mineral drying - summary of best practice 2013-06-18

8/13/2019 Mineral Drying - Summary of Best Practice 2013-06-18

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Mineral Drying

Common& Best Practice

A Short Compilation

Jarrod Hart

June 2013


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Why use water in the first place?

Water may also aid in final application: Paper filling/coatingcolours

Paints & Coatings

Ceramic slips

However

Water has an extraordinarily high latent heat of

vaporization

Its simple: Many mineral processes work better wet:

Grinding, blending, size classification

Beneficiation by:

Flotation, selective flocculation, magnets

Leaching/acid washing/electroseparation

Density/shape (jigs riffles spirals)

Chemical treatments

Surface treatments

Bleaching

Transport

gravity flow, piping

and pumping

Water can manipulate product form:

Granulation (pelletisation, extrusion)

Densification

Jun-13 Performance & Filtration Minerals2


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So Many Types! Which one????

The Basics

Drying may be multiple steps: Dewatering

Thermal Drying

Pulverisation

Note: Some equipment combines these processes

Dewatering is usually a solid-liquid separationprocess, with two major mechanisms Sedimentation (density difference)

Filtration (essentially a size difference)

Drying is usually done by vaporizing the liquid Direct (mix the material with hot gases)

Indirect (radiate or place material in heated vessels)

Other methodsinclude

Vacuum (aids vaporization)

Freeze drying (freeze + sublimation)

Water displacement (e.g. with solvents)



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Dewatering Methods 1: Sedimentation Based

Concept

A separationprocess based on density difference

Calculations rely on Stokes Law



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Dewatering Methods 2:

Filtration

5

Concept

Extract water from fluid through a

membrane that holds the solids back

All concepts play with the compromise

between pressure drop (flowvseffort) and

fineness of exclusion

Mechanical pressure may also be used to

compress resulting cake removing more

fluid

Jun-13 Performance & Filtration Minerals


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Major Types of Filter

Batch Filters

Plate and frame filter

Horizontal pressure filter

Candle/leaf filters (beer)

Tube press

Continuous Filters

Drum/disc filter

Belt press



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Major Types of Dryer

Rotary Dryers (direct or sometime indirect)

Curtain/belt/band/moving tray dryers Fluid Bed Dryers

Spray Dryers

and of course ovens!



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Rotary Driers

Direct fired

Indirect fired Lifter bars common

Pros: Versatile

Cons: HighCapex and footprint

May need a scrubber



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Belt Dryers

Material placed on an air permeable belt which passes through oven

May make multiple passes as below:

Pros Suited to delicate materials

Cons

Need to distribute carefully on belt

No agitation so material drying uneven

Poor efficiency for self-insulating materials



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Fluidised Bed Dryers

Upward air movement suspends and agitates bed of material

Note: Its common to

add cool air at the end,

using up all theembedded heat for

evaporation theres

no added value in

having a hot product!

See

Pros: Fair efficiency

Fair footprint

Fair capex

Cons:

Not suited to all minerals Material must be granular not dusty

May requirebackmixingof dry

product with wet feed



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Inside a Fluid Bed Dryer



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Spray Dryers

Slurry atomisedinto hot gas environment, then recovered withcyclone/baghouse

Pros: Slurry to powder in

just one step

Product has good flow and can

have good density

Cons: High energy requirement

unsuited to low solids slurry



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Flash Dryers

Basics: cake/lump dropped into fast moving

hot gas current, then recovered with

cyclone/baghouse

May be with or without mechanical agitation

Eg. Cell mill or Scott AST

Many, many variations available!

Spin flash

Pulse (hybrid spray dryer/flash dryer)

Pros:

Versatile

Wide range of product

forms and moisturelevels

Fair capex

Fair efficiency

Cons:

Low density products



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Jun-13 Performance & Filtration Minerals


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Selection Process

Methods vary in how water is heated Choice depends on varying importance of:

Starting and ending moisture levels

Form and density of feed and product

Capital cost

Energy efficiency

Footprint

Mineral physical strength and heat sensitivity

Mineral abrasivity

Hybrids and combos also possible

Spin flash

Spray-bed



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Dryer Selection Guide


Source:APV Dryer Handbook


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Dryer Performance

Q: how do you know if your dryer is working?

A: Product is dry! Q: I mean efficiently?

A: Benchmarking!

BenchmarkingDryers come in so many shapes and sizes, how do we compare?

Seek the common features:

Removing water (want this high)

Using energy (want this low)

Hence a good measure is the ratio!

kWh/kg water removed is a good benchmark



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Specific Energy Use

We often measure something like MMBTU per tonneof product

Great for comparing two dryers with the same drying duty

However, no use if anything else changes

Starting moisture, ending moisture, mineral type & mineral PSD

Therefore we prefer energy per unit of water removed

Specific energy use is often in btu/lb, kJ/kg, kWh/t or similar

1 Btu/lb= 2.326kJ/kg

kWH= 3,415 Btu

A key value is theheat of vaporizationof water Defines the limit of drying efficiency

~970 Btu/lb or ~2257 kJ/kgat 100C

~1050 Btu/lbor ~2444kJ/kgat 25C

Alas dryers are usually far off the ideal

Imerys expects 1500Btu/lb or ideally towards 1300Btu/lb, above 2,000 is bad.

