Modern Coal Preparation

Modern Coal Preparation Presented by GRANT GRAN T FLEMING FLEM ING B.E.

Indonesian Mining M ining Conference Conference Jakarta, Indon I ndonesia esia 25 th February, 2009

• What is i s Modern Modern Coal Co al Prepar Prepara ation? • What was w as Modern Modern Coal Prepa Preparation? ration? • The Development of Coal Separation Principles • The Development of Coal Preparation Equipment • What is i s to come come in Coal Prepara Preparati tion on Equipment? De sign • Coal Preparation Plant De

What is Modern Modern Coal Preparati Preparation? on?

Today, “Modern “Modern Coal Preparati Preparation” on” is probably probably best-defin best-d efined ed as a s the the applicatio ap plication n of the most up-t up- todate separation principals and and proven p roven equipment equipment that is best suited for the application in hand.

But without a history, coal preparation can have no future.

What was Modern Modern Coal Preparati Preparation? on? 100 years ago, only screens, screens, manually-operated basket (o ( or pole) po le) jigs, jigs, sluices or troughs and hand picking tables tab les existed existed to beneficiate coal. Continuous (Hartz (Hartz or Luhrig-type Luhrig-type)) jigs and gravity thickeners (Patented (Patented in 1848, 18 48, 1879 and the 1860’s, 1860 ’s, respectively) were were still yet to come. The Baum fully-automatic continuou continuous s coarse coarse coal  jig was introduced in 1892 and by the 1940’s was the world’s most commonly commonly e mployed coal separator.

This was because there was no need or use for fine coal. “If you could not pick a coal particle up with a pitch fork, it was not worth keeping to sell.” Apart from jigging, dry cleaning plants became common throughout the world. For example, in 1934 in England, there were 634 washeries and 150 dry cleaning plants to process only 40% of the country’s total output. The remainder of the coal was either hand picked or manually washed in pole or basket jigs. Dry cleaning plants became popular because they did not lower the thermal value of their product and the Chinese still manufacture a locally-designed plant.

The need to produce more and more steaming coal for industry has always driven coal preparation development. Hence, mainly ‘coarse coal’ separation methods were preferably improved over the period 1900 to 1930 while very little small or fine coal separation research was undertaken at all. It was not until the early 1940’s was it realised that un-recovered fine and small coal was amounting to a significant loss of product (i.e. revenue) and that something had to be done to rectify the situation.

The development of basket centrifuges for dewatering small-sized coal occurred in the 1930’s. These machines immediately improved product quality (and profits) and so the evercontinuing need to improve product yields and revenues commenced in earnest. Over the next 10 years, both vertical and horizontal basket centrifuges were developed. Eventually, these machines were employed for fine coal dewatering as well.

However, even up to as recently as say, 50 years ago (1950’s), coal preparation was quite limited to the crude separating tools and lack of knowledge that existed at the time. The available separators were simply horizontal or inclined screens, a range of pneumatic jigs (Baum, Jefferies, Simon-Acco and McNally-Norton, etc.), plunger-type jigs, water-only classifying cyclones [DSM (1937) or Visman (1958)], Diester or Wilfley concentrating tables, trough-washers (Rheolaveur, Rittinger, etc.), Humphery spirals (1943), classifying cones and thickeners.

It can be seen that mainly jigs and/or dry beneficiation was predominately practiced up to the 1940’s and 1950’s. The separating ‘fluid’ used in these devices was either water or air because they were cheap and readily available. Some coal separator designs like the Blackett barrel (1895), the Chance sand cone (1910) or the Simcar autogenous cyclone (1950) attempted to employ an inexpensive dense media (containing coal, shale or sand) as these materials were inexpensive commonly available to coal miners.

Dense Medium baths of various types (Tromp, Barvoys, DSM, Daniels, Drewboy, etc.) began to appear from the late 1930’s. These used a dense media consisting of water and either magnetite or haematite). The DSM heavy medium cyclone process was first employed in the late-1940’s). The first Compound water-washing cyclones did not arrive until the early 1960’s and flotation (although discovered around 1900) was still yet to be introduced to coal industry until the mid-to-late 1960’s.

Over the next 40-50 years, up to the present, the commercial need to reduce operating costs has further improved coal preparation techniques and separating equipment (for all coal particle size ranges) . This period saw the introduction of high capacity equipment such as Tacub and Batac Jigs, a number of dense medium baths (Teska, Leebar, etc.), multi-sloped (banana) screens up to 4.2m wide x 7.3m long, 1.5m diameter dense medium cyclones, 1.5m and 1.8m diameter coarse coal centrifuges and high-rate tailings thickeners.

In parallel to this, fine coal beneficiation and dewatering finally began to receive the attention it rightfully deserved. Spirals returned to favour (1980) only to be possibly superseded by newer fine coal separators like the Teetered Bed Separator and/or the Reflux Classifier. To dewater fine coal, a range of equipment such as Belt Press, Ceramic disc, Hyperbaric, Plate and Frame and Recessed Plate filters are now commonly employed in preparation plants around the world.

Hence, it can be readily seen that, over the past 100 years, what “was” coal preparation has evolved into what coal preparation is “now”.

The Development of Coal Separation Principles As the world became more coal-dependent following the start of the Industrial Revolution, newer industries arose and different types of coal were needed for specific applications. A way of identification was required and a method to ‘sort out’ these different coals became essential.

Gifted researchers, such as: Sir Isaac Newton the Bernoulli family George Stokes George Westinghouse Osbourne Reynolds Karl Reinhardt, F.W. Meyer, Klaas Frederik Tromp Rosin & Rammler have all contributed to our better knowledge of coal preparation.

