Comparative analysis of demineralization on Indian C

8/3/2019 Comparative analysis of demineralization on Indian C

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COMPARATIVE ANALYSIS OF DEMINERALIZATION OF

DIFFERENT INDIAN COALS DURING CHEMICAL BENEFICIATION

AT ELEVATED TEMPERATURE UNDER ATMOSPHERIC PRESSURE

Document by:Bharadwaj

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ABSTRACT

Coking industries, and especially, blast furnaces, are more sensitive to high ash contents in coals. Asa result, enormous efforts are being extended to make coal a better source of energy. Accordingly,

physical as well as chemical coal cleaning processes have been explored. In contrast to physical coalcleaning, chemical coal cleaning techniques are in an early stage of development. In the present work,

an effort has been made to study the effect of aqueous alkali leaching alone and in combination withacid leaching and washing on the removal of mineral matter from different Indian coals containinghigh ash under elevated temperature at atmospheric pressure. Chemical leaching experiments wereconducted for five different coals of Indian origin at various process conditions using the laboratory

and bench scale experimental set up. The research study revealed that it is possible to reduce the ashcontent in all the coals by this method. The degree of demineralization however varies from coal tocoal. The degree of demineralization was improved by increasing the reaction time and alkaliconcentrations. The effect of this treatment on the alkali, silica, alumina and phosphorous content of

the coals were also evaluated. The mechanism of demineralization was evaluated by analyzing thecoal samples before and after the alkali and acid treatment using XRD technique.

KEY WORDS: coal, leaching, mineral processing

1. INTRODUCTION

Most of the coals are associated with mineral matter, which makes it unsuitable for efficient

utilisation, such as carbonisation, gasification, combustion, or liquefaction. Coking industries, and

especially, blast furnaces, are more sensitive to high-ash contents in coals. As a result, enormous

efforts are being extended to make coal a better source of energy. Accordingly, physical as well as

chemical coal cleaning (beneficiation) processes have been explored. In general, physical coal

cleaning processes involve pulverizing the coal to release the impurities, wherein the fineness of the

coal generally governs the degree to which the impurities are released. Based on the physical

properties that effect the separation of the coal from the impurities, physical coal cleaning methods are

generally divided into four categories: gravity, flotation, magnetic and electrical. In contrast to

physical coal cleaning, chemical coal cleaning techniques are in an early stage of development.

A review of the literature of the chemical cleaning of coals shows that the methods used include acid

leaching, alkali leaching under high pressure and at elevated temperatures and leaching by molten

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caustic baths. Choudhary and Bhaktavatsalam [1]worked in the area of chemical treatment of coal

whereammonia treated coal was reacted with a mixture of concentrated sulphuric acid and calcium

fluoride in a two stage process at a reaction temperature of 350 deg. C.

Demineralization/desulphurization of coal by aqueous or fused sodium hydroxide alone or followed by

mineral acids have been reported by many investigator [2-6]. They achieved different levels of successin removing mineral matter and sulphur from coal from different sources by treatment with sodium

hydroxide alone or in combination with other substances. Sharma and Gihar [4] applied the method ofchemical cleaning of low grade coals through alkali-acid leaching employing mild conditions under

ambient pressure resulting in substantial demineralization. The removal of sulphur and ash from coal

treated with aqueous hydrogen peroxide/sulphuric acid solutions has been studied at ambient

temperature, under a variety of experimental conditions. Almost complete elimination of the sulphate

and the pyritic sulphur was observed. Waugh and Bowling [6] reported 90% reduction of the mineral

matter content of coal from Australia by caustic wash.

In the present work, an effort has been made to study the effect of aqueous alkali leaching alone and in

combination with acid leaching and washing on the removal of mineral matter from five different

Indian coals containing high ash under elevated temperature at atmospheric pressure. The results

obtained in the laboratory scale studies are reported here. The research was carried out keeping in

mind the high ash content and finely disseminated mineral matter content typical of Indian coals.

2. METHODOLOGY/EXPERIMENTATION

Five different coal samples were selected for the chemical leaching experiments using alkali and acid.

The coals used for this study are West Bokaro (WB) clean coal, Jamadoba, WB ROM, WB Middling

and WB Sink-Float coals. Various properties of these five coals are listed in Table 1. WB clean coal is

a physically beneficiated medium coking coal from West Bokaro coal field and Jamadoba coal is aprime coking coal from the Jharia mines located in the eastern part of India. WB ROM coal is a run-

of-mine coal sample collected from the WB collieries. WB Middling and WB Sink-Float coal samples

are artificially generated from the froth flotation feed using sink-float method. Coals at low ash level

are separated out from the original materials to generate these samples at moderate to high ash levels.

