DAVE OSBORNE, SOMERSET INTERNATIONAL AUSTRALIA PTY LTD,

AUSTRALIA, DISCUSSES HOW ULTRAFINE COAL CAN ELUDE PRODUCERS

AND OUTLINES HOW THIS CHALLENGE CAN BE OVERCOME.1

R elatively few new beneficiation technologies have emerged in recent decades – just better designs from existing technology. These designs have been much enhanced by the emergence of new materials, leading to improved maintenance and longer component life. Implementation

of better process control and process design approaches has also contributed significantly, leading to both improved operation and higher efficiency.

| World Coal | Reprinted from April 2015

Reprinted from April 2015 | World Coal |

Most of the more economically viable coal has already been won. However, there has always been the desire to increase recovery when technically possible and cost-effective. Power and water conservation concerns – driven by considerations in regards to the cost of carbon and legal pressure – offer additional incentive.

Difficult to process metallurgical coals are increasingly being encountered, e.g. in Mozambique (highly laminated deposits), India (highly mineralised maceral content) and Australia’s Bowen Basin (multi-banded seams with metallurgical coal content, such as the Fairhill and Girrah coal deposits).

Most of these emerging challenges are demanding greater liberation, resulting in finer particle size distributions. For this reason, new beneficiation solutions are required. At the top of this list is the need to effectively win the ultrafine coal that results from increased degradation or comminution by a combination of effective cleaning, dewatering and size aggregation. This article will review the coal cleaning options on offer before considering other contributing

technology that can be selected to obtain the required outcome for a particular source of coal.

Ultrafine coal cleaningThe most common types of wet coal cleaning processes can be categorised as either flotation separation or enhanced gravity separation. These processes are described below.

Flotation separation Flotation was not extensively practiced until the early 1970s, when it was usually adopted for higher-value metallurgical coals. More recently, it has also been employed for higher-energy thermal coals. Reluctance in adopting this process was mainly due to marketing problems caused by extra fines, the high dewatering cost of fines, an inability to effectively handle clays or water quality problems. However, there is currently an increased emphasis on the process, due to a combination of the following factors:

n Improved overall coal recovery, i.e. combustibles recovery, etc.

n Consequent reduction in users’ costs, arising from reduced ash, sulfur, etc.

n Enhanced coking properties from the increased concentration of vitrinite in ultrafine coal.

n Positive response to environmental pressures on the disposal of raw coal tailings.

n Potential to further reduce the amount of solid waste disposal.

Flotation is distinctly different from gravity-based separation processes employed for coarser fractions. For this reason, operating performance can often be disappointing. This is often due to a lack of understanding or training of operators and is most evident with column and pneumatic cells, which require added knowledge and skill.

Two types of flotation mechanisms are in use: mechanical and pneumatic. Mechanical flotation is often performed in a series of agitated cells – or a bank – that keep particles suspended in a turbulent environment needed to disseminate and disperse air into bubbles. With column and pneumatic cells, the slurry flow is used to keep the particles agitated. Air is either sourced from a compressor (column) or drawn in from the atmosphere (pneumatic).

 

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Fig  1  Mechanical  (Wemco  "Smart"  cell),  Column  (Microcell)  and  Pneumatic  (Jameson)  cells  

Courtesy  FL  Smidth3,  Eries4  and  Glencore  Technology5    Examples  of  three  of  the  most  common  flotation  processes  in  use  are  shown  in  the  above  Fig  1.  For  

the  traditional  mechanical  circuit,  4-­‐8  cells  are  usually  employed  with  no  re-­‐cleaning  of  the  concentrate,  and  scavenging  of  the  tailings  would  be  regarded  as  unusual.  Reagents  are  not  “conditioned”  in  the  slurry  and  only  minimal  control  of  feed  solids  content  is  applied.  For  the  more  

advanced  cell  types,  i.e.,  column  and  pneumatic  cells  circuits,  the  feed  may  be  deslimed  at  100  –  150  microns,  especially  for  thermal  coals,  and  screen-­‐bowl  centrifuges  are  often  selected  because  the  effluent  will  “remove”  more  ultrafine  clays,  etc.  whilst  also  increasing  the  potential  for  controlling  

the  moisture  content  of  the  flotation  concentrate.  Dosing  of  collector  is  carried  out  as  soon  as  practical  with  the  use  of  wash-­‐water  being  common  to  minimize  entrainment.      

Most  of  the  column  and  pneumatic  types  have  been  in  commercial  operation  for  over  two  decades  and  whilst  the  concepts  are  more  or  less  unchanged,  the  designs  have  evolved  to  provide  operators  and  repairers  increased  unit  capacity,  improved  access,  lower  wear  and  deterioration  and  as  a  result  

lower  capital  and  operating  costs.    Fig  2  provides  a  good  example  of  this  evolutionary  process.    

