coal flotation chemistry
TRANSCRIPT
The Science of Coal Flotation
J A Euston*, P White and D G Osborne
Somerset International Australia
* https://au.linkedin.com/in/jeffeuston
The Science of Coal Flotation
J A Euston*, P White and D G Osborne
Somerset International Australia
* https://au.linkedin.com/in/jeffeuston
ABSTRACT: The modern coal preparation plant produces a significant proportion of fines. Recovery and
dewatering of these fines, both cleaned coal and tailings, presents challenges to the coal preparation
engineer. With decreasing particle size, the effects of surface chemistry dominate over macroscopic factors
such as density and diameter. Flotation depends on multiphase processes and the chemistry and physics of
particle surfaces. Dewatering processes such as thickening and filtration rely on the addition of surface
active polymers to increase effective particle size and settling and filtration rates respectively. The reactions
and interactions between coal, slimes, dissolved minerals, flotation and dewatering chemicals are highly
complex, having a seemingly unpredictable effect on fine coal processes such as flotation and thickening.
The accumulation of these in the recirculating process water creates a cycle of increasingly severe plant
performance issues with no apparent causes in the properties of the fresh feed. There is an increasing need
in coal preparation to maximise resources and one of the most effective ways to achieve this is in the area
of fines recovery and dewatering. An improved understanding of the science influencing these processes
can make a significant contribution to the successful development of equipment and techniques for fines
and ultrafines treatment.
INTRODUCTION
Modern coal preparation plants produce a significant proportion – often as much as 20% - of fine and
ultrafine coal. Processes to clean and dewater this coal are intrinsically more complex than those for coarse
and small coal. Flotation is the only currently accepted technique for processing coal finer than around 250
microns and, in the context of coal, the process is poorly understood. As a consequence, there is a tendency
for coal producers to focus on coarse coal and accept what are often significant losses from the fines circuit.
With decreased coal prices and a global trend towards maximising the value from coal resources there is
renewed interest in minimising the coal fines lost to tailings. A closer look at the flotation process clearly
indicates the benefits of multi-stage flotation (Euston 2010i).
As particle size decreases, the difficulty and cost of dewatering increases significantly and the use of
vacuum filtration is required to produce anything close to acceptable product moistures. The contribution
of fine coal moisture, often approaching 30%, to overall product specification can have a significant effect
on coal quality, particularly as it affects thermal coal calorific value. As with flotation, above, it is often
easier to simply discard the fine coal. The alternative to vacuum filtration is to use screen bowl centrifuges
as a low cost, small footprint alternative. However, these devices will always lose a proportion of the
ultrafines in the effluent and further coal is recycled as part of the screen drain. This recirculating load will
quickly build up in the fines circuit with deleterious effects on flotation. Placing low moisture ultrafine coal
on the product belt will have significant economic benefits for the operation as this coal has already
absorbed the costs of mining and processing. In addition to the obvious benefit of increased product tonnes
there will be a large (potentially up to 50%) reduction in tailings and the removal of recirculating loads of
fine coal and chemically laden water can have a marked effect on overall plant performance and stability.
A number of techniques are being developed (Osborne and Walton 2016) to recover and dewater ultrafine
coal and these will undoubtedly lead to a renewed interest in fine coal recovery.
Flotation is based largely on the surface properties of both the coal and the non-coal particles and is
heavily influenced by process water chemistry. Performance predictions and processes based around
density are not relevant and the performance of the flotation circuit often appears unpredictable. Issues with
flotation are often put down to “poorly floating coals”. Similarly, fines dewatering processes such as
thickening and vacuum filtration are heavily dependent on chemicals and fundamentally affected by process
water quality. The chemistry of this process water is determined largely by the chemistry of the coal seams
and the physical and chemical interactions of the mined coal with air and water. A complex series of
physical and chemical reactions occurs which profoundly affects fine coal processes. In this paper these
reactions are discussed with an aim to a better understanding of fine coal processes.
PHYSICS AND CHEMISTRY OF FINE COAL PROCCESES.
Coal is a sedimentary deposit formed from the decay of organic matter in a marine or estuarine
environment and is characterised by coal type, rank, maceral, etc. The non-coal component is similarly
diverse and comprises shales, mudstones and minerals laid down as sedimentary deposits between the coal
forming layers. The formation of coal seams occurs in an oxygen free environment saturated with water.
On exposure to the atmosphere during mining, transport and stockpiling a number of reactions with oxygen
occur. These processes are called weathering and oxidation. Weathering has a general physical connotation
whilst oxidation suggests a more definable chemical process.
