) is defined as



= 

 

(where CECi is cation-exchange capacity of individual ions (i = K+, Na+, Ca2+ and Mg2+)). In the ammonium acetate (CH3COONH4) method [93], these ions are exchanged with ammonium ions. CEC serves as an important parameter to assess the release of nutrients and soil fertility [78,79,110]. Variations of CEC and CECi with milling time are presented in Fig. 12 and Fig. 13, respectively. The CEC of un-milled glauconite was found to be 42.8 meq/100 g glauconite with individual contributions of 2.4, 25.0, 8.7 and 6.7 meq/100 g glauconite arising from K+, Na+, Ca2+ and Mg2+ ions, respectively. The difference between the exchange behaviour of Na+ and K+ was quite remarkable; about 95% of Na+ was exchanged while there was practically no exchange (< 2 %) for K+. The percentage exchange for Ca2+ and Mg2+ was 40 and 5, respectively.

The milling changed the exchange behaviour of all the ions and the effect was ion specific (Fig. 13). The exchange capacity of potassium (CECK+), the nutrient critically relevant to plant growth and crop development, increased remarkably upto 65 meq/100 g of glauconite after 120 min milling – the increase is nearly 30-fold. With further milling, there was no substantial change upto 240 min. On the other hand, CECNa+ decreased to ~19 meq/100 g after 240 min (nearly 24% decrease vis-à-vis un-milled sample). CECCa2+ showed a decreasing trend similar to CECNa+. Interestingly, CECMg2+ displayed an initial increase, reached a maximum and then exhibited a reverse trend with prolonged milling (Fig. 13).

4. DISCUSSION The glauconite sample used in the study primarily consists of glauconite (85.2%) and quartz (12.9 %) as the major phases. The mechanical activation occurs below a critical particle size at which micro-crack length approaches particle size [50,58,111,112]. Due to weak bonding across interlayer region and inherently defect structure, the critical particle size is expected to be much larger for clay minerals (such as glauconite) vis-a-vis quartz which is characterized by a stronger Si-O bonding and critical size of 3-5 µm [58,112]. Therefore, glauconite is more easily activated as compared to quartz (Fig. 9). Quartz gets activated only after prolonged milling and the duration used in this study is order of magnitude lower than reported in literature [58]. Glauconite phase present is chemically heterogeneous in nature and, therefore, the mineral specific physicochemical parameters represent average values. Glauconite is the primary mineral present which participates in the cation-exchange reaction (contribution from minor minerals (< 2%), if any, is expected to be small and not considered). Since the sand sample contains 85.2% glauconite, the value of CEC which are presented in this paper on the basis of per 100 g glauconite sand can accordingly be converted to per 100 g glauconite mineral basis. The discussion presented here focuses on: (a) physicochemical properties of the activated and non-activated samples; (b) cation-exchange and its correlation with the characteristics of ions and the samples; and (c) significance of mechanical activation as greener option towards direct usage of glauconite. 4.1 Physicochemical Properties

Table 3 presents a summary of the physicochemical characteristics of the samples (particulate, structural and surface charge) at different milling time. The significance of various measured parameters is highlighted in the table. The changes occur concurrently with varied contributions at different milling time. 4.1.1 Particulate characterisation During milling, particles undergo breakage and agglomeration as manifested by change in particles size (Fig. 2 and 3) and morphological features (Fig. 4). Comparisons of the particle size distribution and median size for the initial (i.e. tMA=0 min) and 15 min milled (tMA=15 min) samples indicate that breakage dominates during initial stage of milling. The shift of the mode at the coarser size towards finer size (Fig. 2) is a consequence of bigger particles breaking into smaller ones (Fig. 4(a),4(b)). Relative increase in particle population occurring at the submicron mode with increase in milling time indicates that processes (chipping and attrition) responsible for the generation of such submicron particles go unhindered (Fig. 2(b)). With further milling beyond 15 min, shift in the position of mode/size distribution towards coarser size (Fig. 2) results due to agglomeration of particles as depicted by SEM micrographs in Fig. 4. Agglomeration becomes increasingly more pronounced with milling time as evident from the SEM micrographs for 60 and 240 min milled samples (Fig. 4(c) and 4(d), respectively). In general, three stages are observed during milling, namely Rittinger stage (particle size decreases linearly with milling time), aggregation stage (non-linear decrease, particle joining by weak Van der Waals forces) and agglomeration/mechanical activation stage (characterized by deformation, an increase in particle size by fusing together of particles and breakage of agglomerate) [51,58,113]. Interestingly, during glauconite milling it is observed that: (a) the breakage stage is small; (b) aggregation stage is missing; and (c) agglomeration of particles sets in very early (Fig. 3). While no previous report is available on breakage mechanisms in glauconite, similar studies on clays in general indicate that due to weak interlayer bonding involving H2O and –OH ions, delamination and sliding of layer precedes cleavage along basal planes which is followed by breakage perpendicular to basal planes/edge surfaces [58,71]. A plausible explanation for missing aggregation stage and an early initiation of agglomeration stage observed for glauconite milling may be presented based on an analogy with clays where sliding of basal planes is integral to early breakage and preferential breakage occurs along basal planes. The phenomenon of agglomeration and its manifestation in particle size variation with time is a common feature observed during high energy milling in dry mode, especially for minerals having hydroxyl ions in their structure

[71,89,114,115]. The -OH groups and chemically bound water in glauconite structure are anticipated to promote agglomeration as reported for mechanical activation of clay minerals [58]. The behaviour of -OH/chemically bound water is illustrated in the TG-DTG plots of glauconite in un-milled and milled conditions (Fig. 14(a), (b)). With an increase in milling time, the weight loss associated with glauconite dehydroxylation reactions (320 – 640 oC) [15,36] diminishes with a simultaneous increase in weight loss at lower temperatures as manifested by the shift in weight loss (TG) (Fig. 14(a)). The weight between RT-100 oC and 100-200 oC are ascribed to physical water and interlayer/interstitial water, respectively Increase in TG loss below 200 oC point towards milling induced dehydroxylation/weakening of water bonding. Similar behaviour is observed during milling of other clay systems [58,66,116] and hydroxides/oxyhydroxides [89,114]. The liberated water molecules serve as a binder and the particles get welded together as they are repeatedly stressed during planetary milling [51,58,115]. Thus, in-situ water produced during milling plays an important role in promoting agglomeration/mechanical activation. The effect of milling goes beyond particle size. The porous structure of glauconite sample is indicated by large difference in SSAgeo and SSABET (Table 3) and supported by SEM micrographs presented in Fig. 4(a) for glauconite in the un-milled sample. The BET surface area and pore structure change with milling time. The change in surface area during milling may occur due to cumulative effect resulting from the following phenomena: (a) creation of new pores due to milling induced dehydroxylation as revealed by TG-DTG (Fig. 14); (b) pore coalescence (Fig. 6); and (c) breakage/agglomeration as manifested by the change in median size and geometrical surface area (Fig. 3, Table 3). Since geometrical surface area is smaller compared to BET surface area, it may be expected that major contributions arise from (a) and (b); new pore creation dominating initially (say up to 15 min) and increasing effect of pore coalescence with prolonged milling (Fig. 6). It is interesting, inspite of pore coalescence, the average pore width increases with milling (Fig. 7). Since an increase in pore width is expected to promote diffusion, increasing pore width (Dp) and decreasing surface area are expected to have opposing effect on the exchange reactions. 4.1.2 Structural Changes Changes in particulate characteristics (d50, SSAgeo, SSABET and Dp) are associated with a simultaneous progressive structural degradation as manifested by an increase in degree of amorphisation (Am), decrease in microcrystalline dimension (MCD) and increase in

microstrain (ε) as inferred from the detailed analysis of glauconite peaks in powder XRD patterns (Fig. 9). Grinding induced amorphisation and other structural changes (e.g. crystallite size) observed in this study are reported earlier during dry grinding of other phyllosilicates (e.g. muscovite, biotite, kaoline, talc, chlorite, illite, montmorillonite, etc) [58,63,66,69,71–73,76,117]. The increase in structural degradation is expected to enhance reactivity due to stored energy associated with creation of more internal surfaces (lowering of MCD), defects (high ε) and amorphisation [50,58,99]. FTIR spectra of the un-milled and milled samples shed further light on the structural changes occurring during milling. Notable changes occur in bonds associated with interlayer water and –OH ions coordinated with cations in octahedral positions (Fig, 11, Table 2). The –OH groups play an important role during amorphisation. Milling can induce dehydroxylation reaction (–OH + –OH → –O + H2O); thus the activated solid may have –OH groups which are less tightly bound to amorphised solid, gel water, hydrate water composed of water molecules and OH group, interlayer water in liberated or in original position [51,58]. The change in the IR band corresponding to H2O and –OH groups is complex interplay of water molecules and hydroxyl group. Effectively, there is a weakening of water bonding and this is manifested in TG-DTG plot (Fig. 14) as discussed earlier to explain agglomeration phenomenon. While degradation of structure can contribute to increased reactivity due to stored energy, it is important to point out that water as a transport medium can play important role in the migration of ions and consequent structural changes during milling [51]. During milling, the triplet of bands below 500 cm-1 (i.e. 494, 457, and 442 cm-1) which is known to be sensitive to variations in the octahedrally coordinated ions in glauconite is also affected. The bonds in the octahedral layers are weaker as compared to tetrahedral layers and preferentially broken [67]. From the disappearance of the distinctly visible bands 492 cm-1 (Si-O-Fe3+ bending) and 434 cm-1 (Si-O-Mg bending) after milling (Fig.11), it can be inferred that breaking of bond for octahedrally coordinated ions and their redistribution also contribute to the amorphisation. 4.1.3 Surface charge The zeta potential (ξ) in Fig. 8 represent a composite value with major contribution arising from the nature of glauconite phase since presence of quartz in the sample is much smaller. Literature on zeta potential of glauconite is scanty [118,119]. However, based on the published literature on the non-activated and mechanically activated clay minerals