                                                                                                                         3  http://www.flsmidth.com/enUS/Industries/Categories/Products/Flotation/WEMCOFlotation/WEMCOFlotation;    4  http://www.eriezflotation.com/flotation/  5  http://www.jamesoncell.com/EN/Pages/default.aspx;      

Figure 1. Mechanical (FLSmidth Wemco smart cell)1, column (Eriez Microcell)2 and pneumatic (Jameson)3 cells. 1. Source: http://www.flsmidth.com/enUS/Industries/Categories/Products/Flotation/WEMCOFlotation/WEMCOFlotation. 2. Source: http://www.eriezflotation.com/flotation. 3. Source: http://www.jamesoncell.com/EN/Pages/default.aspx.

| World Coal | Reprinted from April 2015

For either type, two phases occur. The pulp phase is where bubble-particle contact occurs in both mechanical and column flotation. However, in pneumatic flotation the pulp phase is a quiescent zone, facilitating separation of the collected coal from the tailings with contacting of bubbles and particles. In all cell types, the froth phase is used to separate bubble-particle aggregates from the pulp. Water in the froth that reports to clean coal can entrain fine clay and reduce coal quality. Entrainment can be reduced by increasing froth depth, by use of wash water or by reducing the amount of reagents; however, this may also reduce coal recovery. Caution and good testing protocols are therefore essential. Entrainment is most effectively overcome by froth washing or, in some instances, via desliming occurring in a separate vessel or chamber.2

Examples of three of the most common flotation processes in use are shown in Figure 1. For the traditional mechanical circuit, 4 – 8 cells are usually employed with no re-cleaning of the concentrate. Scavenging of the tailings would be regarded as unusual. Reagents are not conditioned in the slurry and

only minimal control of feed solids content is applied. For the more advanced cell types – column and pneumatic cell circuits – the feed may be deslimed at 100 – 150 µm, especially for thermal coals. Screen-bowl centrifuges are often selected because the effluent will remove more ultrafine clays, while also increasing the potential for controlling the moisture content of the flotation concentrate. Dosing of collector is carried out as soon as practical. Wash water is commonly used to minimise entrainment.

Most of the column and pneumatic types have been in commercial operation for over two decades and, while the concepts are more or less unchanged, the designs have evolved to provide operators and repairers increased unit capacity, improved access, lower wear and deterioration. This results in lower capital and operating costs. Figure 2 provides an example of this process.

There are other innovations on the horizon including the Concorde flotation process offered by Graeme Jameson and a Reflux flotation cell (Figure 3), which is under development by Kevin Galvin

and his team at Newcastle Innovation in Research and Engineering (NIER) in Australia.3,4

Enhanced gravity separatorsOriginally developed for minerals processing applications, this category is capable of upgrading ultrafine particles. Efficient separations down to below 325 mesh (38 µm) can be achieved for most coals. They are particularly well-suited to the removal of pyritic

Figure 2. Development path for the Jameson cell. Source: Glencore Technology.

 

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Fig.  2  Development  path  for  the  Jameson  Cell  (Courtesy:  Glencore  Technology)  

 There  are  other  innovations  on  the  horizon  including  the  Concorde  flotation  process6  offered  by  Graeme  Jameson  and  a  Reflux  flotation  cell  shown  below  in  Fig  3  which  is  under  development  by  

Kevin  Galvin  and  his  team  at  Newcastle  Innovation  in  Research  and  Engineering  (NIER)  in  Australia7,.    

 Fig  3  Reflux  Flotation  Cell  

                                                                                                                         6  Jameson,  G.J.  2010,  "New  directions  in  flotation  machine  design."  Minerals  Engineering,  V  23,  pp  835-­‐841.  7  Dickinson  J.  E.,  Jiang  K.  and  Galvin  K.  P.  “Fast  Fine  Coal  Flotation  using  a  Reflux  Flotation  Cell”      

Figure 3. Reflux flotation cell.

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sulfur and other dense minerals. Commercially available units include:

n Falcon concentrator. n Mozley multi-gravity separator. n Knelson concentrator. n In-line pressure jig and Kelsey jig. n Galvin’s Graviton – suggests strong

potential.

R&D in the past decade suggests that there could be a steady adoption of some of these units because they offer a small footprint solution and ability to treat quite a wide range of coal feed types as effectively as spirals and teetered bed separators.

Dewatering solutionsExamples of screens include various types of static sieves, several of them variants of the DSM sieve bend. The flowsheets adopted often incorporate the classifying hydrocyclone. There are also several examples of specifically developed fines screens including the Derrick Stack Sizer, Bivitec screen and the Hein Lehmann Liwell screen. Dewatering of ultrafine coal is perhaps the most significant development and includes a wide variety of equipment – most of which is still evolving in terms of lowering achievable moisture levels and increasing particulates recovery. This includes:

n Centrifuges of various types, including screen-bowl and solid-bowl decanters.

n Vacuum and pressure filters: rotary drum and disc types, including hyperbaric, horizontal belt filters, recessed plate and membrane filter presses, belt press units, etc.

n Thermal drying, including fluidised-bed and flash-type driers, and microwave treatment.

