Method for producing a lignite product Field of the invention

The invention relates to a novel method of producing a value-added lignite product. In particular, the present invention relates to a process that provides a comparatively low moisture content product that has improved handling and utilisation compared with unprocessed lignite.

Background of the invention

In this specification where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.

Coal is a fossil fuel formed in swamp environments from the remains of organic matter such as plants that have been preserved (at least in part) by immersion in water and mud. Coal is a non-renewable energy source because it takes millions of years to create. Coal is classified into four main types, or ranks (lignite, sub bituminous, bituminous, anthracite), depending on the amounts and types of carbon it contains and on the amount of heat energy it can produce. The rank of a deposit of coal depends on the pressure and heat acting on the organic matter as it sank deeper and deeper in the earth over millions of years. For the most part, the higher ranks of coal contain more heat-producing energy.

World coal consumption is about 5.3 billion tons annually. Coal is primarily used as a solid fuel to produce electricity and heat through combustion.

Bituminous coal contains 45 to 86 wt% carbon, and has two to three times the heating value of the lowest rank of coal, which is known as lignite or 'brown coal'. Bituminous

coal was formed under high heat and pressure and is typically between 100 and 300 million years old.. Bituminous coal is used to generate electricity and is an important fuel and raw material for the steel and iron industries

Anthracite contains 86 to 97 wt% carbon, and has a heating value slightly lower than bituminous coal. Sub bituminous coal typically contains 35 to 45 wt% carbon. Most sub bituminous coal is at least 100 million years old.

Lignite is the lowest rank of coal with the lowest energy content and used almost exclusively as fuel for steam-electric power generation. It contains 25 to 35 wt% carbon. Lignite deposits tend to be relatively young (around 15 and up to 50 million years old) and have not been subjected to extreme heat or pressure. Lignite is 'crumbly' and has a high inherent moisture content, sometimes as high as 70 wt%, bulk density as low as 1.07 gem ^{' 3} . In many instances lignite has a very high ash content compared with bituminous coal, however Latrobe Valley lignite (deposits in the Gippsland region of Victoria, Australia) has comparatively low ash content. It is also a heterogeneous mixture of compounds of various structural formulae.

Large reserves of lignite are located in Germany, Russia, the USA, Australia, China, and Eastern Europe. Because of its low energy density and therefore energy potential, lignite is inefficient to transport and is not traded extensively on the world market compared with higher coal grades. It is often burned in power stations constructed very close to open cut brown coal mines, such as the power stations located in the Latrobe Valley, Victoria and Monticello, Texas.

Lignite coal is often made into 'briquettes'. Briquette manufacture typically involves drying lignite to about 12 to 18 wt% moisture, optionally adding binders, then ram-

extruding the dried mixture. Briquettes are useful because they render the lignite easier to transport and handle and are primarily used in household and industry.

In the past attempts have been made to create other good quality, readily transportable lignite products. For example, Australian patent application 36096/95 relates to a batch process for manufacturing a low water content lignite product, the process comprising pressing granular lignite on a flat bed, steam heating the pressed coal, then imposing further pressurisation to expel water. The product is apparently a dry granular or powdery mass, which would then be expected, due to its dry nature and high fines content, to require special safe handling and safe storage facilities to prevent spontaneous combustion or explosion, or rehydration of the product. The lignite processing would appear to be carried out separately from the winning, transportation, handling and storage of the coal.

Australian patent application 37292/89 relates to a process for improving the quality of lignite. The process steps include combining it with additives, shearing and attrition, followed by extrusion and drying to create densified coal pellets. The characteristics of the pelletised product depend upon the additives used during the shearing and attrition steps, however the pellets would need to be sufficiently robust to be stored outdoors and subjected to the weather, to avoid high cost storage and handling equipment. The nature of the process requires separate coal winning and transportation to the process plant which may well be away from the coal winning surface.

Australian patent application 63621/86 relates to a process for dewatering lignite to produce a low water content product. The process comprises steam dewatering using a plurality of autoclaves in sequence, in a batch process. This necessarily requires the use of a steam raising plant. Again the question of handling, storage and transport arises.

Australian patent application 68491/74 relates to a process for upgrading lignite. The process steps include grinding the low ranking fuel to a fineness that will pass through a 300 BSS mesh (48μm), sufficient to destroy the cellular structure of the coal, forming a slurry, and then drying the slurry.