[40,71,75,120–128] which include T-O-T clays (Supplementary Figure S1), following general expression can be written for ξ : ξ = ξ  + ξ  + ξ  + ξ 



The charge on the faces (ξ  ) or basal plane (silioxane surface) is permanent charge resulting from isomorphic substitution of Si4+ by Al3+ and is pH independent. In contrast, the contribution from edge (ξ  ) results from broken bond along the edge surfaces (R-OH, R = Si, Al, Mg, Fe, etc) and varies with pH similar to metal oxides; the charge is negative at pH ≥ 3.5. The other two terms (ξ  and ξ 

 )

are of relevance in mechanically

activated clays. Unlike metal oxides, the genesis of charge in clay minerals (with permanent charge from isomorphic substitution in clay crystal lattice and variable pH dependent charge from edge charges) is complicated [122]. The contribution from structural deformation and amorphisation is expected to add further complexity. The 5 min milled sample is characterized by a negative charge in the measured pH range and does not show an iso-electric point. Selim et al [118] made a similar observation for glauconite but no explanation was presented. In other clay systems similar observation is made and attributed to dominance of ξ  over pH dependent contribution from edge surfaces ξ  [71,75,118,121,124,126,129,130]; thus the sample shows a weak dependence

on pH. The ξ potential for the 15 and 240 min milled samples attain increasingly more positive charge and greater dependence on pH. This may be attributed to increasing contribution from edge surfaces. Signs of fragmentation along the basal and edges surfaces are evident in 15 min milled sample (Fig. 4(b)). Interestingly, the potential becomes positive in the 240 min milled sample and it shows an IEP around pH ~ 2.3 (similar to reported values for glauconite and quartz, ~ 2 and 1.9, respectively [131]). The structural disorder (as manifested by MCD, ε and Am) increases with milling time (Fig. 10). An overlapping contribution from ξ  and ξ 



may be expected, however, this could not be

delineated. The nature of contribution is not well understood. Further, the distinction between face and edge surfaces is lost with increasing amorphisation. 4.2 Cation-Exchange Capacity In the glauconite structure, octahedrally coordinated ions (Al3+, Mg2+, Fe3+, Fe2+) are sandwiched between SiO4 tetrahedral layers with partial replacement of Si by Al and, successive group of these three layers are held together by K+ ions which may be partially

replaced by other ions such as Na+ and Ca2+ (Supplementary Figure S1). The cations, K+, Na+, Ca+ and Mg2+ can undergo cation exchange with NH ions. Typical, exchange reaction with NH4+ ions can be expressed as follows: Θ| +  ⇄ Θ| +  Θ|



+

2 ⇄

 Θ| +  



Where Θ| " indicate ions in exchangeable position. The ions have large negative enthalpies of hydration (–350 to –2000 kJ/mol) and in reality surrounded by varying numbers of water molecules [132]. The exchange behaviour is determined by the nature of the ions (e.g. valence, ionic radii, free energy of hydration), occurrence in the structure of the host phase (lattice substitution, site occupancy/coordination number, sorption, etc) and, physicochemical properties of the solid phase (surface area, porosity, surface charge) [40,41,58,71,75–77,128]. The characteristics of the exchangeable ions, namely valance (Z), ionic radius (r), Z/r ratio and free energy of hydration are summarized in Table 4. In principle, NH ions having lowest Z/r ratio and lowest free energy of hydration (least stable) can replace all other ions listed in the table [40,134,135]. In terms of CECi (meq/100 g of sample) of the ions for the un-milled sample, it is observed that CECNa+ (25) >> CECCa2+ (8.7) > CECMg2+ (6.7) > CECK+ (2.4). Similar sequence is followed for the % of ions exchanged; the values (in % exchanged) are 95, 40, 5 and ~ 2 for Na+, Ca2+, Mg2+ and K+, respectively. Except for potassium ions, the CEC of other ions show a broad correlation with the parameter listed in Table 4, that is, a decrease with an increase in Z/r ratio and more negative value of free energy of hydration. Fixation of potassium in clays is well recognized [40] and different fixation mechanisms are offered. Potassium with an ionic diameter of ~ 0.266 nm can fit into the hexagonal cavity of the basal oxygen layer that has about the same size and, this makes the potassium difficult to replace [40,134,136]. Three different types of exchangeable sites are recognized in clays, namely basal surface, edge-interlayer and interlayer sites and, potassium release from interlayer sites is extremely slow [134]. Based on an extensive study on CEC of glauconite, Manghnani and Hower [41] concluded that virtually all the potassium was fixed (CECK+ = 0.7 – 2.5 meq/100 g, average value ~ 1 meq/100 g). Fixation of potassium in well specific crystallographic sites is supported by several studies [137–141]. A significant fraction of the potassium sites may be vacant [141]

and heterogeneity of coordination (octahedral and prismatic) is indicated [137,138]. Thus, the observed distinct behaviour of potassium may be explained from its fixation in the glauconite structure. Based on the variability of the composition of glauconite during X-ray microanalysis (Supplementary Table T2), it was inferred that the elements showing less variability in composition (i.e. K and Mg) may be dominantly present as a part of structure vis-à-vis those showing large variability (Na and Ca) and likely to be present in sorbed state. The low value of CECMg2+ may also be ascribed to its presence as part of structure in octahedral sheets [103,137,138]. The exchangeable ions show ion-specific response to milling (Fig. 13). The CECK+ increases dramatically with milling time. In contrast, a decrease is observed for CECNa+ and CECCa2+. CECMg2+ shows an initial increase and decreases with prolonged milling. The change in CEC may be ascribed to physicochemical changes induced during milling. For phyllosilicates, it has been reported that the CEC is affected by two counteracting processes: (a) fracture of the particles involving an increase in number of exchangeable ions; and (b) agglomeration of the particles which results in the covering of sites by neighbouring particles [58]. Based on this surmise, we can infer that enhancement in the CEC for K+ which is fixed within the glauconite structure is mostly due to fracture of particles and, agglomeration plays a key role for Na+ and Ca2+ present in sorbed state and not tightly bound to the structure. Both the factors contribute in the case of Mg2+, breakage during initial stage and agglomeration with prolonged milling. The effect of milling is not confined only to breakage and agglomeration and, the glauconite sample also undergoes changes in terms of BET surface area, structural degradation and other parameters (Table 3). In order to assess the effect of physicochemical changes during milling, binary correlations between CEC and physicochemical parameters (rxy, where x = CECi or CEC, and y = parameters listed in Table 3) were calculated and the values are given in Table 5. Since zeta potential for the activated samples converged to a common value at pH ~ 4 and above, it was not considered. The correlations were examined in terms of their physical significance. In general, reactivity of a solid increases with specific surface area. Thus, the negative correlation between CECK+ and SSABET is untenable indicating that the CECK+ is controlled by other variables. High positive correlations with particulate characteristics (e.g. SSAgeo, Dp) are quite understandable since increase in external surface and pore diameter are expected to play a positive role in