Oxidation includes the chemical leaching of atoms and molecules from the surfaces of coal and minerals
and the physical degradation of the particles. In addition, evaporation of volatiles leads to an increase in
surface area and porosity. Oxidation of the coal surfaces results in the formation of soluble acids,
collectively known as humic acids, a family of organic acids with molecular weights ranging from 100s to
1000s. Being acids, the primary effect is to reduce the pH of the process water. The rejects associated with
coal seams comprise shales, mudstones, etc. representing the inorganic strata laid down between the coal
beds.
Weathering results in the generation of slimes and the associated dissolution of inorganic salts from the
clays, typically sodium, calcium and magnesium reflecting the marine deposition and often described as
salinity. Slimes are typically composed of clays which break down quickly in the presence of air. These
clays, bentonite and montmorillonite are referred to as swelling clays and have surface areas well in excess
of 500m2/g giving a huge surface area for chemical and physical adsorption. Slimes have a profound effect
on coal preparation plant performance affecting both the chemistry and viscosity of the process water.
All of these processes, the dissolution of mineral inorganics and organic (humic) acids and the
breakdown of clays into slimes contribute to the process water chemistry and influence the performance of
dewatering and flotation unit operations. In most plants process water is recycled and the water chemistry
changes as slimes increase and the pH decreases. Recirculating humic acids accumulate and decrease
process water pH while accumulating slimes eventually create problems for many unit operations outside
of the fines circuit, including dense medium recovery circuits and other issues with increasing slurry
viscosity. De Korte (2001) provides an excellent summary of both the physical and chemical effects of
weathering in coal.
This review focusses on the effects of oxidation and weathering on fine coal processes. A brief mention
should be made of the broader effects of slimes and soluble species. Clean coal product typically contains
10 to 12% total water and this water will obviously have the same chemical composition as the process
water. Slimes will always follow the water. Slimes present as swelling clays have been shown to increase
the total moisture limit of coal through their ability to hold water in the void volume. On the downside as
these clays will absorb water, as little as 0.5% clay may cause a stockpile to swell and slump under heavy
rainfall conditions. Slimes recirculating in the process water can increase viscosity and have detrimental
effects on magnetite circuits.
SURFACE CHEMISTRY AND PROCESS WATER
Most solids acquire an electrical charge when immersed in water. This occurs through ionisation of the
surface, preferential ionic dissolution or the adsorption on the surface of potential determining ions. For
the majority of minerals, the surface charge is negative. In solution, positive ions are attracted to the
negatively charged surface and form a rigid layer of positively charged ions. This is known as the Stern
layer and is essentially fixed as a coating around the negatively charged particle. Figure 1 illustrates the
charge distribution around a particle surface. A second more diffuse layer is known as the Gouy Layer.
The charge at the boundary between these two layers is known as the Zeta Potential. Ultimately, as
distance is increased from the particle the charge approaches that of the bulk solution. Dissolved salts or
electrolytes are known to compress the electrical double layer, reduce the zeta potential and allow closer
approach of particles and bubbles.
Figure 1. Absorption of ions at a particle surface.
A basic understanding of these surface interactions is relevant for a number of reasons:
Dissolved species are absorbed onto coal and slimes particle surfaces and into the charged layer.
This profoundly alters the reaction of particles with flotation and dewatering chemicals.
Ultrafine particles such as clays are strongly influenced by surface charge.
THE EFFECTS OF PROCESS CHEMISTRY ON COAL FLOTATION
Coal is an ideal candidate for flotation. Its organic nature suggests a high level of natural hydrophobicity
and its low density will assist in the levitation of particles in the flotation cell. Figure 2, below summarises
the concept of contact angle. Coal particles are naturally hydrophobic although it is usual to add a collector,
typically diesel or kerosene to increase the effective contact angle.
Figure 2. The Concept of Contact Angle.
As expected contact angles (the measure of floatability) for coals vary significantly with both rank and
type. Arnold and Aplan (1985) indicate that contact angles for coal can vary from 90 to 130 degrees for
exinite, 60 to 70 degrees for vitrinite and 25 to 40 degrees for inertinite.
Flotation variables can be broadly described as belonging to four groups, those related to equipment
design, operating variables (rotor speed, air rate and froth depth), feed properties and the chemistry of the
process water. Polat (2003) discusses these variables at three levels; level I being those which can be
controlled regularly and readily through to the level III, those which cannot be readily controlled. Polat
places mineral and solution chemistry in this third level of variables which are not easily controlled. Firth
et al (1978) investigated the reasons for some coals floating poorly and observed that floatability varies
between seams and even within individual seams. They concluded that coals are “poorly floating” because
of the coal type, rank and petrographic composition, oxidation and the presence of slimes. Lower rank coals
are more prone to oxidation. Finer sizes are obviously more sensitive to oxidation as this is a surface area
phenomenon.