Most recently, ultrafine coal dewatering development has been directed towards higher-g centrifugation and large diameter hyperbaric filters (Figure 4).

The conundrum of selecting the most cost-effective solution for treating the barren tailings from flotation is still the subject of much conjecture. This is made more challenging by the need to move

 

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The  conundrum  of  selecting  the  most  “cost-­‐effective”  solution  for  treating  the  barren  tailings  from  flotation  is  still  the  subject  of  much  conjecture.  This  is  made  more  challenging  by  the  need  to  move  

away  from  the  impoundment  “solution”,  undoubtedly  the  cheapest  one,  towards  a  more  responsible  one  incorporating  “cost-­‐effective”  disposal  of  a  handle-­‐able  paste  or  semi-­‐solid  material.      

 Fig  4  Example  of  tailings  dewatering  and  disposal  options  (After  Honaker,  et  al)8  

 

Fig.  4  illustrates  two  common  options  for  treating  coal  flotation  tailings,  but  geotextile  bags  into  which  the  tailings  sludge  is  pumped  after  “deep”  flocculation  is  another.  An  optimized  solution  for  producing  an  acceptable  product  component  and  satisfactory  tailings  

disposal  is  that  shown  in  Fig  5.  The  question  marks  are  added  to  highlight  the  fact  that  other  options  exist  and  each  flowsheet  must  be  a  bespoke  design.      

   Fig  5  Emerging  fines  treatment  circuit  –  an  “optimized”  beneficiation  approach.  

                                                                                                                         8   Honaker,   R.Q.,   Luttrell,   G.H.   and   Mohanty,   M.   2010.   Coal   Preparation   Research   in   the   USA.   XV1   International   Coal  

Preparation  Congress,  Lexington,  pp864-­‐874.  Proceedings.  Edited  by  R.  Honaker.  Littleton,  CO:  SME.  345-­‐351.    

Figure 5. Example of tailings dewatering and disposal options (after Honaker, et al).5

Figure 6. Emerging fines treatment circuit: an optimised beneficiation approach.

Figure 4. High-g decanter centrifuge for ultrafine coal dewatering.

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away from the impoundment solution – the cheapest – towards a more responsible solution incorporating cost-effective disposal of a handleable paste or semi-solid material.

Figure 5 illustrates two common options for treating coal flotation tailings.

An optimised solution for producing an acceptable product component and satisfactory tailings disposal is shown in Figure 6. The question marks are added to highlight the fact that other options exist and each flowsheet must be a bespoke design.

Many challenges still exist in achieving this combined outcome in a cost-effective way, but the combination shown in Figure 6 is gaining acceptance in both Australia and the US. The most desirable outcomes are those offering the overall lowest cost solutions.

Briquetting and agglomerationFine coal can present storage, handling and transportation challenges, including fugitive dust and self-combustion issues

in addition to unacceptably high moisture content. The briquetting option provides a range of advantages including improved handling, low dust potential, reduced volume (for transportation and storage) and a higher bulk density for the coal charge to coke ovens.

Unit capacities of over 50 tph are now available. The agglomerating option, usually targeting lower rank coals (with high inherent moisture) can also improve combustion performance. Oil-based options have been tried but are often very cost sensitive. Both briquetting and agglomeration processes are regarded as proven technology in coal applications, but need appropriate testing and design to fully justify specific commercial adoption on a wider scale. However, increasing environmental and transportation cost pressures are mounting and creating growing interest.

Contributing technologies The evolution of a true solution follows a long and steady learning curve.

Engineers in the coal preparation business may not be rocket scientists, but they can make use of some rocket science deliverables, such as advanced materials technology, or high technology outcomes from other industries and other fields, such as chemical reagent development, process control and automation.

Figure 7 shows a timeline with some examples of new dewatering equipment, introduction of new materials and process control techniques and adoption of related innovations that have driven improved utilisation and availability.

Computers and process controlR&D is steadily progressing towards a vision known as the Intelligent Plant – as first projected by Firth and others from CSIRO more than a decade ago.6 Much of this has already progressed well beyond the R&D and incubation stages via an operating plant and is now commercially available. A lot of this

Figure 7. Concomitant emergence of supporting technologies.

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work has been focused on improving fines treatment and examples include:

n Plant control and monitoring systems.

n Improved flotation performance. n Sensors and circuit control systems. n Coal quality analysis and

interpretation, i.e. online analysis with control potential.

n Diagnostic wear and maintenance protection.