The knowledge that these scientists passed on to the world’s coal preparation engineers is truly immeasurable. They gave us the tools, such as the Laws of Motion and an understanding of fluid mechanics that are so essential to perform all of the design tasks that we do.

Their work informed us, for example, about particle Settling Velocity Ratios and the relationship between coal density and ash content, etc. This insight led to the development of Washability Curves (1902), the Meyer Curve (1926), the Size Distribution Curve (1933) and the Tromp Partition Curve (1937), etc. These tools are now absolutely essential to design and predict the performance of the modern coal preparation plant of today.

The Development of Coal Preparation Equipment Beginning some 4000 years ago, hieroglyphic drawings show us that materials used to construct the Pyramids in Egypt were washed and separated using jigging poles in the waters of the Nile River. Today’s modern high-capacity jigs (the Tacub and Batac machines) are now, via the Hartz and Baum  jigs, the direct descendents of this ancient process.

The coal industry, prior to the 17th century existed only to mine and sell coal into a very limited market. The 18th century saw the industry expand rapidly with the Industrial Revolution but, whilst sales increased, the technology that was becoming available to it was never employed to the fullest extent .

This attitude persisted into the 19th and early 20th centuries possibly as a result of low prices and the industry entrenched contempt of small/fine coal. Unfortunately, only academics understood the implications of Newton’s late 17th century work into the settling velocity of coarse-sized particles larger than 50mm. His work actually provided the basis off all gravity separations that would be used in coal preparation from then onwards.

In 1851, when Stokes published his work on the settling velocity of smaller-sized particles (smaller than 5mm), the coal industry finally received the last piece of knowledge that it needed to efficiently separate the whole range of material sizes that it beneficiated. In summary, while their conclusions differed slightly, the work of Newton and Stokes clearly demonstrated that of the ‘readily’ available fluids (ie. air, water or a medium) to undertake a gravity separation in, the medium-based fluid was by far the best.

Their independently derived equations for Settling Velocity Ratio (after some simplification) were:Newton Stokes

SVR = [(DH – DF)/(DL – DF)]n = 1 SVR = [(DH – DF)/(DL – DF)]n = 0. 5

To give an example to demonstrate these equations, Let DH = density of the high density particle (shale) = 1.85 DL = density of the low density particle (coal) = 1.45 DF = dens ity of the s eparating fluid (air) = 0.00017 (water) = 1.00 (medium) = 1.45

The table below illustrates the significant Settling Velocity Ratio differences that can result from employing different separating fluids.

Researcher

Medium Newton Stokes

Particle Size

> 50mm < 5mm

Settling Velocity Ratios Separating Fluid Used Air Water 1.28 1.13

1.89 1.37

9.00 3.00

The difference in the settling velocity of a high density particle compared to the settling velocity of a lower density particle is the basis of all gravitybased coal or mineral separations. The greater the difference, the easier the separation task becomes. The table quite clearly demonstrates why a dense medium based separation (ie. by cyclone or bath) will always achieve a more accurate and sharper separation than a water-based separation (ie. by  jig, spiral or water-only cyclone) could achieve.

It has taken another 100 years since Stokes’ time for the coal industry to slowly move away from pneumatic and/or water-based separations to medium-based separations. Today, the transition is almost complete as the number of existing and new jig plants being constructed in the world has fallen considerably. Dense medium cyclones now probably perform most of the world’s coal preparation duties.

Similar conclusions regarding the future of spirals separators in the coal industry. Unfortunately, spirals have always suffered operational and product quality problems since their re-introduction to the coal industry in 1980 and a replacement device is being sought. The equipment that may replace today’s spirals (the Teetered Bed Separator or the Reflux Classifier) both use a coal-based medium to improve their separating capabilities. As a result, their performance and separation efficiencies (Ep) are very encouraging.

What is to come in Coal Preparation Equipment? The move to dense medium-based separation will never stop the development of newer equipment to serve the coal preparation industry. Researchers are currently trialing equipment like: 1.Wider (6.0m) vibrating screens 2.Improved short and tall column flotation cells 3.Hyperbaric centrifuges.

The following research areas are predicted to continue to expand with some expanding more rapidly than others mainly in response to varying environmental pressures:•Physical Separation (mainly of small and fine-sized coal, fine coal and tailings dewatering, hyperbaric horizontal filter, continuous hyperbaric centrifuge) •Chemical Separation (fine coal flotation, mineral matter reduction, fine coal and tailings dewatering, displacement dewatering) •Biological Separation (mineral matter reduction) •Liquefaction •Gasification •Sequestration

Coal Preparation Plant Design Coal preparation plant design has changed dramatically with the introduction of high capacity equipment. Now, single-stream modules of up to 1200t/h are possible. The following table shows the reduction of major equipment items, on a per module-basis, that has occurred over a 40-year period (1970-2010). Now, with only 6 major plant items to maintain (instead of up to 9 in 1970), today’s plant module can process 5 to 6 times more ROM.

‘TYPICAL’ PREPARATION PLANT MODULE DETAILS

Construction Era

1970

2010

Module Feed Capacity t/h

250

1200

D/S Screen (No. x Width)

2 x 2.4m

1 x 4.2m

DM Cyclone (No. x Diam.)

2 x 0.7m

1 x 1.3m

D & R Screens (No. x Width)

3 x 2.4m

2 x 3.6m

Centrifuges (No. x Diam.)

2 x 1.1m

2 x 1.5m

No. of Major Equipment Items

9

6

Thank you for your attention I welcome your questions

The

The