International patent application WO/2007/066191 relates to a method of improving the physical properties of solid fuels such as lignite. The process principally consists of introducing lignite to a hot oil bath to change the handling and storage characteristics of the lignite. International patent application WO/1987/005891 relates to a similar process comprising placing crushed coal in hot oil to dry the coal and change its handling and storage characteristics.

International patent application WO/2006/ 119044 relates to a method of changing the bulk characteristics of lignite using high pressure compaction and comminution processes to eliminate voids in the bulk product.

International patent application WO/2006/043162 relates to a device for processing lignite to form an extruded 'worm like' or pelletised fuel. The device comprises a crushing element and mills with grinding bodies including a low wear grinding device suitable for lignite, mixers, extruders and a pressing worm provided with the crushing elements.

International patent application WO/2005/003255 relates to a method for upgrading low rank coal stocks using acidic aqueous waste mixed with coal to remove high ash material. Removal of ash is useful in raising the value of the fuel, but acid treatment of lignite coal typically creates a light, fluffy coal product that is unlikely to create a product having desirable hard, dust free and good weathering characteristics.

International patent application WO/2001/054819 is a further process for upgrading low rank coal by formation of a plastic mass under the shear action of Tollable surfaces. In order to achieve the desired degree of cell or pore destruction, multiple crushing passes may be required.

Many of the processes disclosed in prior patents relate to drying lignite. However the products of these processes typically consist of a dry powder which is potentially hazardous due to the risk of powder explosion, difficult to handle because of the high fines content and raises occupational health issues for workers who may inhale the fines. Accordingly, the products typically require special handling and costly storage facilities. For example, International patent application WO/1997/035944 relates to a process for comminuting and drying lignite using multiple comminution steps and a fluidised bed drying process. International patent application WO/1997/031082 relates to a process for reducing the water content of lignite by exposing the lignite to thermal energy and pressure on a flat bed. International patent application WO/ 1996/010064 relates to a method for reducing the water content of lignite by the application of heat and pressure to granular brown coal spread out in a bed. International patent application WO/ 1992/014801 relates to a process for drying hydrous solid fuels, in particular lignite using a drying container linked into the steam circuit of a steam generating plant.

The aforementioned processes are likely carried out at a purpose-built facility remote from the seam from which the lignite is won, necessitating transportation and storage of the coal at a processing location. None of the aforementioned processes appear to be readily adapted for mobile performance or location at the coal digging face. Several require expensive fixed plant or steam/heat generation facilities.

Furthermore, typically the dried granular, powder or oily lignite product is unsuitable for storage outdoors (for example, on the ground in windrows) exposed to atmospheric condition. Most require special storage and handling facilities due to their inherent explosibility or risk of spontaneous combustion due to high fines content, or environmental concerns with oily products in contact with the ground, or their potential for degradation under outdoor storage.

During the 1980's studies were carried out in Victoria's LaTrobe Valley in an attempt to establish an industrial process for solar drying of lignite to create a high quality product for carbonisation, and as a possible replacement for briquettes. As a result of these studies a purpose built pilot plant was built and operated at Hazelwood in the Latrobe Valley for a period of around three years.

The pilot plant was located remotely from the mine so lignite was transported from the mine to the plant using trucks. Inside the plant the lignite was passed through a grinding mill for reduction to small particles, and added to water in a slurry mixer. The resulting slurry was then pumped into specialised concrete lined above-ground ponds where it was left to dry under the sun. Once the material in the pondage had dried sufficiently it would be harvested and broken into storable lumps of hard lignite similar in characteristics to briquettes. The pilot plant produced approximately 2000 tonnes per annum of product.

However, the pilot plant revealed a number of drawbacks that rendered it impractical to scale up to a full sized plant.

The mass of handling equipment, the need to transport lignite from the mine leads to significant cost and efficiency questions. The need for purpose built ponds to contain the slurry for drying also added to both the fixed cost and committed large areas of land for this purpose. The bases of these ponds needed to be built of a non contaminating material

so that no non burnable, abrasive or other contaminant material could enter the slurry, and that harvesting was performed easily.

Another problem encountered was the fact that only three harvests per year were achieved. In order to provide commercially viable quantities of product a full sized plant would have had to either provide five harvest per year or alternatively, the pondage area would have had to be significantly increased. The capital cost of a full sized, purpose-built plant and the need for large scale drying ponds was significant and rendered the process commercially unviable. Furthermore, the cost of transporting the coal from the mine bench to a remotely located purpose-built plant also imposed significant costs.