terms of number of K+ ions that are exposed and favourable condition for mass transfer, respectively. Interestingly, the observed correlations with Am, MCD and ε indicate that, in addition to particulate characteristics (SSAgeo, Dp), parameters associated with structural degradation significantly influence the CECK+. It is likely that the energy accumulated due to an increase in grain boundary areas and the defects weakens potassium bonding and promote its exchange [77]. Similar increase in CEC with decrease in crystallite size has been reported for kaolin samples of varied origin [142]. The correlations observed for CECNa+ and CECCa2+ are similar and quite opposite of CECK+. In the first glance, only the correlation with SSABET appears to be physically significant. This means that the mesoporous structure of glauconite (Fig. 6) plays an important role in the exchange of Na+ and Ca2+ ions. The negative correlation of CECNa+ and CECCa2+ with average pore diameter (Dp) is unexpected since an increase should favour mass transfer and consequent improvement in exchange capacity. It is likely that the increase in Dp is offset by decrease in SSABET during prolonged milling (Fig. 7, Table 3) or the entire range of porosity is not relevant from the point of view of the diffusion of these ions. External surface (as manifested by SSAgeo) and structural degradation (as manifested by Am, MCD and ε) apparently show correlations which defies common wisdom as indicated by the relevance of various physicochemical properties (Table 3). Such anomalous correlations are possible if the physicochemical properties have counteracting roles. For example, Kumar [143] and Kumar and Das [144] observed an anomalous negative correlation between specific surface area and removal of sorbed/ion-exchanged ions present in the sheet structure manganese minerals. The correlation was explained by dual counteracting roles of surface area; a positive role as is customary (i.e. increase in reactivity with an increase in surface area) and, a negative dominating role arising from the enhanced destruction of sheet structure and formation of a more compact structure in which interlayer region becomes complex and hinders the migration of ions from solid to solution phase. Laws and Page [68] reported that dry grinding leads to a decrease in exchangeable Na+ and Ca2+ from kaolinite. They established that even though Na+ and Ca2+ are liberated from the structure, the ions subsequently get fixed in a new phase (permutite precursor) formed during milling. In the present study, no new phase formation was indicated in the powder X-ray diffraction patterns of milled samples (before and after ion-exchange with ammonia). However, the migration of Na+ and Ca2+ from easily exchangeable sites to newer sites from where exchange becomes difficult is a possibility. During milling, it may occur through dehydroxylation and structural collapse [125],

redistribution of ions among edge, face and interlayer sites [145] and masking by adjoining particles during agglomeration [58]. The migration during ion-exchange may be favoured by increasing structural disorder or stored energy. This hypothesis gets credence from the CEC determinations in which the equilibration time was increased from the standard 5 min to 3600 min (6 h). CEC decreased with an increase in equilibration time indicating reverse migration of ions into the solid phase. The decrease for Na+ and Ca2+ was found to increase with milling time; as the milling time increased from 0 to 180 min, decrease in CEC for Na+ changed from 48 to 60%, respectively and, correspondingly for Ca2+ it changed from 37 to 52 %. Such decrease in CEC is attributed to hysteresis in exchange with ammonia resulting from site specificity for different ions [145]. For the sake of completion, it may be added that the decrease was also observed for K+ and Mg2+, however, the decrease was less and close to 30 % in all the cases. The correlations observed for CECMg2+ are similar to that of CECK+ except that values of correlation coefficients are lower (Table 5). This is expected as Mg2+ ions substitute Al3+ in the octahedral sheets and occur as part of basic T-O-T framework. The correlation between CECtotal and physicochemical properties of the samples are also similar to CECK+. This may be ascribed to dominant contribution arising from the exchange of K+ ions (Fig. 13).

4.3 Green Chemistry Perspective As evident from Fig. 12, the CEC of glauconite can be tailored using mechanical activation. The CEC results were further validated for another sample with a different chemistry (Supplementary Figure S4). Thus, mechanical activation offers the possibility for wide spread direct applications of glauconite as a source of potash and other nurtients [3,10,11,13,14,20,27]. Table 6 compares and highlights the importance of mechanical activation based scheme vis-a-vis hitherto reported processes; namely, high temperature volatization processes, pre-processing (e.g. roasting, sulphation, etc) and leaching and, direct chemical leaching with additives or under hydrothermal conditions (Table 1). The parameters used for comparison include: resource (chemicals, water), process, energy, effluents and environmental impact, etc. Based on Table 1 and 6, the superiority of mechanical activation of glauconite from the green chemistry perspective is understandable. The process offers advantages in terms of resource conservation, process simplicity, room temperature operation, and, no effluent generation. Further versatility can be added to the process by simultaneous milling of glauconite with the source of N and P (e.g. ammonium phosphate) and any other need based nutrient [15]. Having established mechanical activation of glauconite as a ‘proof of concept’ it is necessary to chalk-out a full strategy for its implementation. The planetary mill used in this study has limited prospect from the points of view of scale-up. This barrier may be overcome by using other mills, such as eccentric vibratory mills which are available in several tonnes per hour capacity [54,146]. The efficacy of these mills can be further improved by judicious coupling with traditional ball mill and improved beneficiation schemes. Since no chemicals are required, the mechanical activation on mine site requires consideration to save transportation cost. Large scale field trials and long term impact assessment would also be necessary for implementation.

5. CONCLUSIONS Following are the major findings/conclusions that follow from this study: 1.

During mechanical activation by planetary milling, glauconite is preferentially activated vis-à-vis quartz which is present as another major phase in the glauconite sand used in this study.

2.

Milling induces physicochemical changes in glauconite/glauconite sand. The changes include: (a) agglomeration as manifested by change in morphology and particle size distribution; (b) surface area and porosity; (c) structural changes in terms of amorphisation, microcrystallite dimensions and microstrain; (d) dehydroxylation and change in the nature of structural water; and, (e) surface charge.

3.

The cation exchange capacity can be tailored by mechanical activation. (a)

Significant increase in CEC is possible from 43 to 105 meq/100 g with an increase in milling time to 120 min.

(b)

The increase in total CEC with mechanical activation is essentially attributed to increase in CEC for K+ ions, which showed ~ 30-fold increase from a negligible value.

(c)

The effect of milling time on the exchange behaviour was ion-specific. CEC showed an increase for K+ ions, decease for Na+ and Ca2+ and, an initial increase and decrease on further milling for Mg2+.

4

The total CEC and CEC of individual ions (K+, Na+, Ca2+, Mg2+) are correlated with physicochemical properties of activated samples. Explanations for the observed correlations are presented in terms of the occurrence of ions and plausible mechanisms of cation-exchange.

5

Mechanical activation of glauconite is established as a ‘proof of concept’ and ‘green alternative’ to hitherto developed pyro- and/or hydro- metallurgical processes. The strategies required for the implementation of the process are presented.

REFERENCES [1]

J. Pretty, W.J. Sutherland, J. Ashby, J. Auburn, D. Baulcombe, M. Bell, J. Bentley, S. Bickersteth, K. Brown, J. Burke, H. Campbell, K. Chen, E. Crowley, I. Crute, D. Dobbelaere, G. Edwards-Jones, F. Funes-Monzote, H.C.J. Godfray, M. Griffon, P. Gypmantisiri, L. Haddad, S. Halavatau, H. Herren, M. Holderness, A.-M. Izac, M. Jones, P. Koohafkan, R. Lal, T. Lang, J. McNeely, A. Mueller, N. Nisbett, A. Noble, P. Pingali, Y. Pinto, R. Rabbinge, N.H. Ravindranath, A. Rola, N. Roling, C. Sage, W. Settle, J.M. Sha, L. Shiming, T. Simons, P. Smith, K. Strzepeck, H. Swaine, E. Terry, T.P. Tomich, C. Toulmin, E. Trigo, S. Twomlow, J.K. Vis, J. Wilson, S. Pilgrim, The top 100 questions of importance to the future of global agriculture, Int. J. Agric. Sustain. 8 (2010) 219–236. doi:10.3763/ijas.2010.0534.

[2]

D.M. Oosterhuis, D.A. Loka, E.M. Kawakami, W.T. Pettigrew, The Physiology of Potassium in Crop Production, Adv. Agron. 126 (2014) 203–233. doi:10.1016/B978-012-800132-5.00003-1.

[3]

C. Zörb, M. Senbayram, E. Peiter, Potassium in agriculture - Status and perspectives, J. Plant Physiol. 171 (2014) 656–669. doi:10.1016/j.jplph.2013.08.008.

[4]

S.K. Jena, N. Dhawan, D.S. Rao, P.K. Misra, B.K. Mishra, B. Das, Studies on extraction of potassium values from nepheline syenite, Int. J. Miner. Process. 133 (2014) 13–22. doi:10.1016/J.MINPRO.2014.09.006.

[5]

W.O. Santos, E.M. Mattiello, A.A. Pacheco, L. Vergutz, L.F. da Silva Souza-Filho, D.B. Abdala, Thermal treatment of a potassium-rich metamorphic rock in formation of soluble K forms, Int. J. Miner. Process. 159 (2017) 16–21. doi:10.1016/J.MINPRO.2016.12.004.

[6]

W.O. Santos, E.M. Mattiello, L.M. da Costa, W.A.P. Abrahão, R.F. de Novais, R.B. Cantarutti, Thermal and chemical solubilization of verdete for use as potassium fertilizer, Int. J. Miner. Process. 140 (2015) 72–78. doi:10.1016/J.MINPRO.2015.05.003.