Process and plant modelling remains a very active research area, applied to both advanced process control and process simulation, as well as to aspects of plant design. However, robotics for routine tasks, such as sampling and preparation and analysis, currently in use in iron ore and other minerals processing applications, have not yet been adopted in coal preparation applications.

Materials science and engineeringNew materials have played an important part in improving adoption, acceptance and performance of equipment in coal preparation. An insight is provided in the pathway diagram in Figure 1 and includes wear resistant lining materials, flow improvement materials and lightweight materials replacing steel and concrete.

Chemicals and reagentsA plethora of chemicals and reagents have been introduced to create improved operating conditions and a variety of health and safety applications. These include flotation reagents, flocculants and coagulants, fugitive dust suppressants and collection aids, binding and agglomeration aids, lubrication and surface protection.

Plant design and engineeringThe development of advanced computer software and process modelling software has contributed enormously to plant design and engineering improvements, safety enhancement and significant cost saving.

Future directionIn response to a request made to several prominent Australian coal preparation

specialists7 the following collective thinking was offered as a pathway for required innovation:

n Initial stages of project development demands that so-called front-end project needs must be met, i.e. no shortcuts for cost savings that ultimately jeopardises the project feasibility.

n Effective gravity-based cleaning processes for fine coal down to 50 µm, e.g. dense medium options are still tempting despite many failed attempts.

n Significant improvements to fine coal classification, whether it be approximately 0.1 mm or 0.25 mm: i.e. better hydrocyclones, sieve bends or something new.

n More predictable and controllable froth flotation: specific reagent assessment process/procedures and bubble size control, and enhanced dosing control and response.

n Full suite of online sensing and control devices that enable optimised operation.

n Centrifuge technology – screen-bowl and especially solid-bowl – that will get closer to effective dewatering by zero.

n High-pressure filters or high-g solid-bowl centrifuges for tailings that offer much higher unit capacity and operate reliably and at lower cost.

In the longer term, a step-change towards improved liberation and recovery will need the following:8

n Efficient grinding/pulverising systems to get to a size that cost-effectively liberates coal grains from most of the impurities.

n Range of physical, surface, chemical and other techniques to beneficiate the pulverised coal.

n Techniques to agglomerate, store, handle, transport and dewater the upgraded pulverised coal.

n Robotic and auto-control solutions for tedious tasks and safety risk applications.

ConclusionThere are many success stories where processes, designs and equipment have

evolved to meet identified needs because of breakthrough technologies emerging in several fields. These include material science, chemical/reagent development, instrumentation and control, information and data processing, computer simulation and modelling, and sampling and analysis, including robotics. Such technologies have rarely emerged specifically for coal applications, but most have emerged from chemical engineering and mineral processing innovations that have proved adaptable. These, and many other areas of innovation hold great potential for meeting the future needs of ultrafine coal beneficiation in a changing world where greater challenges are emerging, threatening the future of coal utilisation .

Acknowledgements The author would like to acknowledge the Coal Preparation Society of America for the opportunity to present these concepts at the Coal Prep International 2014 in Lexington; and also to his former employer, Glencore, for supporting attendance at this conference and for permission to include content as noted in the article. Finally, the contributions made by Australian peers Andrew Swanson, Andrew Vince, Bruce Firth, Kevin Galvin, Frank Mercuri, and Jeff Euston is gratefully recognised.

References1. Article derived in part from a keynote

presentation made at Coal Prep International 2014, Lexington, Kentucky, US, on 29 April 2014.

2. OSBORNE, D.G., HUYNH, L., KOHLI, I., YOUNG, M. and MERCURI, F., ‘Two Decades of Jameson Cell Installations in Coal’, presentation given at the XVII International Coal Processing Congress, Istanbul, Turkey (2013).

3. JAMESON, G.J. ‘New Directions in Flotation Machine Design’, Minerals Engineering, vol. 23 (2010), pp. 835 – 841.

4. DICKINSON, J.E., JIANG, K. and GALVIN, K.P., ‘Fast Fine Coal Flotation using a Reflux Flotation Cell’.

5. HONAKER, R.Q., LUTTRELL, G.H. and MOHANTY, M., ‘Coal Preparation Research in the USA’, in HONAKER, R.Q. (Ed.), Proceedings of the XVI International Coal Preparation Congress (Society of Mining, Metallurgy and Exploration; 2010) pp. 345 – 351.

6. FIRTH, B. A., ‘The Intelligent Plant – Measurement Requirements in Fine Coal Cleaning and Dewatering Circuits’, ACARP Project Report C11069 (2008), p. 93.

7. Andrew Swanson (QCC), Andrew Vince (Elsa Consulting), Bruce Firth (CSIRO), Kevin Galvin (NIER) and Frank Mercuri (Anglo American).

8. Andrew Swanson (QCC), April 2014.

| World Coal | Reprinted from April 2015