International Journal of Minerals, Metallurgy and Materials Accepted manuscript, https://doi.org/10.1007/s12613-020-1994-3 © University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Chlorination roasting coupled water leaching process for potash recovery from waste mica scrap using dry marble sludge powder and sodium chloride

Sandeep Kumar Jena*, Jogeshwar Sahu, Geetikamayee Padhy, Swagatika Mohanty, and Ajit

Dash

Mineral Processing and Central Characterisation Department

CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India-751013

Abstract

The present paper reports the effective utilisation of marble sludge powder (MSP) for the

recovery of potash values from waste mica scrap using chlorination roast-leaching method.

Characterisation studies indicate the presence of dolomite as major mineral phase in MSP,

whereas it is muscovite and quartz in mica sample. The acid leaching studies suggest a

maximum of 22% potash recovery under conditions like 4M H2SO4 acid, particle size 100µm,

stirring speed 600rpm, leaching temperature 75°C and time 90 min. In order to achieve a

lowest-level of 80-90% potash recovery, the chlorination roasting-water leaching process was

adopted. The optimum conditions for recovery of ~93% of potash from mica (K2O ~ 8.6%)

requires 900°C roasting temperature, 30 min roasting time with a ratio of Mica: MSP: NaCl

1:1:0.75.The roasting temperature and % NaCl is found to be the most important factor for

the recovery process. The reaction mechanism suggests the formation of different mineral

phases like sylvite (KCl), wollastonite, kyanite, enstatite during roasting, which confirmed

from the XRD phases and SEM morphologies. The effectiveness of the MSP blended NaCl

for potash recovery was found to be better as compared with other reported commercial

roasting additives.

2

Keywords: Potash recovery, mica scrap, marble sludge powder, chlorination roasting and

water leaching.

*Corresponding author:

sandeepjena@immt.res.in / sandeepimmt@gmail.com (Dr. Sandeep Kumar Jena)

CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, Odisha, India-751013

Tel. No.: 91-674-2379211; Fax No: 91-674-2581160

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1. Introduction

The generation and steady increase of ineffectual solid waste materials due to industrial,

mining, agricultural and commercial activities owing to increasing urbanisation and growth

of population, creates lots of problems in its disposal and recycling. So, there is a need to

reduce, recycle, reuse or recover the generated solid wastes as far as possible. Marble sludge

powder (MSP) is one of the major industrial solid wastes, generates in the marble processing

industries during sawing and polishing processes of marble. The marble sludge, in the form

of semi-liquid or slurry form, contains around 24-45 % water along with dust marble particles

and it is estimated that 20% of the processed marble goes to the sludge waste [1, 2].

Practically 70% of the total mine’s production, discarded as waste during the mining,

processing and polishing stages which creates many significant environmental issues [2- 4].

As per the available data the world’s major marble exporters are Turkey, Italy, China, Greece,

Spain, and India, which produces a substantially very high amount of marble sludge as waste

material [5,6].

Most of the waste is dumped unscientifically and improperly in nearby pit or vacant spaces,

which deteriorate the local environment in many aspects like, reduces the soil fertility,

deplete water permeability and also increases the suspended particulate matters (SPM) in air

[7-10]. Reports are available regarding some serious human health problems due to inhalation

of dry marble sludge powder polluted air [11-13]. In addition, the handling and disposal of

the generated marble waste supplements an extra financial burden for marble/dimension

stone companies [14, 15]. Hence, value addition to this waste by recycling through a proper

method is a challenge. However, apart from land filling [16-18] many other reports are

available in literature regarding the use of marble/granite waste for production of building

4

and construction raw materials [19-23], for different class ceramic products [24-26], for

removal of heavy metal ions [27-30], for neutralisation of acidic soil [27,31,32] and as

inorganic filler materials in tyre and polymer production [33-35].