[7]

P. van Straaten, Rocks for Crops - Agrominerals of Sub-Saharan Africa, ICRAF, Nairobi, Kenya, 2002. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.471.316&rep=rep1&type=p

df (accessed April 30, 2019). [8]

D. Ciceri, M. de Oliveira, R.M. Stokes, T. Skorina, A. Allanore, Characterization of potassium agrominerals: Correlations between petrographic features, comminution and leaching of ultrapotassic syenites, Miner. Eng. 102 (2017) 42–57. doi:10.1016/J.MINENG.2016.11.016.

[9]

J. Priyono, R.J. Gilkes, High‐Energy Milling Improves the Effectiveness of Silicate Rock Fertilizers: A Glasshouse Assessment, Commun. Soil Sci. Plant Anal. 39 (2008) 358–369. doi:10.1080/00103620701826498.

[10] T. Skorina, A. Allanore, Aqueous alteration of potassium-bearing aluminosilicate minerals: from mechanism to processing, Green Chem. 17 (2015) 2123–2136. doi:10.1039/C4GC02084G. [11] D.A.C. Manning, Mineral sources of potassium for plant nutrition. A review, Agron. Sustain. Dev. 30 (2010) 281–294. doi:10.1051/agro/2009023. [12] J.I. Sanz Scovino, D.L. Rowell, The use of feldspars as potassium fertilizers in the savannah of Colombia, Fertil. Res. 17 (1988) 71–83. doi:10.1007/BF01050458. [13] P. van Straaten, Farming with rocks and minerals: challenges and opportunities, An. Acad. Bras. Cienc. 78 (2006) 731–747. doi:10.1590/S0001-37652006000400009. [14] P. van Straaten, ROCKS FOR CROPS: THE USE OF LOCALLY AVAILABLE MINERALS AND ROCKS TO ENHANCE SOIL PRODUCTIVITY, in: V. de Melo Benites, A. de Oliveira Jr, P.S. Teixeira, M.F. Moraes, R.M.V.B. de Campos Leite, R.P. de Oliveira (Eds.), Technol. Innov. A Sustain. Trop. Agric., International Scientific Centre for Fertilisers (CIEC), Rio de Janeiro, RJ, 2014: pp. 55–57. http://www.16wfc.com/images/fertilizer/downloads/Proceedigns_16WFC_Medium_re solution.pdf (accessed December 23, 2017). [15] G.S. Kuanysheva, B.D. Balgysheva, A.B. Asilov, F.K. Urakaev, Modified Glauconites of Republic Kazakhstan and Their Sorption Properties, Chem. Sustain. Dev. 22 (2014) 37–42. http://www.sibran.ru/upload/iblock/d9f/d9f6c263821a8a2dabea40719e3d1bcf.pdf (accessed December 22, 2017).

[16] E. Srasra, M. Trabelsi-Ayedi, Textural properties of acid activated glauconite, Appl. Clay Sci. 17 (2000) 71–84. doi:10.1016/S0169-1317(00)00008-9. [17] D.L. Sparks, Chemistry of soil potassium in Atlantic coastal plain soils: A review, Commun. Soil Sci. Plant Anal. 11 (1980) 435–449. doi:10.1080/00103628009367051. [18] M.K. Abd El-Rahman, Degritting of gluaconite clay by different techniques for use in water treatment and as fertiliser, Miner. Process. Extr. Metall. 115 (2006) 145–149. doi:10.1179/174328506X109095. [19] E. Karimi, A. Abdolzadeh, H.R. Sadeghipour, A. Aminei, The potential of glauconitic sandstone as a potassium fertilizer for olive plants, Arch. Agron. Soil Sci. 58 (2012) 983–993. doi:10.1080/03650340.2011.557369. [20] M. Rudmin, S. Banerjee, A. Mazurov, B. Makarov, D. Martemyanov, Economic potential of glauconitic rocks in Bakchar deposit (S-E Western Siberia) for alternate potash fertilizer, Appl. Clay Sci. 150 (2017) 225–233. doi:10.1016/J.CLAY.2017.09.035. [21] G. El-Habaak, M. Askalany, M. Faraghaly, M. Abdel-Hakeem, The economic potential of El-Gedida glauconite deposits, El-Bahariya Oasis, Western Desert, Egypt, J. African Earth Sci. 120 (2016) 186–197. doi:10.1016/J.JAFREARSCI.2016.05.007. [22] Y. Tokunaga, Potassium silicate: A slow-release potassium fertilizer, Fertil. Res. 30 (1991) 55–59. doi:10.1007/BF01048827. [23] N-P-K World, N-P-K World, 2016, (2016). https://npk101.wordpress.com/category/potash-fertilizers/. [24] S.K. Jena, N. Dhawan, D.S. Rao, P.K. Misra, B. Das, Extraction of potash values from pyrophyllite mine waste, Sep. Sci. Technol. 51 (2016) 269–277. doi:10.1080/01496395.2015.1086376. [25] S. Shekhar, D. Mishra, A. Agrawal, K.K. Sahu, Physico-chemical treatment of glauconitic sandstone to recover potash and magnetite, J. Clean. Prod. 147 (2017) 681– 693. doi:10.1016/J.JCLEPRO.2017.01.127. [26] S. Shekhar, D. Mishra, A. Agrawal, K.K. Sahu, Physical and chemical characterization and recovery of potash fertilizer from glauconitic clay for agricultural application,

Appl. Clay Sci. 143 (2017) 50–56. doi:10.1016/J.CLAY.2017.03.016. [27] M.K. Soni, On possibility of using glauconitic sandstone as a source of raw material for potash fertilizers, Indian Min. Eng. J. 1 (1990) 3–10. [28] V.P. Yadav, T. Sharma, Leaching of Glauconitic sand stone in acid lixiviants, Miner. Eng. 5 (1992) 715–720. doi:10.1016/0892-6875(92)90066-I. [29] C.S. Rao, A.S. Rao, Characterization of indigenous glauconitic sandstone for its potassium‐supplying potential by chemical, biological, and electroultrafiltration methods, Commun. Soil Sci. Plant Anal. 30 (1999) 1105–1117. doi:10.1080/00103629909370271. [30] U. Bansal, S. Banerjee, K. Pande, A. Arora, S.S. Meena, The distinctive compositional evolution of glauconite in the Cretaceous Ukra Hill Member (Kutch basin, India) and its implications, Mar. Pet. Geol. 82 (2017) 97–117. doi:10.1016/J.MARPETGEO.2017.01.017. [31] TIFAC Technology Roadmaps, Technologies for Agricultural Application of Glauconite- A Potash Mineral, (2001). http://tifac.org.in/index.php?option=com_content&view=article&id=729&Itemid=205. [32] S. Banerjee, U. Bansal, K. Pande, S.S. Meena, Compositional variability of glauconites within the Upper Cretaceous Karai Shale Formation, Cauvery Basin, India: Implications for evaluation of stratigraphic condensation, Sediment. Geol. 331 (2016) 12–29. https://www.sciencedirect.com/science/article/pii/S0037073815002304 (accessed December 22, 2017). [33] S. Banerjee, S.L. Chattoraj, P.K. Saraswati, S. Dasgupta, U. Sarkar, Substrate control on formation and maturation of glauconites in the Middle Eocene Harudi Formation, western Kutch, India, Mar. Pet. Geol. 30 (2012) 144–160. doi:10.1016/J.MARPETGEO.2011.10.008. [34] R.K. Rawlley, Mineralogical investigations on an Indian glauconitic sandstone of Madhya Pradesh state, Appl. Clay Sci. 8 (1994) 449–465. doi:10.1016/01691317(94)90039-6. [35] Mineral Data Publishing, Glauconite, (2001).