All these coal samples are ground to desired size (-0.5 mm), preserved in desiccators, and used for allsubsequent analyses under this study. The ash content of these coals varies in between 15% for WB

coal to 26.4% incase of WB ROM coal. Aqueous solution of sodium hydroxide (NaOH) in

combination with and HCl of various concentrations were prepared and used for carrying out the

experiments.

Table 1: Properties of Various Coals

Properties WB Clean

Coal

Jamadoba

Clean Coal

WB ROM

Coal

WB

Middling

WB

Sink-Float


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Proximate Analysis (db), %

Ash

Volatile MatterFixed Carbon

15.0

27.2

57.8

17.9

21.1

61.0

26.4

25.3

48.3

19.0

26.1

57.8

25.0

27.2

57.8

Ash Composition, %Total Alkali (Na2O + K2O)

SiO2Al2O3Phosphorous (P)

Sulphur (S)

0.282

8.120

3.670

0.133

0.850

0.309

9.710

4.000

0.134

0.670

0.560

12.170

6.160

0.196

1.100

-

-

-

-

-

-

-

-

-

-

Calorific Value (CV), kcal/kg 6992 6344 5408 - -

Crucible Swelling Number (CSN) 6.5 6.5 2.5 - -

The experiments were carried using the laboratory scale setup as shown in Figure 1. The equipment

assembly consisted of a Reactor which is three-necked 1000 ml round bottom flask. The main centreneck was utilized for assembling the motor and stirrer. The speed of the motor for stirring was

controlled through a variac. A condenser was connected to one of the other two necks which, in turn,

was connected to a conical flask with water for trapping the volatile gases generated during the

reaction. The third neck is used to facilitate addition of reagents using a dropping funnel and also tomonitor the temperature of the reaction bath using a thermometer. Rotamantle was the source of heat

input for the reactor to raise the temperature to the reaction temperature of 80-85 C. This rotamantle

does a two-fold job of controlling the temperature as well as speed of the stirring (when the magnetic

stirrer is used). Both these parameters can be controlled through the variacs. The experiments werealso repeated in a bench scale reactor. The feed coals were treated 10-40% aqueous NaOH solution for

5.0 hr at 80-85 deg. C with stirring. The alkali treated coal is first filtered and washed with only water

or water followed by 10% HCl solution and then dried. The dried product coal samples after chemical

treatment were analyzed for their properties.

Fig. 1: Laboratory Scale Experimental Setup

LEGEND

A Rotamantle

B Three necked round

bottomed flask

C Water condenser

D Motor & stirrer

E Reagent addition and

temperature measurementF Conical flask containing water

D

C

FA

BE


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3. RESULTS AND DISCUSSION

3.1 Effect of Alkali Concentration followed by Acid Treatment on the Demineralization

To study the effect of chemical leaching process involving alkali treatment followed by acid washing

on demineralization of different coals, the coal samples were treated with NaOH of 10-40%

concentrations at 85

C under atmospheric pressure maintaining a coal to reagent ratio of 1:10. All the

experiments were conducted for 2.5 h. After the alkali treatment the coals samples were washed first

with water and then using 100 ml 10% HCl solution. The product coal samples, after filtration, were

analyzed for their ash contents. The test results are shown in Fig. 2. From Fig. 2, it can be seen that

alkali treatment, depending on the concentration of alkali, decreases the ash content of different coals

to various degrees. The percentage ash removal increases with the increase of NaOH concentration up

to a critical value. At higher NaOH concentration, the extent of demineralization becomes invariant ofthe NaOH concentration. The reduction in ash content on acid treatment of the alkali treated coal was

found to increase with increase in concentration of alkali. Increase in alkali concentration exhibits

positive influence on the ash removal up to 30% NaOH concentration incase of WB, Jamadoba and

WB ROM coals. Further ash reduction is insignificant above 30% NaOH concentration incase of these

coals. The rates of reduction in ash percentage incase of WB Middling and WB Sink and Float coal

samples are less than that of the rest of the three coals. Even if the rate of removal of ash decreases at

higher concentrations of alkali for these coals, the ash removal continues even after 30% NaOH

concentration. It shows that the mineral matter are firmly bound and finely distributed inside the coal

matrix for these coals. Hence the ash reduction rate is low as compared to other three coals.

