The influence of surface phenomena on the dewatering of fine clean coal

The Influence of Surface Phenomena on the Dewatering of Fine Clean Coal B.P. Singh Regional

Research

Laboratory,

Bhubaneswar

751013, Orissa, India

The paper presents results of experimental work carried out to study the role of surface active agents in the dewatering of fine clean coal. The fundamental phenomena which determine their mode of action are examined, and the mechanism of surfactant-enhanced dewatering is investigated in terms of the surface chemical phenomena and the physical processes involved in the dewaterlng. The performance and possible mode of action are compared between two surfactant dewaterlng aids. The results of dewatering experiments show, that the presence of the sodium dodecyl sulphate leads to a very significant reduction in the final residual cake moisture content. A direct and strong correlation exists between the point of zero charge, surface tension reduction in the coal system and residual moisture reduction in the filter cake. These improvements are attributed to the chanaes in the wettina characteristics of the coal particles in the presence of effective dewatering ai&. t is common practice in the coal industry to dewater the fines, or froth flotation product. Coarse ( > 0.75 inch) coal I is easily dewatered to 3% to 4% moisture using vibrating screens and centrifugal dryers. The main problem of excess product moisture occurs with fine (-28 mesh) clean coal and refuse. Water in clean coal reduces the heating value of the coal, creates handling and pulverising problems, and increases transportation costs. From a utility viewpoint, a 1% increase in moisture can offset a 4.5% decrease in ash content.t’l The economics of coal utilisation are critically dependent on solid/liquid separation technology. In particular, as energy costs rise, the need for better solid/liquid separation theory and practice becomes increasingly urgent. In recent years, however, attention has increasingly focused on the use of chemical pretreatments as a means of reducing moisture content in the final filter cake.t2-51 Flocculant filter aids and surfactant dewatering aids are two distinct types of proprietary chemical additives generally used in industry. Conventional dewatering aids are based on the dialkyl esters of succinic acid, commonly known as the sulphosuccinates. r3] However, surfactant-aided dewatering aids can influence product properties, and therefore the following advantages are possible in filtration processes?

Although surfactant-enhanced dewatering is of economic and technical importance, the fundamental surface chemical phenomena which decide their mode of action are not fully understood. At this point it is prudent to mention the structural and functional groups of various types of dewatering aids about which we have first-hand information. In this field, two distinct classes of proprietary chemicals are available for enhancing the filtration dewatering properties of slurries. Flocculant

filter

aids

These are linear, tong-chain, water-soluble erally based on polyacrylamide. Typical ionicities are: -CH-CH-

--CH-CH-

--CH-CH-

c=o $,J@

7=” NH

-CH-CH-

c=O NH

B N+R3) Cationic

Anionic

polymers formulas

genand

-CH-CHc=o Cl’

k H2 Nonionic

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The basic function of a flocculant filter aid is to adsorb at the solid/liquid interface, and aggregate particulate matter. Flocculants increase the filter yield of solids, and produce a permeable filter cake amenable to rapid dewatering. In this case, the long-chain polymers bridge between individual particles, to produce a multi-particle aggregate.

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Surfactant

dewatering

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P=

2 TGL

COS e sL rc

(1)

where P is the pressure (kPa), r, the surface tension (mN/m), rcL the capillary radius (m) and 6 the solid/liquid contact angle (de0

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CH-COOR ; 2 2%

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where R is a hydrocarbon group. This allows filter cake capillaries to drain more readily, and thus reduces the residual moisture content of the cake. Both classes of chemical additives can be used separately or in combination. The aim of this paper is to show how surfactants act in the dewatering of filter cakes, and to throw light on their mode of action. The surfactants chosen for this study were sodium dodecyl sulphate (SDS) and dodecyl ammonium bromide (DAB).

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The mode of action of this type of aid is not really understood, despite their successful application worldwide. They typically consist of amphiphilic molecules composed of a hydrophilic and a hydrophobic grouping. Conventional types are normally based on sulphosuccinate:

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The use of flocculants as a dewatering aid is well documented,“’ but very little is known about the mode of action of surfactant dewatering aids, despite their successful application worldwide. The proposed mechanism of surfactant action involves adsorption at the gas/liquid interface, leading to a reduction in liquid surface tension and an increased solid/ liquid contact angle,t5V71 which is often represented by the Laplace-Young relationship in the following form:

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0 Improved dewatering as a result of lower capillary retention forces at a given pressure potential in a vacuum or pressure filter. 0 Savings in thermal drying costs. fJ Improved filtrate recovery in hydrometallurgical processes. 0 Extended application of continuous filtration processes towards finer particles.