http://www.handbookofmineralogy.org/pdfs/glauconite.pdf (accessed December 23, 2017). [36] S.G. McRae, Glauconite, Earth-Science Rev. 8 (1972) 397–440. doi:10.1016/00128252(72)90063-3. [37] J.M. Huggett, Minerals: Glauconites and Green Clays, in: Ref. Modul. Earth Syst. Environ. Sci., Elsevier, 2013. doi:10.1016/B978-0-12-409548-9.02893-1. [38] H.A. Buckley, J.C. Bevan, K.M. Brown, L.R. Johnson, V.C. Farmer, Glauconite and celadonite : two separate mineral species, Mineral. Mag. 42 (1978) 373–382. http://www.minersoc.org/pages/Archive-MM/Volume_42/42-323-373.pdf (accessed December 23, 2017). [39] M.S. Hassan, H.M. Baioumy, Structural and chemical alteration of glauconite under progressive acid treatment, Clays Clay Miner. 54 (2006) 491–499. doi:10.1346/CCMN.2006.0540410. [40] B.P. Robinson, Ion-exchange minerals and disposal of radioactive wastes-- A survey of literature, 1962. https://pubs.er.usgs.gov/publication/wsp1616 (accessed December 23, 2017). [41] M.H. Manghnani, J. Hower, Glauconites: Cation exchange capacities and infrared spectra, Part I. Cation Exchange Capacity of Glauconite, Am. Mineral. 49 (1964) 586– 598. http://www.minsocam.org/ammin/AM49/AM49_586.pdf (accessed December 23, 2017). [42] S.K. Sharma, A.K. Roy, A.T. Balagopal, H. Roy, High temperature interaction of glauconite with calcium salts, Fertil. Technol. 16 (1979) 41–46. [43] A.. Mazumder, T. Sharma, T.. Rao, Extraction of potassium from glauconitic sandstone by the roast-leach method, Int. J. Miner. Process. 38 (1993) 111–123. doi:10.1016/0301-7516(93)90068-L. [44] P. Jing, Process for preparing potassium sulfate, CN 1309086 A, 2001. [45] W.M. Bruce, R. Ray, Altered glauconite and method of altering, US 2139299, 1939. [46] B.R. Rao, L.S. Rao, A.K. Mazumdar, T.C. Rao, Fluoride aided potassium extraction

from glauconitic sandstone for liquid fertilizer, Miner. Eng. 6 (1993) 405–413. https://ac.els-cdn.com/089268759390019J/1-s2.0-089268759390019Jmain.pdf?_tid=c944cae6-18e3-42c5-9ee40328e87fe48e&acdnat=1521912117_363943e38cdcfe76d286d49debee16c8 (accessed March 24, 2018). [47] M.K.H. Siddiqui, Bleaching Earths, Pergamon, Oxford, UK, 1968. [48] V.A. Sontakkey, R.S. Aehdi, I. Mohanram, V.A.J. Aruna, S.M. Lal, I. Ravindran, Beneficiation of a glauconite sand stone sample, Kurchha – Barwadih area, Sonbhadra district, Uttar Pradesh, in: Int. Semin. Miner. Process. Technol. 2017, Indian Institute of Mineral Engineers, Chennai, India, 2017: p. 7. https://www.researchgate.net/publication/314258048_BENEFICIATION_OF_A_GLA UCONITE_SAND_STONE_SAMPLE_KURCHHA__BARWADIH_AREA_SONBHADRA_DISTRICT_Uttar_Pradesh. [49] T.C. Alex, R. Kumar, S.K. Roy, S.P. Mehrotra, Towards ambient pressure leaching of boehmite through mechanical activation, Hydrometallurgy. 144–145 (2014) 99–106. doi:10.1016/J.HYDROMET.2014.01.017. [50] P. Baláž, Mechanochemistry in nanoscience and minerals engineering, SpringerVerlag, Heidelberg, 2008. doi:10.1007/978-3-540-74855-7. [51] P. Baláž, M. Achimovičová, M. Baláž, P. Billik, Z. Cherkezova-Zheleva, J.M. Criado, F. Delogu, E. Dutková, E. Gaffet, F.J. Gotor, R. Kumar, I. Mitov, T. Rojac, M. Senna, A. Streletskii, K. Wieczorek-Ciurowa, Hallmarks of mechanochemistry: from nanoparticles to technology, Chem. Soc. Rev. 42 (2013) 7571. doi:10.1039/c3cs35468g. [52] S. Kumar, R. Kumar, A. Bandopadhyay, Innovative methodologies for the utilisation of wastes from metallurgical and allied industries, Resour. Conserv. Recycl. 48 (2006) 301–314. doi:10.1016/j.resconrec.2006.03.003. [53] R. Kumar, S. Kumar, T.C. Alex, S. Srikanth, S.P. Mehrotra, Process Innovations using Mechanical Activation of Minerals and Wastes, in: M. Gabriele, D. Francesco (Eds.), Exp. Theor. Approaches to Mod. Mechanochemistry, Transworld Research Network, Chennai, 2010: pp. 255–272.

[54] R. Kumar, S. Kumar, S.P. Mehrotra, Towards sustainable solutions for fly ash through mechanical activation, Resour. Conserv. Recycl. 52 (2007) 157–179. doi:10.1016/j.resconrec.2007.06.007. [55] R. Kumar, T.C. Alex, Z.H. Khan, S.P. Mahapatra, S.P. Mehrotra, Mechanical activation of bauxite potential and prospects in the bayer process, in: TMS Light Met., 2005: pp. 77–79. http://www.scopus.com/inward/record.url?eid=2-s2.023244432903&partnerID=tZOtx3y1. [56] R. Kumar, T.C. Alex, M.K. Jha, Z.H. Khan, S.P. Mahaptra, C.R. Mishra, Mechanochemistry and the Bayer process of alumina production, in: A.T. Tabereaux (Ed.), Light Met. 2004, TMS (The Minerals, Metals & Materials Society), 2004: pp. 31–34. http://www.scopus.com/inward/record.url?eid=2-s2.02442571197&partnerID=tZOtx3y1. [57] L. Takacs, The historical development of mechanochemistry, Chem. Soc. Rev. 42 (2013) 7649. doi:10.1039/c2cs35442j. [58] A.Z. Juhasz, L. Opoczky, Mechanical Activation of Minerals by Grinding: Pulverizing and Morphology of Particles, Ellis Horwood Limited, New York, 1994. [59] G.E. Christidis, F. Dellisanti, G. Valdre, P. Makri, Structural modifications of smectites mechanically deformed under controlled conditions, Clay Miner. 40 (2005) 511–522. doi:10.1180/0009855054040188. [60] F. Dellisanti, G. Valdré, Study of structural properties of ion treated and mechanically deformed commercial bentonite, Appl. Clay Sci. 28 (2005) 233–244. doi:10.1016/j.clay.2003.12.036. [61] F. Franco, L.A. Pérez-Maqueda, J.L. Pérez-Rodríguez, Influence of the particle-size reduction by ultrasound treatment on the dehydroxylation process of kaolinites, J. Therm. Anal. Calorim. 78 (2004) 1043–1055. doi:10.1007/s10973-004-0469-5. [62] R.L. Frost, É. Makó, J. Kristóf, E. Horváth, J.T. Kloprogge, Modification of Kaolinite Surfaces by Mechanochemical Treatment, Langmuir. 17 (2001) 4731–4738. doi:10.1021/la001453k. [63] R. Hamzaoui, F. Muslim, S. Guessasma, A. Bennabi, J. Guillin, Structural and thermal

behavior of proclay kaolinite using high energy ball milling process, Powder Technol. 271 (2015) 228–237. doi:10.1016/j.powtec.2014.11.018. [64] J. Hrachová, J. Madejová, P. Billik, P. Komadel, V.Š. Fajnor, Dry grinding of Ca and octadecyltrimethylammonium montmorillonite, J. Colloid Interface Sci. 316 (2007) 589–595. doi:10.1016/j.jcis.2007.07.085. [65] P. Huang, Z. Li, M. Chen, H. Hu, Z. Lei, Q. Zhang, W. Yuan, Mechanochemical activation of serpentine for recovering Cu (II) from wastewater, Appl. Clay Sci. 149 (2017) 1–7. doi:10.1016/j.clay.2017.08.030. [66] A.Z. Juhasz, Colloid-chemical aspects of mechanical activation, Part. Sci. Technol. 16 (1998) 145–161. doi:10.1080/02726359808906792. [67] E. V Kalinkina, A.M. Kalinkin, W. Forsling, V.N. Makarov, Sorption of atmospheric carbon dioxide and structural changes of Ca and Mg silicate minerals during grinding: I. Diopside, Int. J. Miner. Process. 61 (2001) 273–288. doi:10.1016/S03017516(00)00035-1. [68] W.D. Laws, J.B. Page, Changes produced in kaolinite by dry grinding, Soil Sci. 62 (1946) 310–336. http://journals.lww.com/soilsci/citation/1946/10000/changes_produced_in_kaolinite_b y_dry_grinding.6.aspx (accessed December 22, 2017). [69] V.A. Nikashina, A.N. Streletsky, I. V. Kolbanev, I.N. Meshkova, V.G. Grinev, I.B. Serova, T.S. Yusupov, L.G. Shumskaya, Properties of mechanically activated natural clinoptilolite and chabazite, Clay Miner. 46 (2011) 329–337. doi:10.1180/claymin.2011.046.2.329. [70] J. Ondruška, Š. Csáki, V. Trnovcová, I. Štubňa, F. Lukáč, J. Pokorný, L. Vozár, P. Dobroň, Influence of mechanical activation on DC conductivity of kaolin, Appl. Clay Sci. 154 (2018) 36–42. doi:10.1016/j.clay.2017.12.038. [71] I. Sondi, M. Stubičar, V. Pravdić, Surface properties of ripidolite and beidellite clays modified by high-energy ball milling, Colloids Surfaces A Physicochem. Eng. Asp. 127 (1997) 141–149. doi:10.1016/S0927-7757(96)03893-9. [72] G. Suraj, C.S.P. Iyer, S. Rugmini, M. Lalithambika, The effect of micronization on