The present study reports a novel application of the waste marble sludge powder for the

recovery of potash values from low-priced/waste mica scrap material. The potassium value is

commercially expressed as its oxide content i.e. potash (K2O). The major industrial

application of potassium is for making potassic fertiliser (~ 95%) apart from glass, ceramic,

pigment and pharmaceuticals productions [36, 37]. The demand of potassic fertiliser is

growing persistently at 3 to 3.5 annually [38, 39]. Many agricultural-based countries of

Africa, South Asia, Central Europe and Oceania having no commercial source of potash

entirely depend on the import value. However, the presence secondary mineral sources like

feldspar (4-12%, K2O), mica (6-14%, K2O), glauconitic sandstone (4-11%, K2O) and

nepheline syenite (5-10%, K2O) which contains a good amount of potash values is the only

option, to utilise for potash recovery. Mica is one of such highly available mineral in India

with a production of 14000MT making it a top 8th global producer [40]. However, owing to

the technological development of new materials, the global demand for mica is declining

continuously [41]. Therefore, an attempt has been made here to extract the potash values

from mica, which will support the fertiliser industry.

Literature studies suggests the three major routes of potash recovery from mica/other potash

bearing minerals like feldspar, nepheline syenite, glauconitic sand stone are bioleaching,

chemical leaching and chlorination roasting coupled water leaching processes. Dependency

of various critical parameters such as pΗ, temperature, complexity of mineral structure,

presence of CO2 and O2 on bioleaching extraction processes, makes it unsuitable due to low

extraction efficiency [42-44]. Similarly, the chemical leaching of mica

5

(muscovite/biotite/phlogopite) depends on the nature of mineral acids used, concentration,

leaching temperature, time, and particle size of the sample [45-47]. Variation of acids plays

an important role and affects the potash recovery value. Potash (K) release from the complex

biotite mica structure completely depends on the chain length of the phosphoric acids used

and the degree of dehydration of the system [45], whereas the release of K from muscovite

mica depends of concentration of H2SO4 and the leaching temperature [48]. In a similar

approach Varadachari (1997), also reported a maximum of 14% potash recovery from biotite

mica using 9.4N HCl at 80⁰C after 4 hours of leaching. The low recovery of potash using

BaCl2 solution, Mehlich Solution (H2SO4 + HCl), NaOH and HNO3 are reported [46, 47, 49]

and it is due to the positioning of K in the complex structure.

However, the chlorination roasting followed by water leaching is the most efficient method to

recover potassium from these rocks/minerals. Various experimental factors like the roasting

temperature, roasting time, additive ratio with respect to mica scrap/similar K-bearing rocks,

largely influences the extraction efficiency [50,51]. CaCl2 is the most commonly used

roasting additive for extraction of potassium from mica, feldspar, glauconitic sandstone [47,

52-54]. The comparison of CaCl2 with other alkali /alkaline earth metal chloride indicates the

cheaper, non-toxic, and highly efficient character; hence preferable reported in many pieces

of literatures. The Ca+2 ion replaces the K+ ion from the complex crystal structure of mica and

subsequently the released K+ combines with the Cl- to form KCl. The present work reports

the use of MSP (due to high calcium content), as a very low- cost roasting additive to recover

potassium from the mica scrap. Various characterisation studies have been carried out to

establish the plausible extraction mechanism.

2. Materials and Methods

2.1 Materials and its pre-treatment

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The off-white coloured slippery and flaky mica scrap sample (Fig 1a) used in this study was

collected from one of the mica sheet manufacturer and exporter located in the state of

Jharkhand, India and further processed in a mortar grinder (Retch model 200M) for size

reduction purpose. Varying the grinding timings of the mortar grinder, the ground products

are size analysed using Indian standard sieves which indicate 80% of the product

will pass through 100μm after 6 minutes of grinding. However the as received dried marble

sludge powder (Fig 1b) from Rajasthan state, India was used as such without any further

treatment. The size analysis results for both the samples are presented in Fig 1(c, d). The

other chemicals and standards (for ICP-OES and flame photometer) were purchased from

Avantor (India) Ltd, New Delhi.

0

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0 100 200 300 400 500 600

Cum

mla

tive

wt.

%

Particle Size (µm)

2 min 4 min6 min 8 min10 min

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0.1 1 10 100 1000Particle Size (µm)

(a) Mica Scrap (b) Marble Sludge

(c) (d)

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Fig. 1. The as-received mica scrap (a) and its mortar ground products at different timing (c),

the dry marble sludge powder (b) and its particle size analysis (d).

2.2 Extraction methods

2.2.1 Acid leaching

The acid leaching studies were performed with respect to different mineral acids like H2SO4,

HNO3, HCl and H3PO4 in a glass beaker using a digital magnetic stirrer. The major leaching

parameters like acid concentration, particle size of mica scrap, leaching time and temperature

were optimised for maximum recovery of potash values. The agitation rate of the slurry

sample was maintained at 600±10 rpm. Both flame photometer (Systronic 128µc) and

Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES Perkin Elmer 8300)

instruments were used to analyse the leached liquor to note the potash value.