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TABLE 1. CHARACTERISTICS OF COAL SAMPLE. Ash content

19.4%

BETsurfacearea with adsorbate N2 Pore size distribution

3.0 m’lg < 0.02 pm

pH of point of zero charge

5.3

Analysis: Moisture

0.06%

Volatiles

19.55%

Ash

19.40%

Fixed carbon

60.97%

Hydrogen

6.22%

Nitrogen

1.75%

Chlorine

0.03%

Oxygen

11.63%

TABLE 2. X-RAY FLUORESCENCE OF FINE CLEAN COAL.

Manometer

lgure 1. Bench-scale dewatering equipment set-up.

Percentage

Fe

0.09 - 0.6

cu

0.07

Ti

< 0.02

Ca

0.06

Si

0.50 -0.70

EXPERIMENTAL

Slurry sample: The froth flotation product was obtained from the coal washery. The characteristics of the coal sample are summarised in Table 1. The product contained 20%‘solids (19.4% ash), with an average particle size of 30 pm. The BET surface area with adsorbate N2 was 3.8 (m*/g). XRD analysis showed minor quantities of Fe, Si, Ca, Cu, Ti etc. (see Table 2). Reagents: The reagents used for dewaterability studies were kindly supplied by Merck, Germany and Rishfloc, India. All the other reagents were water-soluble, and were prepared following the methods suggested by the manufacturers. Sodium dodecyl sulphate and dodecyl ammonium bromide were selected as examples of anionic and cationic surfactants, with the ability to appreciably reduce the slurry surface tension. Chemically well defined materials were used instead of proprietary brand surfactants, in order to gain some insight into the process fundamentals. Blectrokinetic behaviour The electrokinetic mobilities of the coal sample were measured with the Rank Brothers Mark II microelectrophoresis apparatus, using the two-electrode mode and a flat quartz cell. A detailed description of this apparatus and the procedures for preparing test suspensions have appeared elsewhere.f*] Contact

angk

msasurement

The contact angles were measured using the captive bubble technique. Is1 A gas bubble was formed at the tip of a microsyringe immersed in the test solution which also contained the coal particles. The whole system was equilibrated for 12 min

isotherms

The coal sample (0.5 g) was weighed in a series of cleaned and steamed glass-stoppered test tubes. To each tube was added the surfactant solution (15 ml) of known strength. These were then placed in a thermostatic bath maintained at the desired temperature. Equilibrium is attained once the concentration of the solution remains constant with time. At the end of an experiment a sample of the supernatant solution was centrifuged at 15009 for 28 min, and the liquid above the settled coal layer was analysed for the residual concentration of surfactant. Abstraction of the surfactant was calculated from the difference between the initial and final values. The equilibrium surfactant concentration at the end of an experiment was determined by the method developed by Epton.f”] Filtration

Materials

I

before bringing the gas bubble into contact with the coal particle surface. Then the whole system was equilibrated for 12 min before measuring the contact angle. Adsorption

Element

1

and

dewatering

experiment

The filtration and dewatering studies were carried out using laboratory bench-scale vacuum filtration equipment. A schematic diagram of the filtration set-up is shown in Figure 1. For the dewatering studies a known quantity (normally 30%) of the slurry was conditioned in a beaker with reagents, using a constant-speed stirrer for 10 min, and subjected to filtration using Whatman No.1 filter paper. The filtration rate was calculated by monitoring the filtrate volume collected at intervals of 5 s. The drying period was kept at 30 s following each experiment. At the end of each drying period, the cake was carefully removed from the Buchner funnel, weighed and dried in an electric oven at 105°C overnight. The dried filter cake was allowed to cool in a dessicator, and then weighed. The final moisture content was calculated from the difference in the weight of the filter cake. RESULTS Baseline

dewatering

AND DISCUSSION

studies

Baseline dewatering data, without using any reagents, are shown in Figure 2. After 4 min of filtration the residual moisture content was 21%, and there was no significant reduction in the moisture content after 4 min. The effect of pH on the residual cake moisture and on the electrophoretic mobility of the coal fine is shown in Figure 3. The point of zero charge (PZC) was near pH 5.2. The cake moisture content was found to be minimum at the PZC for the test conditions shown. This is attributed to minimisation of electrostatic repulsive forces near the PZC, which induces flocculation.t’31

I -d

0

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Filtration

Time

I b (Minutes)

0

Figure 2. Effect of filtration time on residual cake moisture of clean coal. Drying time 30 s, vacuum 72 cm Hg, pH 5.2, solids 30%.