Inorganics in Coal Flotation
The presence of salinity has been shown to increase froth stability. In at least one coal preparation plant
in the Bowen Basin of Queensland, the high salinity is sufficient to produce a stable froth and no synthetic
frother is required. It has been noted that this effect only exists when the salinity is present with the coal at
source, addition later in the process has no effect. This is likely to be because the salt reaction must occur
as the coal weathers and new reactive sites are opened up. Ofori at al (2005) in the ACARP report on the
impact of saline water on coal flotation present an excellent summary on the positive effects of salts on coal
flotation performance.
Organics
The pH of the pulp has been shown to affect flotation as it alters the surface charge of coal and mineral
particles. In addition, the solubility and reactivity of species which have an independent effect on coal
flotation such as humic acids are also affected by pH. Maintaining the process water at neutral or alkaline
pH will generally improve flotation performance. There are a lot more (very fine) clay particles than coal
particles and consequently a much larger surface area for reaction and absorption. At alkaline pH no
collector is absorbed. At acid pH a large amount of collector is absorbed onto the much higher available
surface area. Collector is therefore not available for the slower floating coal particles which may still have
clays attached to their surfaces. This mechanism which results in excessively high collector requirements
is referred to as “collector starvation” (Firth and Nicol 1981).
Slimes
Much has been written over the years on the effects of slimes on coal preparation plants and specifically
coal flotation. Three phases exist; air, solid (both coal and mineral matter) and liquid (water and the often
immiscible oil used as a collector). Coal is naturally hydrophobic but usually requires the addition of a
collector, typically diesel or other organic chemical. The collector coats and enhances the hydrophobic coal
surfaces enabling the attachment of bubbles and allows them to float in the flotation cell. Jowett (1956)
noted that the presence of slimes effectively reduces the contact angle to zero with little or no subsequent
flotation. There are two distinct mechanisms by which slimes can interfere negatively with flotation,
product ash increases through entrainment of slimes in the froth but more importantly slimes can form a
coating on both coal and bubbles inhibiting bubble attachment.
Typical coal froth will contain around 20% solids w/w; the remaining 80% is process water which will
contain essentially the slimes content of the feed slurry. In addition, ultrafine slimes can be trapped in the
irregular surfaces of the coal particles. This phenomenon is common to all flotation processes but there are
further mechanisms which occur specifically with coal by which excess slimes report to the froth. Slimes
particles can physically attach to the coal particles through surface charges, forming a slime coating on the
coal surfaces. In general terms slimes will also follow the process water.
This mechanism through which slimes report with the coal as a form of entrainment increases product
ash. Of more importance is the effect of this slime coating on floatability and recovery. Ultrafine slimes
can attach to both coal particles and, interestingly to bubbles.
The swelling clays such as bentonite and montmorillonites commonly associated with coal seams are a
significant source of slimes. Bentonite has been shown to reduce coal yield, actively depressing coal
floatability with as little as 2% bentonite increases pulp viscosity significantly, increasing the entrainment
and carryover of slimes in the froth concentrate. Swelling clays absorb organics and combined with their
large surface area can absorb collector and frother significantly reducing the availability of these chemicals
for flotation. Not only are the coal particles starved of collector but the slime particles with collector
attached will become hydrophobic and float.
Clay platelets can also form a slime coating on bubbles causing the froth to become stable and resist
attachment to particles. Clays can also attach to coal particles reducing hydrophobicity. This is significant
for coarse particles as surface coating is reduced.
Xu et al (2003) have investigated the effect of a number of clays on coal flotation. Figure 3 shows the
effect of montmorillonite clays on coal flotation under acid and alkaline conditions.
Fig. 3. The Effect of Clays on Coal Combustibles Recovery at pH 5 (left) and pH 10 (right) (Xu et
al, 2003).
The authors used the measurement of zeta potential and demonstrated the negligible effect on flotation
performance of kaolinite but the profound effect of montmorillonite. These figures show the rapid
flotation of coal, with over 80% recovery after only 100 seconds. In the presence of montmorillonite
recovery was around 50% after the same time and failed to reach 60% recovery even with extended
flotation times. This depression of flotation was particularly evident under acidic conditions.
THE EFFECTS OF PROCESS CHEMISTRY ON DEWATERING PROCESSES
As particle size decreases the difficulty of dewatering increases considerably. In sedimentation processes
particle settling rates are so low that it is necessary to add polymer flocculants to achieve the settling rates
required in modern high rate thickening. Similarly, in filtration processes it is generally necessary to
flocculate the feed to prevent cloth blinding and maintain effective filtration rates.