Prolonging the treatment time or increasing the alkali concentration may even lead to continuous gain

in ash of some coals as stated in literature. The gain in ash content or decrease in degree of

demineralization of the coal samples with increase in alkali concentration is attributed to sodium

aluminosilicate formation. Alkali also reacts with silica and alumina in clay and other silica and

alumina-bearing materials present in the coal forming soluble silicates and aluminates as per the

following simplified reactions.

SiO2 + 2 NaOH Na2SiO3 + H2O

Al2O3 + 2 NaOH 2NaAlO2 + H2OThe concentrations of silicate and aluminate ions in the solution gradually increase with alkali

concentration and treatment time till the same exceed the solubility product of sparingly soluble

sodium aluminosilicates. The later is precipitated as per the following simplified reaction:

Na2SiO3 (aq) + NaAl(OH)4 (aq) + NaOH (aq) + H2O [Naa(AlO2)b(SiO2)c . NaOH . H2O]

Continuous decrease in ash removal or gain in ash with alkali concentration and reaction period is

therefore attributed to progressive precipitation and accumulation of aluminosilicates. This can happen

when more and more amount of silica and alumina from the coal dissolves out.

3.2 Effect of Reaction Time with Alkali on the Demineralization of Different Coals

The major factors, on which degree of demineralization depends, are alkali concentration and the

treatment time. To study the effect of reaction time on the demineralization of different coals,

chemical leaching experiments were conducted for 2.5 h and 5.0 h reaction time. The experimental

results plotted in Fig. 3 shows that regardless of the type of coal, the maximum reduction of ash

content is achieved within 2.5 h. However, smaller improvement in demineralization is obtained even

at longer reaction time.

Sodium-aluminosilicate gel


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3.3 Effect of Chemical Leaching Process on Various Properties of Coal

The effect of the chemical leaching process on various properties of West Bokaro (WB) clean coal has

been depicted in Table 2. There is an overall increase of nearly 5% in the calorific value. There is no

significant change in the alkali content either after alkali treatment or after the acid treatment provingthere is no accumulation of Na2O or K2O during the process. The silica and alumina content reduced

by nearly 50% and 70% respectively with 40% NaOH treatment followed by acid treatment. Since

percentage of alumina removal is more than that of silica, a positive change is noticed in the Alumina

(Al2O3) to Silica (SiO2) ratio which plays an important role during the steel making process. A

marginal reduction in sulphur content is observed. There is no significant change in the phosphorous

content of the coal samples after the first step where the treatment is done only with alkali. However, a

sharp decrease in phosphorous content is noticed after 10% HCl washing.

Table 2: Properties of Med. Coking Coal after alkali treatment and acid washing

10% NaOH 20% NaOH 30% NaOH 40% NaOH

10 % NaOH

+ 10% HCl

20 % NaOH

+ 10% HCl

30 % NaOH

+ 10% HCl

40 % NaOH

+ 10% HCl

Ash (db), % 12.6 11.4 10.2 9.4 9.9 7.5 6.5 6.4

C.V., kcal/kg 7012 7073 7122 7220 7044 7112 7220 7312

Alkali, % 0.344 0.315 0.307 0.300 0.326 0.279 0.271 0.238

SiO2 Removal, % 11.8 21.3 29.6 37.6 26.8 41.1 48.0 49.5

Al2O3 Removal, % 17.7 31.6 41.4 52.3 36.8 59.7 65.9 69.2

Al2O3/SiO2 0.42 0.39 0.38 0.35 0.39 0.31 0.30 0.28

P, % 0.124 0.12 0.114 0.12 0.04 0.032 0.028 0.028

S, % 0.8 0.8 0.8 0.75 0.8 0.79 0.75 0.68

3.4 Analysis of the feed and product coal samples using XRD technique

Analysis of the Feed and Product Coal Samples using XRD TechniqueAsh samples were prepared from the coal samples generated during the experimentation. The ash

samples were analyzed with X-ray Diffraction (XRD) method. X-ray Diffraction was done with the

Philips diffractometer using Co radiation from 10

to 80

, 2, with a 0.05

step size. Each scan is

Fig. 2: Effect of Alkali Concentration on the

Demineralization of Various Coals

Fig. 3: Effect of Reaction Time with Alkali

on Demineralization


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processed using standard JCPDS XRD search match programs for identification of mineral phases

present and analyzed using Braggs Law. Qualitative analysis on various minerals present in WB clean

coal was carried out and various mineral phases were identified using the XRD plots. Si and Al are by

far the most abundant elements in these samples due to the high abundance of kaolinite, quartz and

other clay compounds. These are usually the minerals present in coals of Indian origin.Montmorillonite was also found to be present. Fe is the next most abundant mineral element present.