2.2.2 Roast-leaching

The roast-leaching experiments of mica scrap sample involve the high temperature roasting

followed by water leaching process. Batch scale roasting experiments were performed in a

laboratory muffle furnace taking a well-mixed and fixed ratio of mica scrap powder(~10g)

and roasting additives in an iron crucible. Roasting parameters like roasting time, temperature,

particle size, mica to additive ratio were varied to recover maximum potash values from the

feed sample. The different roasting additives used here are NaCl, CaCl2, CaCO3, NaCO3 and

MSP. The collected hard roasted mass, after completion of roasting time was made into

powder form using a hand mortar and allowed to leach with water only. The water leaching

experiments are just similar to the method as described in section 2.2.1. At the end of water

leaching, the solid-liquid phases are separated using laboratory grade filter papers and the

residual parts were washed carefully 5-6 times. The scheme of the extraction process is

presented in Fig. 2.

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Fig. 2. Experimental scheme for potash recovery from mica scrap using MSP and NaCl.

2.3 Characterisation studies

The mineralogical characteristics of the feed and processed product samples were studied

using X-ray diffraction (PANalytical X’pert PRO-XRD) equipped with Cu -Kα radiation (40

kV, 20 mA) source and ZEISS (EVO) make scanning electron microscope coupled with an

Energy-Dispersive Spectroscopy (SEM-EDS).The thermogravimetric analysis of the samples

was conducted from room temperature up to 1200°C in 80ml/min Ar flow using Netzsch

STA 449F3 Jupiter instrument. However, the chemical composition of the samples are

determined using standard wet chemical analysis methods and also using instruments such as

Flame photometer ( Systronic flame photometer , Model 128µc) and inductively coupled

plasma optical emission spectroscopy (Perkin Elmer Optima 8300 , ICP-OES).

3. Results and discussion

3.1 Sample characterization

The chemical composition of the feed mica scrap and marble sludge powder is presented in

Table 1. The mica sample majorly consists of three components such as silica, alumina and

M.S.DRY POWDER (MSP)

MICA SCRAP

MICA SCRAPNaCl

MUFFLE FURNANCE

MIX

ING

IN

CR

UC

IBL

E

Roasted Mass

Ground Roasted Mass

Water Leaching

Residue Filtrate

ROASTING PART LEACHING PART

Filtration

9

potassium with a Loss on Ignition value of 5.3 %. This high LOI value is due to adsorbed

moisture in the mica sample and the corresponding thermogravimetric analysis graph is

presented in Fig.3. However, the chemical analysis of all the mortar ground mica products

indicates that potassium is equally distributed in all fractions (Fig.1c). Similarly the dry

marble sludge powder has two prime components (CaCO3 and MgCO3) with a very high LOI

value of 40.4% (Fig.3) and it is due to the removal of CO2 after heating of the sample at

900⁰C for 1 hour.

The XRD of the mica scrap sample and marble sludge powder as presented in Fig. 4(a, b)

indicates the presence of various phases. The mica sample XRD indicates the presence of

muscovite and quartz phases, whereas the marble sludge XRD confirms the presence of

dolomite and quartz as major phases. The SEM images along with the semi-quantitative

analysis and EDS pattern of mica and marble slurry powder is shown in Fig. 5 and 6. The

typical flaky structure of the mica sample as observed and the semi-quantitative data

confirms the presence of muscovite (K-bearing hydrated aluminium silicate phase). Similarly

the marble sludge sample has no definite structure with unclear particle morphology, but the

semi-quantitative data analysis indicates the presence of calcium and magnesium oxides as

the major constituents.

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Table 1Chemical analysis of the feed mica and marble sludge powder sample.

Mica Marble Sludge Powder (MSP) Composition Wt., % Composition Wt., % SiO2 48.60 SiO2 4.67 Al2O3 27.82 Al2O3 0.41 Fe2O3 4.12 Fe2O3 0.52 K2O 8.62 K2O 0.017 Na2O 0.81 Na2O 0.014 CaO 2.06 CaO 32.64 MgO 1.05 MgO 21.12 MnO 0.12 MnO 0.020 TiO2 0.16 TiO2 0.029 LOI 5.38 LOI 40.45

Fig. 3. Thermogravimetric (TGA) curve of mica and marble sludge powder.