Somelosses aresystemic, but many are the result of poor practice.

This number is a good tool to check for issues and benchmark for comparison with other

machines and facilities!

Confounding factors

Some minerals bind the water so

temps well over 200C may be

required to obtain dryness

Moisture content is deceptive

To dry from a 10% solids slurry means

removing 9mt of water permt of dry

product

Variations multiply

if your crude drops from 55% solids to

45% solids, drying requirementincreases by 50%!


Some Efficiency Benchmarks

Maytag tumble dryer: 1,720Btu/lb

Competitor 2,240Btu/lb source

Typical spray drier1,500 Btu/lb Common range for older equipment

is 1,600-2,200 Btu/lb water


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Energy Efficiency In Drying: A Useful Trick

And we know that:

energy in = energy out

Wewant to: separate mineral from water

The best ways are mechanical (filtration, centrifugation,etc) but in a dryer we do it by vaporizing the

water

It takes a certain amount of energy to make water turn to steam- everything else is waste!!

Thus the perfect dryer would not waste energy onany other task

Heating the mineral, heating the gases, heating the equipment

Or even heating the water vapor produced

Yet the mass balance tells us these are the only possible places for our energy to go.

This means dryer efficiency can be easily monitored using the temperature of the

output streams!

Try to minimize these flows and their temperatures (they are energy flows ormoneyflows)

The enthalpy (contained energy) of these streams is the product of their mass, temperature

difference andheat capacity.



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Energy Efficiency In Drying: Towards the Ideal Dryer

The ideal dryer would vaporize water at room

temperature

Solar drying! Highly efficient. Highly slow.

The ideal dryer would discharge cold minerals, cold

gases and even cold (liquid) water

Hence the after-cooling on fluid bed dryersHence the heat exchangers common on dryers

Heat exchangers may look like they are heating feedbut

another way to see it is they are cooling the product to avoid

energy escaping the system

Some heat exchangers actually re-condense the steam thus

the output is not only colder but lower in energythis is the trick used in condensingboilers.



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Energy Efficiency In Drying: Towards the Ideal Dryer

Unavoidable heat loss

Alas we cannot have all the outputs of a dryer at room temp!

A dryer is a bit like a heat exchanger:

We need a temperature difference to act as a driving force

Smaller temp differences mean slower evaporation

Slower evaporation means more residence time is required

More residence time means a bigger dryer

bigger = more efficient

But bigger means more expensive

But we can learn from heat exchanger best practice:

Dryers can be designed to be counter or co-current

Counter-current can transfer more of the energy

Con-currentcan be smaller

But neither is perfect

Also: sometimeswe cannot tolerate condensation

this may mean we need a hot exhaust and/or hot baghouses

This hot exhaust is money down the drain



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Consider Recirculation

If your system uses large volumes of

gases and the output is not saturated,

consider a recirculating setup

Bleed off air at controlled humidity

Lowers volumes of hot gas released



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Common problems

By far most common problem:

Poor or uneven exposure of mineral to dry airPoor distribution onbelt orwithinfluidisedbed

Nofluidisation

Blocked vents / dirty trays & belts

Symptoms

Instability

Hot gas discharge (know long term trends & compare

temp with peers)

Dust

Overdryingor uneven drying

Instability

Swing between too dry and too wet

Examine process control

Integral and derivative control needs to consider reaction time of system

Process reaction time (lag) needs to be reduced

Stabilize inputs



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Focus: Drying for Dispersibility

Some drying methods lead to the formation of grit/small

lumps Some dryers need to be followed by strongpulverisation,

Some customers need to mill the slurry they make

Some minerals, such as bentonitesor nano-sized pccs

cannot be re-dispersed to original PSD

What to do?

Best practices include:

Avoid high heat

Avoid compressing damp mineral too hard

Avoid meniscus effects Dynamic vsstatic drying

Freeze drying or supercritical drying are the ultimate

Ensureyou are dryingfrom clean water

Minimisedispersants, salts, etc.



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Drying: Summary

Most efficient as a multistep process Dewater as much aspossible

Minimise temperaturesof outputs

Dryer choice depends on Form and density of feed andproduct

Starting and ending moisture levels

Capital cost

Energy efficiency Footprint

Gentleness

Common Issues Poor contact of heat with moisture

Lostheat due to poor operation

Instability

Advice: Monitor specific energy use

Per tonneproduct

Per tonnewater removed

Do an energy balance

Experiment! But only if you measure the effects!

Talk to others with similar equipment Get their data

Look at their outlet temperatures, gas flowrates ideally compare all energy flows

Consider heat recovery Heat exchangers

Condensers

Finally: get help! Talk to us: Jarrod Hart (energy balances),

Kevin Jones (operational excellence), BrianBurns (automation)

Talk to other experts such as Pascal Bizarro(thermal processes)

Or even better: be an expert for others. Letpeople know your skills via your Galaxy profile


mineral drying - summary of best practice 2013-06-18

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