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isL---.

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Distilled

I

8

6

,

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40

go

IDewatering

120

aids

160

200

dosage,(g)

240 t)

Figure 4. Performance of sodlum dodecyl sulphate (SDS) and dodecyl ammonium bromide (DAB) as dewaterbg aids on coal slurry. Drying time 20 s, vacuum 72 tm Hg, pH 5.3, solids 80%, fbcoubnt 40 g1 tonne.

Water

It has been observed that sodium dodecyl sulphate (SDS) was much more effective than dodecyl ammonium bromide (DAB). The effect of cationic surfactant on the cake moisture was marginal, whereas the anionic surfactant showed a considerable reduction in moisture to about 11% at a dosage of 80 g/tonne. The moisture content profile, which is highly reproducible, shows that a concentration of SDS as low as 80 g/tonne was optimum, and above this there was no change in the residual moisture content of the cake. The reduction in surface tension with these two surfactants at optimum dosages was also recorded. The effects of reducing the liquid surface tension by addition of either an anionic or a cationic surfactant was quite different. The results reveal a dramatic fall in retained moisture with anionic surfactant addition of the order of 80 g/tonne, but the corresponding change in the case of cationic surfactant was much smaller. The corresponding changes in slurry surface tension with SDS and DAB were of the order of 30,and 47 mN/ m, respectively.

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10

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12

PH

igure 3. Residual . cake. moisture and ebctrophoretic clean coal as recelveo.

52

mobility of

+-

DAB

-b

SDS

1

The fine size fraction has a dominant influence in determining the filtration characteristics of a coal slurry. The agglomeration of fine particulates played a significant role in determining the cake moisture. The basic mechanism that contributes charge to the surface of the coal particles in aqueous media is the dissociation of ionogenic groups on the coal surfaces, such as those having carboxylic, phenolic and hydroxylic functionality. For such groups the degree and sign of the charge developed depend on the pH of the liquid phase. Since H+ and OH- are potential determining ions, considering a surface containing two acidic groups, one relatively strong (carboxylic, for example) and other one weak (for example, phenolic), the surface charge is governed by the ionisation of these groups.t”5 “I Effeot of surfaotanto on filter cake moisture content Studies were carried out on varying the amount of surfactant, to determine the optimum amount of surfactant needed. Figure 4 shows the variation in the residual moisture content of a typical filter cake for various concentrations of anionic and cationic surfactants added to the feed slurry prior to cake laydown.

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P”

Figure 5. Ebotrophoretb mobility and filter cake moisture of oban coal In presence of SDS (80 g/tonne) and DAB (60 g/tonne).

Effect

of SDS

and

DAB

on electrophoretic

mobility

The effect of sodium dodecyl sulphate and dodecyl ammonium bromide on the electrophoretic mobility and filter cake moisture content is shown in Figure 5. In general, the electrophoretic mobility becomes more negative with increasing surfactant concentration.[2~ I41 Furthermore, in this case it was observed that the filter cake moisture is significantly lower in the region of the PZC. For both systems, the lowest filter cake moisture was obtained at the PZC. There was little shift in the PZC in the presence of either SDS or DAB surfactants (from pH 5.3 to approximately pH 6), and the electrophoretic mobility was more negative compared to the case without surfactants. In this case, the amount of surfactant was kept constant (SDS 80 g/tonne, DAB 60 g/tonne). From adsorption isotherm measurements, it was found that monolayer coverage of these surfactants corresponds to these concentrations. For the anionic surfactant, the lowest moisture (11%) in the filter cake was obtained at a dosage of 80 g/tonne. However, for the cationic surfactant, the lowest moisture (18.0%) was obtained at a dosage of 60 g/tonne. The explanation for the better performance of SDS in comparison to DAB was that the negative charge present at the coal surface prevents any significant adsorption of SDS, because of electrostatic repulsion. Thus most of the SDS was adsorbed at the gas/liquid interface, causing a large reduction in the surface tension, to which the greater reductions in the residual cake moisture were attributed. In the case of DAB, however, a significant amount of DAB was adsorbed at the solid/liquid interface (via electrostatic attraction), and very little surfactant was available for adsorption at the gas/liquid interface. As a result of this, only a small reduction in the surface tension was attained, and hence the poorer performance. This is also corroborated with the finding of Nicol.[4. I51 Adsorption at the coal/ water interface The adsorption isotherms for SDS and DAB are shown in Figure 6. They exhibit typical Langmuir-type behaviour. The isotherm for DAB appears to reach a plateau at about 60 mg/l concentration, whereas with SDS this is reached at 80 mgll concentration. In both cases, the maximum adsorption