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Wei

ght (

%)

Temperature (°C)

40.42

5.20

11

Fig. 4.XRD patterns of (a) feed mica and (b) marble sludge sample.

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Fig 5. (a) SEM image; (b) EDS analysis; (c) Elemental mapping images of the mica sample.

Fig 6. (a) SEM image; (b) EDS analysis; (c) Elemental mapping images of the MSP sample.

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3.2 Acid leaching studies

Mineral acids like H3PO4, HCl, HNO3 and H2SO4 were used to study the leaching behaviour

of the ground mica sample. Preliminary studies taking these acids of 4M concentration at a

fixed leaching temperature of 75°C, leaching time of 90 min with a stirring speed of 600 rpm

and particle size of 100 μm is presented in Fig. 7(a). The results of this study indicate that a

maximum of around 22% potassium could be leached with H2SO4, which is better than the

other three acids used for the purpose. In addition to this another, set of experiments using

H2SO4 were attempted varying the particle size of mica sample, leaching temperature and

leaching time to increase the potassium extraction efficiency. The results are presented in

Fig.7, which implies that whatever the conditions of leaching parameters may be, the

extraction efficiency will be in the maximum range of 24-26% even at 45μm particle size,

95°C leaching temperature and 180 min leaching time. So, these results conclude that acid

leaching process is not a successful method for the recovery of potassium from mica.

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Fig.7. Acid leaching of mica (a) Effect of mineral acids[4M acid , 75°C, 90 min, -100 μm] (b)

Effect of mica particle size [4M H2SO4 , 90°C, 60 min] (c) Effect of leaching time [4M

H2SO4 , 90°C, -100 μm] and (d) Effect of leaching temperature [4M H2SO4 , 60min, -100

μm]. Stirring speeds of all the experiments were fixed at 600rpm.

3.3 Roast-leaching studies

3.3.1 Preliminary roast-leaching experiments

As per the available literatures, potassium recovery from mineral sources like mica, feldspar,

nepheline syenite, glauconitic sandstone is superior using the chloridising roasting-water

9.210.8

16.4

22.1

0

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H₃PO₄ HCl HNO₃ H₂SO₄

Pota

sh e

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n (%

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Mineral Acids Used

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(a)

(d)(c)

(b)

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leaching method as compared with the acid leaching process [50, 52, 55, 56]. Reports on use

commercial roasting additives like CaCl2, NaCl, CaCO3, and Na2CO3 are widely available for

the extraction study of potassium from these minerals. The free energy-temperature diagram

of metal-chloride system suggests that the formation of KCl is favourable than any other

metal chlorides [57], which indicate that, it is possible to recover potassium as KCl by

roasting with metal chlorides like NaCl and CaCl2. However, calcium based compounds are

better due to the isoelectronic nature of Ca2+ and K+ ions, which easily replaces the K+ ions

form the complex structure comfortably [67- 69]. The probability for the release of K+ ions

from the complex structure using Mg2+ ions is less as the difference in ionic sizes of both K+

and Mg2+ is very large and also they are not isoelectronic species. Reports are available

regarding the use of MgCl2 for the purpose, but comparison with CaCl2, suggests the

effectiveness of CaCl2 over others [70]. So based on these conclusions, few preliminary

experiments were carried out using different additives and is presented in Fig. 8.

Fig. 8. Comparison of potash recovery using different commercial roasting additives.

[Roasting conditions: roasting temperature 900°C, roasting time 60 min, mica particle size

100μm, mica: additive ratio 1:1].

0

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NaCl Na₂CO₃ CaCO₃ CaCl₂ M.S.P M.S.P+NaCl

%,P

otas

h ex

trac

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Additives Used

16

The results of this preliminary study indicate that using CaCl2, potash extraction is close to 98%

and is the highest one compared to other additives whereas CaCO3 results in very low

recovery value of around 3%. This big difference in extraction value may be attributed to the

presence of Cl-ion. The presence of Ca2+ ion using both the additives has the possibility to

replace the K+ ion (due isoelectronic nature of Ca2+ and K+ ions) from the complex mica

structure, however a chloride source is necessary to combine with the replaced K+ ion to form

KCl, which is not possible in case of CaCO3 and hence low potash recovery efficiency.