It is important to distinguish between the processes of coagulation and flocculation in chemically
enhanced sedimentation. Coagulation involves the absorption of charged species into the double layer to
reduce the repulsive forces between particles. Flocculation involves the use long chain polymers to form
bridges between particles and the formation of a three dimensional network. These are historically salts of
calcium, magnesium and aluminium such as alum, lime and gypsum but may be short chain amine based
polymers. Bridging flocculants are manufactured with a range of charge densities through anionic through
non-ionic to cationic and the optimum product will be matched to the process.
Inorganics
As mentioned above coagulation proceeds by the addition of cations to the process water. High levels
of dissolved salts intrinsic to CHPP process waters are likely to have an overall positive effect on dewatering
processes. The issue of salinity causing corrosion in the CHPP is a serious concern in many operations.
Calcium and magnesium salts act as natural coagulants and will help to pre-treat clay particles prior to
flocculation. Take care should be taken to use high quality water for the primary dissolution of polymer
flocculants as water with high dissolved solids can retard solution.
Organics
Humic acids traditionally have a detrimental effect on flocculation processes. Standard flocculants used
in coal preparation have an anionic charge from 20 to 30%. The optimum pH range for this product is 5 to
7. At low pH caused by the presence of humic acids flocculant effectiveness will decrease. Humic acids are
often evidenced by the presence of a brown stain in process water, in itself not an issue but an indication
that the pH may be low and a warning that the effects on flotation, as described above, might be expected.
Slimes
As mentioned above the slimes associated with coal seams contain varying amounts of fine clays. Many
of these swelling clays assume charged surfaces, not the same on the edges as on the plates. The addition
of a coagulant such as lime or gypsum is most effective when the addition is at the mining stage or on the
ROM stockpile. Swelling clays such as the sodium montmorillonites common in the Hunter Valley of NSW
have different charges on their flat and edge surfaces. While these will respond to a combination of
coagulant and flocculant and will agglomerate with good supernatant clarity, the resultant floccs will often
by high in residual moisture, best described as “fluffy”, and will not settle quickly with settling rates close
to, or higher than, the design rise rate of a high rate thickener. Recirculation of these floccs in the process
water has a severe effect on many unit operations. Simply increasing flocculant dose will have no positive
effect. In addition to the above, high level of slimes recirculating in the process water will increase the
percent solids of the thickener feed above the 3 to 5% w/w optimum for effective sedimentation.
MANAGING THE FINES RECOVERY PROCESS
Hopefully the above discussion has presented some clarity as to the multiplicity of variables which
affect coal flotation and coal and tailings dewatering. It is common, certainly in Australia, for flotation
yields to vary from being comparable to coarse coal yields down to essentially zero for what are referred
to as “poorly floating coals”. Controlling the entry of slimes into the CHPP and their quick and effective
removal can have a profound effect on both flotation and dewatering unit operations. In addition to the
control of slimes in flotation the use of multiple flotation circuits, analogous to the roughers, scavengers
and cleaners in mineral flotation can improve flotation recovery.
Control of Slimes
Slimes arrive on the ROM stockpile as relatively hard shale and clay based rocks. Only after exposure
to the elements on the stockpile do they break down to form slimes. Preventing the slimes from entering
the CHPP can be achieved by:
Minimising the length of time coal is stockpiled.
Separation of the coarse rejects at the mine by dry sorting.
Desliming prior to the coal entering the CHPP (Bain 2012).
Multiple Flotation Circuits
In mineral flotation the use of multiple flotation stages for roughing, cleaning and scavenging is
accepted practice to maximise the resource recovery and minimise the entrainment of unwanted gangue
and slimes. Firth et al (1978) provided an excellent review of options for treating poorly floating coals
with a view to maximising yield and reducing product ash and these are reproduced in Figure 4, below.
Figure 4. Options for the Treatment of Poorly Floating Coal (Firth et al, 1978).
Desliming of the flotation feed understandably produced the lowest product ash but all the techniques
offered advantages over a single stage. Without debating these options in detail it is easy to conclude that
careful attention of the flotation process can provide insights into the potential to improve both yield and
product ash.
At Stratford Coal in NSW, Australia, a single mechanical flotation cell has been installed as a
scavenger to treat Jameson cell tailings (Euston, 2010i). Figure 5, below, summarises the improved
combustibles recovery achieved with secondary flotation at Stratford Coal. Euston (2010ii) has reviewed
the development of two stage flotation in Australia.
Fig 5. Two Stage Flotation at Stratford Coal (Euston, 2010(i))
CONCLUSIONS
Many of the pressing issues in coal preparation are concerned with the processing of the finest coal
fractions and the subsequent difficulties of dewatering both fine coal product and tailings. New processes
are being developed and existing ones refined to increase the recovery of ultrafine coal. In order to get the
greatest benefit from these innovations a full understanding of the physical and chemical reactions and
interactions is required.
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