kaolinites and their sorption behaviour, Appl. Clay Sci. 12 (1997) 111–130. doi:10.1016/S0169-1317(96)00044-0. [73] H. Takahashi, Effect of Dry Grinding on Kaolin Minerals, Bull. Chem. Soc. Jpn. 32 (1959) 235–245. http://www.clays.org/journal/archive/volume 6/6-1-279.pdf (accessed December 22, 2017). [74] R.M. Torres Sánchez, Mechanochemical effects on physicochemical parameters of homoionic smectite, Colloids Surfaces A Physicochem. Eng. Asp. 127 (1997) 135– 140. doi:10.1016/S0927-7757(97)00105-2. [75] N. Vdović, I. Jurina, S.D. Škapin, I. Sondi, The surface properties of clay minerals modified by intensive dry milling — revisited, Appl. Clay Sci. 48 (2010) 575–580. doi:10.1016/J.CLAY.2010.03.006. [76] M. Xia, Y. Jiang, L. Zhao, F. Li, B. Xue, M. Sun, D. Liu, X. Zhang, Wet grinding of montmorillonite and its effect on the properties of mesoporous montmorillonite, Colloids Surfaces A Physicochem. Eng. Asp. 356 (2010) 1–9. doi:10.1016/j.colsurfa.2009.12.014. [77]

von H.G. Reichenbacht, C.I. Rich, Potassium release from muscovite as influenced by particle size, Clays Clay Miner. 17 (1969) 23–29. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.597.7208&rep=rep1&type= pdf (accessed December 22, 2017).

[78] D. Carroll, Ion exchange in clays and other minerals, GSA Bull. 70 (1959) 749–779. doi:10.1130/0016-7606(1959)70[749:ieicao]2.0.co;2. [79] L.S. Sonon, D.E. Kissel, U. Saha, Cation Exchange Capacity and Base Saturation, (2017). https://secure.caes.uga.edu/extension/publications/files/pdf/C 1040_1.PDF (accessed March 26, 2018). [80] G.H. Jeffery, J. Bassett, J. Mendham, R.C. Denney, Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed., Longman Scientific & Technical, 1989. [81] J.S. Fritz, G.H.J. Schenk, Quantitative Analytical Chemistry, 4th ed., Allyn and Bacon, Boston, MA, 1979. [82] AAS, GFAAS, ICP or ICP-MS? Which technique should I use? An elementary

overview of elemental analysis by Thermo Elemental, available at oliver.chemistry.ucsc.edu/122/Lab5%20Handout.pdf (last accessed on 20-08-2019). [83] A. Mills, Silver as a removable conductive coating for scanning electron microscopy, Scanning Microsc. 2 (1988) 1265–1271. [84] Silver as a Removable Coating for Scanning Electron Microscopy, Electron Microsc. Sci. (n.d.). https://www.emsdiasum.com/microscopy/technical/datasheet/silver_for_SEM.aspx (accessed April 28, 2019). [85] ASTM, Grain size analysis of soils, in: ASTM, Philadelphia, 1965: p. ASTM Designation D422-63. [86] Malvern Instruments Ltd., Wet or liquid dispersion method development for laser diffraction particle size distribution measurements, Worcestershire, UK, 2015. http://www.malvern.com/. [87] K.S.W. Sing, Commission on Colloid and Surface Chemistry Including Catalysis Subcommittee on Reporting Gas Adsorption Data - Reporting Physisorption Data for Gas / Solid Systems, Pure Appl . Chem. 54 (1982) 22–22. doi:10.1351/pac198254112201. [88] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting Physisorption Data for Gas/Solid Systems, Pure Appl. Chem. 57 (1985) 603–619. [89] T.C. Alex, R. Kumar, S.K. Roy, S.P. Mehrotra, Anomalous reduction in surface area during mechanical activation of boehmite synthesized by thermal decomposition of gibbsite, Powder Technol. 208 (2011) 128–136. doi:10.1016/J.POWTEC.2010.12.010. [90] T.C. Alex, R. Kumar, S.K. Roy, S.P. Mehrotra, Mechanically induced reactivity of gibbsite: Part 2. Attrition milling, Powder Technol. 264 (2014) 229–235. doi:10.1016/j.powtec.2014.05.029. [91] M. Kitamura, M. Senna, Electrorheological properties of mechanically activated gibbsite, Int. J. Inorg. Mater. 3 (2001) 563–567. doi:10.1016/S1466-6049(01)00068-X. [92] J. Langford, R. Delhez, T. de Keijser, E. Mittemeijer, Profile Analysis for

Microcrystalline Properties by the Fourier and Other Methods, Aust. J. Phys. 41 (1988) 173. doi:10.1071/PH880173. [93] D. Warncke, J.R. Brown, Potassium and Other Basic Cations, in: J.R. Brown (Ed.), Recomm. Chem. Soil Test Proced. North Cent. Reg., Agricultural Experiment Stations of Illinois, Indiana, Iowa, 1998: pp. 31–33. https://www.mendeley.com/library/# (accessed December 22, 2017). [94] T. Li, H. Wang, Z. Zhou, X. Chen, J. Zhou, A nano-scale study of the mechanisms of non-exchangeable potassium release from micas, Appl. Clay Sci. 118 (2015) 131–137. doi:10.1016/J.CLAY.2015.09.013. [95] L. Lutterotti, MAUD : Materials Analysis Using Diffraction, (2015). http://maud.radiographema.eu/ (accessed December 24, 2017). [96] F. Rouquerol, J. (Jean) Rouquerol, K.S.W. Sing, Adsorption by powders and porous solids : principles, methodology, and applications, Academic Press, 1999. https://www.sciencedirect.com/science/book/9780125989206 (accessed December 22, 2017). [97] U. Kuila, M. Prasad, Specific surface area and pore-size distribution in clays and shales, Geophys. Prospect. 61 (2013) 341–362. doi:10.1111/1365-2478.12028. [98] E.P. Barret, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380. [99] C. Sasikumar, S. Srikanth, R. Kumar, T.C. Alex, S.P. Mehrotra, Where does the energy go in high energy milling?, in: R. Kumar, S. Srikanth, S.P. Mehrotra (Eds.), Front. Mechanochemistry Mech. Alloy., CSIR-National Metallurgical Laboratory, Jamshedpur, India, Jamshedpur, 2011: pp. 240–246. http://eprints.nmlindia.org/5123/1/240246_India_09_Srikanth_et_al__with_jpg_fig_.dot.pdf (accessed April 1, 2018). [100] I. Aliatis, D. Bersani, E. Campani, A. Casoli, P.P. Lottici, S. Mantovan, I.-G. Marino, F. Ospitali, Green pigments of the Pompeian artists’ palette, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 73 (2009) 532–538. doi:10.1016/J.SAA.2008.11.009.

[101] M.H. Manghnani, J. Hower, Glauconites: Cation exchange capacities and infrared spectra, Part II. Infrared Absorption Characteristics of Glauconites, Am. Miner. 49 (1964) 1631–1642. http://www.minsocam.org/ammin/AM49/AM49_1631.pdf (accessed December 23, 2017). [102] L.M. Moretto, E.F. Orsega, G.A. Mazzocchin, Spectroscopic methods for the analysis of celadonite and glauconite in Roman green wall paintings, J. Cult. Herit. 12 (2011) 384–391. doi:10.1016/J.CULHER.2011.04.003. [103] R. Newman, Some Applications of Infrared Spectroscopy in the Examination of Painting Materials, J. Am. Inst. Conserv. 19 (1980) 42–62. doi:10.1179/019713679806028977. [104] M. V Slonimskaya, G. Besson, L.G. Dainyak, C. Tchoubar, V.A. Drits, Interpretation of The IR Spectra of Celadonites and Glauconites in the Region of OH-Stretching Frequencies, Clay Miner. 21 (1986) 377–388. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.588.5942&rep=rep1&type= pdf (accessed December 22, 2017). [105] V.C. Farmer, J.D. Russell, The infrared spectra of layer silicates, Spectrochim. Acta. 20 (1964) 1149–1173. [106] J.D. Russell, V.C. Farmer, B. Velde, Replacement of OH by OD in layer silicates, and identification of the vibrations of these groups in infra-red spectra, Mineral. Mag. 37 (1970) 869–879. [107] B.J. Saikia, G. Parthasarathy, Fourier Transform Infrared Spectroscopic Characterization of Kaolinite from Assam and Meghalaya, Northeastern India, J. Mod. Phys. 1 (2010) 206–210. doi:10.4236/jmp.2010.14031. [108] R.K. Rath, S. Subramanian, T. Pradeep, Surface Chemical Studies on Pyrite in the Presence of Polysaccharide-Based Flotation Depressants, J. Colloid Interface Sci. 229 (2000) 82–91. doi:10.1006/jcis.2000.6990. [109] Z. Zhang, H. Wang, J.L. Provis, Quantitative study of the reactivity of fly ash in geopolymerization by FTIR, J. Sustain. Cem. Mater. 1 (2012) 154–166. doi:10.1080/21650373.2012.752620.