Similar results for MSP as an additive has been observed as in case of CaCO3 roasting.

However, roasting of mica with a combination of MSP and NaCl, shows a sharp rise in

extraction value of around 93%. The details of extraction studies and its possible mechanism

using the waste MSP and NaCl is presented in Section 3.2.2 and Section 4 respectively.

3.3.2 Roasting using MSP and NaCl

To evaluate the effect of MSP and NaCl for the extraction of potash values from mica scrap,

experiments were performed under different conditions. Also, the effect of other vital

roasting parameters such as roasting time, temperature, particle size has been carried out and

is presented in Fig 9.

[a]. Effect of roasting temperature

The roasting temperature plays the most important role during the roasting process as shown

in Fig. 9(b). An equimolar mixture ratio of mica, MSP and NaCl (1:1:1) were roasted for 60

min using a particle size of <100µm varying the temperatures from 600°C to 1100°C. The

roasting temperature plot shows that experiments at 600°C has lowest potash recovery of

around 2% , whereas it is highest close to 94% at 900°C and a further increase in temperature

results in slow decrease in extraction value. The results so obtained can be explained by

considering the melting point of NaCl (801°C) and CaCO3 (825°C). The very low potash

17

recovery below 800°C was due to the higher melting point of the two additives NaCl and

CaCO3, which is the minimum temperature required to initialize potassium extraction

mechanism. However, as the temperature rises above 800°C to 900°C, there is a sharp change

in extraction value and attains its maximum at 900°C. Further increase in temperature beyond

900°C, shows a slight decrease in potash recovery due to formation of water-insoluble

potassium phase leucite (KAlSi2O6) at such high temperature as reported earlier by Santos et

al., 2017 [58]. There is another possibility, is that the formed sylvite (KCl) may get

volatilized at high temperature showing a low value of potassium in the leach liquor [59].

[b]. Effect of roasting time

The effect of roasting time, keeping all other parameters as constants is shown in Fig 9(a). A

set of experiments has been carried out with a time gap of 10 min which shows that for

almost 90% recovery of potassium a minimum roasting time of 30 min is required. However

increase in roasting time above 30 min, seems to have an insignificant effect on potash

extraction.

[c]. Effect of particle size

Similarly, the Fig 9(c) shows the effect of particle size on the extraction efficiency of

potassium from mica scrap. The particle size of the feed sample was varied during the

roasting process under a fixed conditions of temperature 900°C, time 60 min using

mica:MSP:NaCl 1:1:1. The results indicate that lower the particle size higher is the extraction

value and it is due to generation of larger surface area of reactivity. Particle size <100 µm has

a similar recovery as in case of <75 µm and <45 µm particle size. Therefore, it suggests that,

100 µm particle size is the optimum size for recovery of potassium from mica.

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[d]. Roasting additives ratio

The effect of roasting additives was studied taking different ratios of NaCl and MSP at 900°C

for 60min and the results are shown in Fig 9(d). In the first part of this study, the mica and

MSP are kept at fixed ratio of 1:1 varying NaCl and in second part the mica and NaCl are

fixed at 1:1 varying MSP. The result of this study shows that without the addition of NaCl,

extraction of potassium is negligible which indicates the necessity of Cl-ions [57]. However,

a continuous rise in extraction value appears as the addition of NaCl increases. Similarly the

use of NaCl only as roasting additive can extract 56% of potassium and this value goes up to

93% upon subsequent addition of MSP. Variation of NaCl and MSP has shown almost the

same results of extraction [around 84%] after 50% of additive addition with respect to mica

weight. So, the optimum conditions of extraction when compared all the three curves of

Fig.9(d) can be predicted as Mica:MSP:NaCl 1:1:0.75, 900°C roasting temperature, 30 min

roasting time and particle size of<100 µm.

[e]. Water leaching

The roasted mass generated in the high temperature furnace was cooled to room temperature

and ground to fine powder before water leaching. The effect of various leaching parameters

like temperature, time, % of solid, stirring speed has been found to have negligible effect on

leaching behaviour of the roasted mass. It is due to the highly water soluble nature of sylvite

(KCl) formed during this roasting process.