This is evident from the presence of pyrite, hematite and siderite minerals in these coal samples whichcontain Fe. Trace quantities of fluorapatite was also detected in the WB clean coal sample which

shows the presence of calcium and phosphorous in coal. K, Ca, Na and Mg are the next most abundant

elements. Trace amounts of potassium feldspar, gypsum, dolomite and natrojarosite may also occur in

few coal samples. Al, Si, Fe, Ti, Ca, Mg, K, S and Na are often the most abundant mineral elements in

coal. It was observed that intensity of the peaks belonging to the mineral matters Kaolinite, Quartz and

Montmorillonite-Chlorite start decreasing after alkali treatment for 2.5 hr and new peaks appear at 28

degree, 53.8 degree and 37.4 degrees. These new peak corresponds to sodium aluminum silicate

hydroxide (sodalite). The coal samples were washed with 10% HCl after treatment with alkali.

Sodalite has a tendency to get dissolved in acidic solutions. The filter cake was washed with the water

to remove the dissolved salts. The peaks pertaining to sodalite gets removed during the acid washing

which causes further reduction in ash content during the acid treatment.

4. CONCLUSION

It is possible to reduce the ash content physically beneficiated Indian coal by removing about 60% of

the mineral matter by treatment with caustic solution followed by acid washing. For the present case,

the coal yields are above 70% in most of the experiments. A positive change is noticed in the Alumina(Al2O3) to Silica (SiO2) ratio. A marginal reduction in sulphur content and significant reduction in

phosphorous content in envisaged after the acid treatment. The degree of demineralization improved

by increasing the reaction time, alkali and acid concentrations, temperature during experiment and

decreasing the particle size and coal to reagent ratio. The mechanism of demineralization was

evaluated by analyzing the coal samples before and after the alkali and acid treatment using XRD

technique.

5. ACKNOWLEDGEMENTS

The authors are grateful to Dr. Debashish Bhattacharjee, Director (R&D and Technology), Tata Steel

Group for providing all the support to continue the research work.

6. REFERENCES

1. Choudhury, R. and Bhaktavatsalam, A.K. (1997), Beneficiation of Indian coals by chemical

techniques,Energy Convers. Mgmt., Vol. 38, No. 2, pp. 173-178.

2. Culfaz, M., Ahmed, M. and Gurkan, S. (1996), Removal of mineral matter and sulphur from

lignites by alkali treatment,Fuel Processing Technology, Vol. 47, pp. 99-109.

3. Mukherjee, S. and Borthakur, P.C. (2003), Effect of leaching high sulphur sub-bituminous

coal by potassium hydroxide and acid on removal of mineral matter and sulphur, Fuel, Vol.82, pp. 783-788.

4. Sharma, D.K. and Gihar, S. (1991), Chemical cleaning of low grade coals through alkali-acid

leaching employing mild conditions under ambient pressure,Fuel, Vol. 70, pp. 663-665.


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5. Steel, K.M. and Patrick J.W. (2003), The production of ultra clean coal by sequential

leaching with HF followed by HNO3,Fuel, Vol. 82, pp. 1917-1920.

6. Waugh, A.B. and Bowling, K. McG. (1984), Removal of mineral matter from bituminouscoals by aqueous chemical leaching,Fuel Processing Technology, Vol. 9, pp. 217-233.

CONTACT DETAILS:

Name: Dr. Pratik Swarup Dash

Position: Researcher

Company: Tata Steel Limited

Mailing Address: Raw Materials & Coke Making Research Group, R&D, Tata Steel,

At/PO: Burmamines, Jamshedpur, Jharkhand, India

Post/Zip Code: 831007

E-mail: [email protected]; [email protected]

Telephone: +91 - 657 - 6642596/6647421

Mob: +91 9234549790

Document by:Bharadwaj

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Comparative analysis of demineralization on Indian C