[110] P.A. Hazelton, B.W. Murphy, Interpreting soil test results : what do all the numbers mean?, CSIRO Publishing, 2016. [111] D. Tromans, J.A. Meech, Enhanced dissolution of minerals: stored energy, amorphism and mechanical activation, Miner. Eng. 14 (2001) 1359–1377. doi:10.1016/S08926875(01)00151-0. [112] K. Tkáčová, Mechanical activation of minerals, Elsevier, NY, 1989. https://trove.nla.gov.au/work/17016861?q&versionId=19967049 (accessed April 2, 2018). [113] V.V. Boldyrev, Polov, S.V., E.L. Goldberg, Interrelation between fine grinding and mechanical activation, Int J Min. Process. 44–45 (1996) 181–185. [114] T.C. Alex, R. Kumar, S.K. Roy, S.P. Mehrotra, Mechanically induced reactivity of gibbsite: Part 1. Planetary milling, Powder Technol. 264 (2014) 105–113. doi:10.1016/j.powtec.2014.05.028. [115] T.C. Alex, R. Kumar, S.K. Roy, S.P. Mehrotra, Mechanical Activation of Aloxyhydroxide Minerals–A Review, Miner. Process. Extr. Metall. Rev. 37 (2016) 1–26. doi:10.1080/08827508.2015.1055626. [116] Z. Juhász, MECHANO-CHEMICAL ACTIVATION OF KAOLIN MINERALS, in: Acta Mineral. XXIV/1980, Suppl. Proc. 10th Kaolin Symp. Budapest 3 Sept. 1979, IGCP Proj. № 23, Budapest, 1979: pp. 121–145. [117] É. Makó, R.L. Frost, J. Kristóf, E. Horváth, The Effect of Quartz Content on the Mechanochemical Activation of Kaolinite, J. Colloid Interface Sci. 244 (2001) 359– 364. doi:10.1006/jcis.2001.7953. [118] K.A. Selim, R.S. El-Tawil, M. Rostom, Utilization of surface modified phyllosilicate mineral for heavy metals removal from aqueous solutions, Egypt. J. Pet. (2017). doi:10.1016/J.EJPE.2017.07.003. [119] W. Lu, E.H. Smith, Modeling potentiometric titration behavior of glauconite, Geochim. Cosmochim. Acta. 60 (1996) 3363–3373. doi:10.1016/00167037(96)00180-9. [120] S.J. Anderson, G. Sposito, Proton Surface-Charge Density in Soils with Structural and

pH-Dependent Charge, Soil Sci. Soc. Am. J. 56 (1992) 1437. doi:10.2136/sssaj1992.03615995005600050017x. [121] S.A. Hussain, Ş. Demirci, G. Özbayoğlu, Zeta Potential Measurements on Three Clays from Turkey and Effects of Clays on Coal Flotation, J. Colloid Interface Sci. 184 (1996) 535–541. doi:10.1006/JCIS.1996.0649. [122] A. Kriaa, N. Hamdi, E. Srasra, Proton adsorption and acid-base properties of Tunisian illites in aqueous solution, J. Struct. Chem. 50 (2009) 273–287. doi:10.1007/s10947009-0039-6. [123] L. Lavikainen, The structure and surfaces of 2 : 1 phyllosilicate clay minerals The structure and surfaces of, 2016. [124] P. Leroy, A. Revil, A triple-layer model of the surface electrochemical properties of clay minerals, J. Colloid Interface Sci. 270 (2004) 371–380. doi:10.1016/J.JCIS.2003.08.007. [125] R.A. Schoonheydt, C.T. Johnston, Chapter 3 Surface and Interface Chemistry of Clay Minerals, Dev. Clay Sci. 1 (2006) 87–113. doi:10.1016/S1572-4352(05)01003-2. [126] I. Sondi, J. Bišćan, V. Pravdić, Electrokinetics of Pure Clay Minerals Revisited, J. Colloid Interface Sci. 178 (1996) 514–522. doi:10.1006/jcis.1996.0146. [127] F. Thomas, L.. Michot, D. Vantelon, E. Montargès, B. Prélot, M. Cruchaudet, J.. Delon, Layer charge and electrophoretic mobility of smectites, Colloids Surfaces A Physicochem. Eng. Asp. 159 (1999) 351–358. doi:10.1016/S0927-7757(99)00291-5. [128] E. Tombácz, M. Szekeres, Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes, Appl. Clay Sci. 27 (2004) 75–94. doi:10.1016/j.clay.2004.01.001. [129] Y. Yukselen, A. Kaya, Zeta Potential of Kaolinite in the Presence of Alkali, Alkaline Earth and Hydrolyzable Metal Ions, Water. Air. Soil Pollut. 145 (2003) 155–168. doi:10.1023/A:1023684213383. [130] M. Chorom, P. Rengasamy, Dispersion and zeta potential of pure clays as related to net particle charge under varying pH, electrolyte concentration and cation type, Eur. J. Soil Sci. 46 (1995) 657–665. doi:10.1111/j.1365-2389.1995.tb01362.x.

[131] S.R. Rao, Electrical Characteristics at Interfaces, in: Surf. Chem. Froth Flotat. Vol. 1 Fundam., Second, Springer US, 2013: p. 240. [132] R.A. Schoonheydt, C.T. Johnston, The surface properties of clay minerals, in: EMU Notes Mineral., 2011: pp. 337–373. doi:10.1180/EMU-notes.11.10. [133] Y. Marcus, Thermodynamics of solvation of ions. Part 5.—Gibbs free energy of hydration at 298.15 K, J. Chem. Soc., Faraday Trans. 87 (1991) 2995–2999. doi:10.1039/FT9918702995. [134] B.L. Sawhney, Selective Sorption and Fixation of Cations by Clay Minerals: A Review, Clays Clay Miner. 20 (1972) 93–100. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.468.51&rep=rep1&type=pdf (accessed December 22, 2017). [135] S. Nir, D. Hirsch, J. Navrot, A. Banin, Specific Adsorption of Lithium, Sodium, Potassium, and Strontium to Montmorillonite: Observations and Predictions1, Soil Sci. Soc. Am. J. 50 (1986) 40. doi:10.2136/sssaj1986.03615995005000010008x. [136] J.B. Page, L.D. Baver, Ionic size in relation to fixation of cations by colloidal clay, Soil Sci. Soc. Am. Proc. 4 (1939) 150–155. [137] B.A. Sakharov, G. Besson, V.A. Drits, M.Y. Kameneva, A.L. Salyn, B.B. Smoliar, Xray study of the nature of stacking faults in the structure of glauconites, Clay Miner. 25 (1990) 419–435. [138] G.R. Thompson, J. Hower, The Mineralogy of Glauconite, Clays Clay Miner. 23 (1975) 289–300. http://www.clays.org/journal/archive/volume 23/23-4-289.pdf (accessed April 3, 2018). [139] W.D. Keller, Diagenesis in clay minerals : a review, Clays Clay Miner. 13 (1963) 136– 157. [140] J.W. Gruner, The structural relationship of glauconite and mica, Am. Mineral. 20 (1935) 699–714. [141] J.F. Burst, Mineral heterogeneity in “Glauconite” pellets, Am. Mineral. 43 (1958) 481–497. http://www.minsocam.org/ammin/AM43/AM43_481.pdf (accessed April 7, 2018).

[142] K. Tone, M. Kamori, Y. Shibasaki, Y. Takeda, O. Yamamoto, EFFECT OF THE SURFACE POTENTIAL ON THE CATION EXCHANGE CAPACITY OF KAOLIN MINERALS, Clay Sci. 10 (1998) 327–335. https://www.jstage.jst.go.jp/article/jcssjclayscience1960/10/4/10_4_327/_pdf (accessed April 7, 2018). [143] R. Kumar, Anomalous dependence of leachability on surface area during sulphuric acid leaching of nickel- and copper-doped birnessites, Hydrometallurgy. 52 (1999) 71– 79. doi:10.1016/S0304-386X(99)00004-3. [144] R. Kumar, S. Das, Physicochemical and structural factors in the sulfuric acid leaching of nickel- and copper-bearing synthetic birnessites, Metall. Mater. Trans. B Process Metall. Mater. Process. Sci. 29 (1998) 527–540. http://www.scopus.com/inward/record.url?eid=2-s2.00032094860&partnerID=tZOtx3y1. [145] K. Verburg, P. Baveye, Hysteresis in the Binary Exchange of Cations on 2:1 Clay Minerals: A Critical Review, Clay Clay Miner. 42 (1994) 207–220. [146] S.P. Mehrotra, Realm of Mechanochemistry in Extractive Metallurgy - Scientific Curiosity to Commercial Reality, Trans. Indian Inst. Met. 60 (2007) 1–13.