19

Fig. 9. Effect of different roasting parameter on potash recovery. The effect of (a) roasting

time; (b) roasting temperature; (c) Particle size; (d) % of roasting additives.

[f] Product Recovery

The roasted product obtained (at 900⁰C, Mica: MSP: NaCl 1:1:0.75, 30 min roasting time)

was water leached with 40% solid at room temperature for 15 minutes using a mechanical

stirrer. As the roasted product contains sylvite and halite, it easily comes to the solution,

whereas the water insoluble phases remains with the residual part. Then the filtrate was

evaporated at 80-90ºC till the formation of white shining crystals followed by cooling at

room temperature. The chemical analysis of the dried salt was found to be composed of only

KCl (24.9 %) and NaCl (74.7 %) whereas the major chemical composition of the residue is

presented in Table 2. Further characterisation studies by XRD and SEM confirms the

presence of these two phases as shown in Fig 10 and 11 respectively.

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0 100 200 300 400 500Pota

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Additives (%)

Effect ofM.S(M:NaCl 1:1) (d) (c)

(b) (a) 0

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500 600 700 800 900 1000 1100 1200

Pota

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Temperature (°C)

(b)

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Fig. 10. XRD patterns of the crystallised salt after roast-leaching experiments.

Fig 11. (a) SEM image; (b/c) EDS analysis; (d/e/f) Elemental mapping images of the crystallised salt.

21

Table 2. The major chemical composition of the roast –leached residue.

Roast-leached Residue Composition Wt., % SiO2 36.75 Al2O3 18.30 Fe2O3 2.96 K2O 0.42 Na2O 0.26 CaO 21.34 MgO 14.90 TiO2 0.10

4. Mechanism of Extraction

The mechanism of the extraction process has been established using the XRD and SEM

studies of the roasted product and the corresponding roast-leached residue produced under the

optimum conditions (Mica:MSP:NaCl 1:1:0.75, 900°C roasting temperature, 30 min roasting

time and particle size of <100 µm).The extraction mechanism can be discussed in two ways.

Mechanism I:

The heating of mica scrap fine powder at temperature ranging between 600°C-900°C, results

in dehydroxylation followed by decomposition of the sample as presented in Eq 1-3. The

comparison of the XRD peaks of both feed and calcined mica sample, indicates that the peak

intensity of calcined mica sample have been lowered and also maximum number of small

peaks have been vanished indicating the physical distortions of AlO6 octahedron and SiO4

tetrahedron as reported in literatures [60,71]. The appearance of some broad peaks may be

due to the generation of small sized mica particles or conversion of crystalline phases to

amorphous phases [61]. Also the SEM images shows the breaking of flaky mica structure to

finer fractions indicating a clear differentiation between the mica and calcined mica sample.

22

The XRD and SEM images are presented in Fig. 12 and 13 respectively for comparison

purpose.

K2O.3Al2O3.6SiO2.2H2O = K2O.3Al2O3.6SiO2 + H2O↑ …………….1

K2O.3Al2O3.6SiO2 = K2O+ 3Al2O3 + 6SiO2 ……………..2

3Al2O3 + 6SiO2 = 3Al2SiO5 .……………..3

This change in physical property of the mica due heating, possibly generates micro-cracks

with loose binding of K unlike the untreated feed sample and the K present in it gets easily

converted into its chloride form (as KCl) when treated with external chloridizing roasting

additives at the same calcination conditions.

Fig. 12. Comparison of XRD patterns of feed mica and calcined mica sample.

23

Fig.

13. SEM micrographs of (a) mica and (b) calcined mica sample.

The simultaneous decomposition of dolomite takes place within the temperature range of

687°C-916°C [62], which combines with the molten NaCl (melting point 801°C) to produce

the MgCl2 and CaCl2 along with Na2CO3 and Na2O as per the reactions presented in Eq 4-

6.However, the presence of NaCl, further lowers the decomposition temperature of dolomite

to 20-100°C facilitating the reaction process [63].