01 0

Ice

1

80

240

1CQ

Equilibrium

concentration

320

(mg/t)

igure 7. Effect of SDS and DAB on contact angle of coal partisle (PM 5.3). Contact

angle

Contact angle measurements for SDS and DAB are shown in Figure 7. The highest angles (most hydrophobic, using water) were measured for SDS. It was felt that contact angles on coal are difficult to measure, because the advancing angle may change by as much as 30” as the drop moves from the hydrophobic to a more hydrophilic region of the surface. In general, the contact angle measurement results are good agreement with the dewatering results.

r -C-

Cake

/ 9.5

Moisture

I -c-

Filtration

flow

rate

7.5 i

6.5 5.5 4.5 3.5

Concentration

(g/t)

Figure 6. Effect of SDS and DAB on filtration flow rate and cake moisture.

O20’ Equilibrium

concentration

100 140 ( mglt)

Figure 6. Adsorption isotherms of SDS and DAB from aqueous phase en coal at 25°C (pH5.3). densities were achieved at an equilibrium concentration well below the bulk solution cmc value. The most important phenomenon to be noted was that the adsorption density was much higher for DAB than for SDS. This is because of the affinity of DAB for greater adsorption at the negatively charged coal surface than the negatively charged SDS (electrostatic repulsion). As a result of this there was a smaller reduction in surface tension for DAB, and hence poorer results in dewatering.

Filtration

rate

and

cake

moisture

Figure 8 shows the relationship between the filtration flow rate and the cake moisture content in the presence of SDS and DAB. It was found that, in the case of SDS, the maximum filtration rate was observed to be 6.89 x lo3 l/m2h, whereas with DAB it was significantly less (5.5 x lo3 l/m2h). Similarly, the residual moisture content was minimal with SDS (11%) and 18% with DAB. Dewatering

kinetics

Figure 9 shows the dewatering kinetics under different experimental conditions. It was observed that the dewatering kinetics without flocculant and surfactant were very slow, but after the addition of flocculant it was fairly fast in comparison to without any flocculant. However, as soon as surfactant was used, the dewatering kinetics were very fast.

-X+ -o+

Without Flocculant SDS DAB

face. The electrophoretic data suggest that the coal surface is negatively charged. Consequently, the concentration loss from solution arising through adsorption is likely to be greater in the case of the cationic surfactant, and therefore higher dosages are required to produce a given equilibrium surface tension than in the case of the anionic surfactant. The mechanism of dewatering aids has been elucidated, and it has been shown that adsorption on the coal surface, leading to a hydroplhobic surface and an increased solid/ liquid contact angle, is important in reducing the filter cake’s moisture content, in addition to reducing the air/liquid surface tension.

reagent

XKNOWLEDGMENTS The author expresses sincere thanks to Professor H.S. Ray, Director, Regional Hesearch Laboratory, Bhubaneswar for giving permission to publish this paper. The author is also thgnkful to Professor N.N. Roy, RIT, Jamshedpur for providing contact angle measurement data for the coal sample. Time

(minutes)

I

L Figure 9. Dewatering kinetics.

In particular, with SDS it was very fast compared with DAB, because of the greater effectiveness of SDS in reducing the cake moisture content in the dewatering process. The kinetics study was carried out under optimum experimental conditions. CONCLUSIONS Basic investigations as well as laboratory batch-scale filtration dewatering tests with froth flbtation products of clean coal fines have shown that the addition of surfactants reduces the suspension’s surface tension, which allows improved dewatering. There is a direct correlation between the point of zero charge (PZC) with the surfactant solution/coal system and moisture reduction in the filter cake. Froth flotation provided a clean coal with an average particle size of 45 pm that could be dewatered to 11% moisture using vacuum filtration in the presence of surfactant dewatering aids. A clear-cut correlation exists between the surface tension reductions, final cake moisture content, PZC and mineral hydrophobicity, which could be predicted from the LaplaceYoung relationship in Eqn. 1. The more pronounced effect of the anionic surfactant compared to the cationic one can be explained in terms of adsorption at the solid/solution inter-

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