Table 1: Chemical processes for the preparation of potash fertiliser from glauconite

Process

Steps

Chemicals

Drawbacks/

References

Limitations

High

KCl volatilization

CaCl2,

High temperature,

temperature

(1200-1300 oC)

CaCO3

chemicals, suitable

Processes

[27,42]

collection mechanism

High temp

Chloridizing

CaCl2, water High temperature,

operation +

roasting, 800 oC,

harmful chemicals,

leaching

water leaching

evaporation,

[43]

crystallization, Reduction

Coke, HCl,

roasting 700 °C,

water

effluent treatment

[25]

HCl leaching, 80 °C

Sulphation

H2SO4,

roasting, water

water

[26]

leaching

Calcination (500-

H2SO4

[44]

Reduction

Reductant,

[45]

roasting (450 to

NaOH

750 °C), sulfuric acid leaching

650 °C) and treating 5-15% NaOH solution

Chemical

Leaching at

leaching

105°C for 6h

high pressure /

(<152 µm)

difficult to handle

HCl

Harmful chemicals,

[28]

additives, Leaching in

H2SO4,

evaporation,

presence of

fluorides,

crystallization,

additives, 75 µm

hydrides

effluent treatment

[46]

Table 2. Assignment of bands observed in the FTIR spectra of un-milled glauconite Wavenumber, cm-1

Assignment

3539

R-OH stretching (R = octahedrally [38,103,105,106]

References

coordinated ions, Al3+, Mg2+, Fe2+, Fe3+) 3417

-OH stretching/H2O

[104]

2922

C-H symmetric stretching of –CH2

[107,108]

group 2850

C-H asymmetric stretching

1637

H-O-H bending

[102,103]

1028

Si(Al)-O-Si asymmetric stretching

[102,103,109]

808

Fe23+ OH/ Fe2+Fe3+OH bending

[38]

675

Si-O bending

[101], [100]

R-OH bending 492

Si-O-Fe3+ bending

[102,103]

458

Si-O-Si bending

[34,76,103]

434

Si-O-Mg bending

[16,103]

Table 3: Summary of physicochemical characteristics (particulate, structural, surface charge) of glauconite samples after different milling time Property

Relevance

Milling time (tMA), min 0

15

30

60

120

180

240

d50, µm

66.3

5.9

8.7

14.2

12.4

11.4

11.0

SSAgeo,

0.64

1.31

1.18

1.28

1.64

1.62

1.76

50.1

74.9

-

64.3

-

-

28.3

Particulate

reactivity

lower SSABET , decrease in reactivity

m2/g Dp, nm

with

increase in tMA, lower

m2/g

SSABET,

agglomeration

58.1

72.9

-

76.2

-

-

101.7

increase in pore width facilitate mass transfer

Structural Am, %

0

5.9

10.4

23.6

44.6

57.2

67.6

MCD, nm

192

179

168

124

106

82

81

ε x10-3

4.45

4.77

5.33

7.24

8.24

11.67

13.45

higher Am, ε and lower MCD - indicate greater stored energy and higher reactivity

Surface charge (ζ, mV) pH 2

-15.7*

pH 5

*

-27.5

-3.5

2.2

2.5

increase

of

ζ,

lower

electrostatic dispersion/ -30.7

*ζ for 5 min milled sample

-26.9

-31.5

attraction for cations

Table 4. Characteristics of the cations involved in the exchange reactions [133]

Cation

Valance

Ionic radius, nm

Charge/size

Free energy of

(Z)

nonhydrated

hydrated

(Z/r)*

hydration, kJ/mol

NH4+

1

0.148

0.213

6.75

-285

K+

1

0.138

0.212

7.25

-295

Na+

1

0.102

0.218

9.80

-365

Ca2+

2

0.100

0.271

20.00

-1505

Mg2+

2

0.072

0.299

27.70

-1830

* r for non-hydrated ions

Table 5: Correlation coefficients (rxy) between physicochemical properties of samples (x) and cation exchange capacity (y) Binary correlation coefficient (rxy) Physicochemical y = CECi or CEC, meq/100 g

property (x) CECK+

CECNa+

CECCa2+

CECMg2+

CEC

d50, µm

-0.619

0.508

0.500

-0.702

-0.670

SSAgeo, m2/g

0.924

-0.893

-0.898

0.561

0.908

SSABET, m2/g

-0.504

0.736

0.699

0.263

-0.359

Dp, nm

0.745

-0.856

-0.826

0.100

0.651

Am, nm

0.937

-0.989

-0.992

0.337

0.873

MCD, nm

-0.976

0.963

0.988

-0.503

-0.940

ε x 10-3

0.873

-0.969

-0.958

0.225

0.793

Table 6: Comparison of chemical processing schemes (Table 1) with mechanical activation based process to highlight its importance as a green option Parameter

High temperature process

Pyroprocessing & leaching

Direct leaching With Hydroadditives thermal

Mechanical activation

Yes No

Yes Yes

Yes Yes

Yes Yes

No No

Few

Many

Many

Many

Least

No; high temperature vapor handling Difficult

No; high No; high temperature pressure materials handling Yes Yes

No; hazardous nature of additives Yes

Yes; simple operations

-

-

-

-

Yes; energy can be optimized by suitable combination of mills

High temperature

Yes (1200-

-

-

-

-

Pyro-

-

Resource Chemicals Water Process Number of operations Ease of operation

Scalability Energy Mechanical activation

1300 oC)

processing Hydro-

Possible

Yes (450-

-

800 oC) -

Yes

processing Environmental consideration Effluents and Yes; gaseous Yes; environmental emissions gaseous impact emissions, leach residues, liquid effluents

Yes

Yes

-

Yes; leach residues, liquid effluents

Yes; leach residues, liquid effluents

None

Fig. 1

Photographs showing : (a) typical as received glauconite sand lump (width ~ 10 cm); (b) initial powdered sample (quartz particles appear whitish) (-7 mesh / - 2.8 mm); and (c) beneficiated glauconite powder (-72 mesh / - 200 µm)

Fig. 2

Effect of milling time (tMA) on particle size distribution: (a) cumulative undersize; and (b) frequency distribution

Median size (d50), mm

100

10

0

60

120

180

Milling time (tMA), min

Fig. 3

Variation of median size (d50) with milling time (tMA)

240

Fig. 4

SEM micrographs of glauconite after different duration of milling: (a) un-milled sample (tMA = 0 min); (b) tMA = 15 min; (c) tMA = 60 min; and (d) tMA = 240 min

Fig. 5

N2 adsorption-desorption isotherm (at 77 K) of un-milled (tMA = 0 min) and milled (tMA= 15, 60 and 240 min) glauconite samples

Fig. 6

Pore size distribution for un-milled (tMA = 0 min) and milled (tMA = 15, 60 and 240 min) glauconite samples

Fig. 7

Variation of average pore width with milling time

Fig. 8

Variation of zeta (ζ) potential with pH for glauconite sand samples milled for different duration (tMA = 5, 15 and 240 min)

Fig. 9

Powder X-ray diffraction patterns of un-milled (tMA = 0 min) and milled (tMA = 15, 30, 60, 120, 180 and 240 min) samples: (a) full patterns; and (b) amplified view of (131) reflection

Fig. 10

Structural changes with milling: (a) degree of amorphisation (Am); and (b) microcrystallite dimension and strain (MCD and ε)

Fig. 11

FTIR spectra of of un-milled (tMA = 0 min) and milled (tMA = 15, 30, 60, 120, 180 and 240 min) samples

Fig. 12

Effect of milling on total Cation Exchange Capacity (CEC)

Fig. 13

Effect of milling on Cation Exchange Capacity of individual ions (CECi) (i = K+, Na+, Ca2+ and Mg2+)

Fig. 14

Effect of milling on: (a) thermogravimetric weight loss (TG); and (b) differential thermogravimetric (DTG) plots

On Mechanical Activation of Glauconite: Physicochemical Changes, Alterations in Cation Exchange Capacity and Mechanisms Rashmi Singla1, Thomas C. Alex1,2, Rakesh Kumar1,2 1

Academy of Scientific and Innovative Research (AcSIR), CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India 2

CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India

HIGHLIGHTS • • • •

Extensive physicochemical characterisation of mechanically activated glauconite Tailoring of cation exchange capacity of glauconite with Mechanical activation Plausible explanations for cation exchange mechanisms in the activated glauconite With mechanical activation, 30-fold increase in CEC of K+ ions (a macro-nutrient)

•

Mechanical activation - a green alternative to chemical methods of potash extraction