Early-stage of heating (below 801°C)

CaCO3.MgCO3 = CaCO3 + MgO +CO2↑ ……….(4)

At above 801°C

CaCO3 +2NaCl = CaCl2 + Na2CO3 ………. (5)

MgO+ 2NaCl = MgCl2+ Na2O ………. (6)

The CaCl2/MgCl2 formed during this initials stages of heating plays the most important role

for the extraction of potassium. The reaction between the CaCl2 and MgCl2 with the silica

(SiO2) (generated as per Eq 2) produces two new mineral phases such as wollastonite

943X

943X(b)(a)

24

(CaSiO3) and enstatite (MgSiO3) which are confirmed form the XRD study of roasted and

water leached products. The appearance of triclinic (wollastonite) and orthorhombic (enstatite)

crystal structures in its SEM images of both the products also substantiates its formation. The

XRD and SEM images are presented in Fig 14 and 15. The appearance of sylvite (KCl)

mineral in the roasted product and its disappearance in the water leached residue is also

observed in their XRD and SEM study along with other insoluble phases like Kyanite

(Al2SiO5) of acicular bladed structure. The possible reactions are presented in two sets of

reactions as presented in Eq 7 and 8.

CaCl2+SiO2+0.5O2= CaSiO3+Cl2

CaCl2+SiO2+H2O= CaSiO3+HCl (7)

HCl +K2O = KCl +H2O

MgCl2+SiO2+0.5O2= MgSiO3+Cl2

MgCl2+SiO2+H2O= MgSiO3+HCl (8)

HCl +K2O = KCl +H2O

25

Fig. 14. XRD patterns of roasted mica and roast-leached residue of sample.

Mechanism II

There is another possibility that the, muscovite [KAl3Si3O10(OH)2] reacts directly with the

quartz present in the mica and MSP sample at high temperature to form K-Feldspar

(KAlSi3O8) and aluminium silicate (Al2SiO5) with loss of all the adsorbed moisture [64,65].

Then the CaCl2 formed (by reaction of MSP and NaCl), reacts with the K-feldspar to produce

anorthite (CaAl2Si2O8), which further dissociate in next step to form wollastonite (CaSiO3)

and Kyanite (Al2SiO5) [51,55,66,67]. The appearance of different possible phases are verified

by the XRD and SEM micrographs of the roasted and roast-leached residue as presented in

Fig. 14 and 15 respectively. The possible reactions are presented in a set of Eq. 9.

KAl3Si3O10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O

KAlSi3O8 + CaCl2 = CaAl2Si2O8 + 4SiO2 + 2KCl [9]

CaAl2Si2O8 = CaSiO3 + Al2SiO5

26

Fig.15. Scanning electron micrographs of (a) roasted product (b) water leached residue

showing the formation of different mineral phases (S-sylvite; K-Kyanite; E- enstatite; W-

wollastonite).[ Mica:MSP:NaCl 1:1:0.75, 900°C roasting temperature, 30 min roasting time

and particle size of <100 µm).

Conclusion

The generation of huge amount solid waste (MSP), during the processing of marble creates

many environmental problems like reduction of soil fertility, depletion of water permeability,

increase of suspended particulate matters in air including many health related issues. So,

there is a need to reuse the generated MSP for value addition rather than land refilling. In this

connection, the MSP wastes were used to recover potash values from a low-cost mica scrap

waste using chlorination–roasting method. Preliminary studies using H2SO4 acid leaching

process indicates to recover a maximum of 22% potassium from mica scrap using particle

size, 100µm ; stirring speed 600rpm; leaching temperature 75°C and time 90 min. However,

the roast-leaching experiments using MSP and NaCl, results in recovery of almost 93% of

potash values from the mica scrap under the optimum conditions like Mica:MSP:NaCl

27

1:1:0.75, roasting temperature 900°C, time 30 min, particle size 100µm.There is a substantial

increase in potash recovery when the temperature rises from 800°C to 900°C. Further, rise in

temperature results in lowering of recovery value owing to the formation of water insoluble

potassium phases like leucite (KAlSi2O6) or volatilization of KCl. The formation of different

water soluble and insoluble phases like sylvite, wollastonite, halite, enstatite, and kyanite

were evident from their XRD phases and SEM micrographs of the roasted and roast-leached

products. The present report discusses an encouraging process to recover potash values for

fertiliser application utilising two cheap industrial waste materials.

Acknowledgements

The authors are thankful to the Head, Mineral Processing Department and Director, CSIR-

IMMT, Bhubaneswar for his kind permission to publish this paper. The authors also wish to

gratefully acknowledge Mr Sapan K. Kandi and Mr Debadatta Sahoo for useful discussions

during the manuscript preparation.

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