Accordingly, there is an ongoing need for a cost effective technological and economically viable solution to production of value added lignite products.

Summary of the invention

The present invention relates to a method for producing a lignite product comprising the use of a mobile device to dig lignite from a seam, grind the lignite and form a slurry for transportation to a locus for drying, such as the slot from which the lignite was dug.

Control of the particle size during digging and grinding together with careful choice of any useful additives concomitantly provides control of the characteristics of the slurry and the product.

Typically the mobile device consists of a single unit, or alternatively two or more mobile units that are integrated or contiguous.

In a particularly preferred embodiment, the present invention provides a method for producing a lignite product, the method comprising the steps of,

(a) digging lignite from a lignite seam, the average particle size of the lignite dug from the seam being less than 100 mm,

(b) grinding and, optionally, screening to reduce the average particle size of the lignite to less than 10 mm,

(c) forming a slurry by adding water to the lignite particles and further grinding to reduce the average particle size,

(d) transporting the slurry to a locus for drying, and

(e) drying the slurry to form the lignite product,

wherein steps (a) to (c) are carried out by a mobile device.

In a particularly preferred embodiment, the ground lignite is acid washed between steps (b) and (c) to reduce ash and/or other contaminants in the product.

In a further preferred embodiment, the slurried lignite is alkali treated during step (c) to modify the characteristics of the slurry and the resulting product.

The present invention further provides a mobile device suitable for carrying out the method of the present invention.

In one preferred embodiment the mobile device consists of a single unit. In another preferred embodiment the mobile device consists of two or more mobile units that are integrated or contiguous. Each unit may comprise wheels, tracks, skis or the like for traversing a surface such as the surface of a mine bench. The mobile device, or one or

more of the units comprising the mobile device may be capable of locomotion. One or more of the units comprising the mobile device may be capable of being pulled along by a vehicle or machine capable of self locomotion.

It may be necessary to have large hose reels included with the mobile device to supply sufficient volumes of water to form the slurry. Suitable commercial hoses will be readily available and known to those skilled in the art.

The present invention further provides a method for producing a lignite product, the method comprising the steps of,

(a) digging lignite from a lignite seam to create a slot, the average particle size of the lignite dug from the slot being less than 100 mm,

(b) grinding and optionally screening to further reduce the average particle size of the lignite to less than 10 mm,

(c) forming a slurry by adding water to the lignite particles and further grinding to reduce the average particle size,

(d) transporting the slurry into the slot formed by digging, and

(e) drying the slurry to form the lignite product,

wherein steps (a) to (d) are carried out by a mobile device,

and wherein the average size of the lignite particles in step (c) is proportionally related to the distance the slurry is transported in step (d) by the mobile device.

The slot dug out of the lignite seam is typically 10 to 40 cms in depth and of any convenient length and width. In this embodiment the sun drives the moisture out of the

slurry so that the slot contains a long, continuous length of hard lignite product. The hard lignite product typically undergoes a cracking process due to shrinkage as it dries, causing the product to break up into pieces that can be readily collected. Alternatively, or in addition, the product can be broken up by manual or automated means.

Typically the mobile device is capable of carrying out the digging, grinding, forming a slurry and casting. It may also be used for reclaiming the cast, hard lignite product or pieces of product once it is dry. For example, typically the mobile device has a cutter head with integrated grinding ability. In a further embodiment it may have a harvesting head.

Grinding

Step (b) of the process of the present invention may include one or more grinding steps either integral to the digging step (a) or contiguous to it, or both. For example, The digging device may arrange its cutter head and cutter box to achieve a grinding step within the cutterbox or it may pass the dug material to a single grinding device, or alternatively, multiple different grinding devices.

The lignite particles may be ground any suitable milling or grinding apparatus known to those skilled in the technology such as hammer mill, attrition mills, stirred ball mill, powder grinders, Dispax mill, roller mill or fine grinders. The optimal particle size and size range will depend on the particle sizes appropriate to the input requirements of subsequent stages of the process and the final product characteristics. Screening

Step (b) may optionally include screening the lignite. Typically one or more screens will be used. The screens may be rotary screens, flat screens or inclined screens although other suitable types of screens will be readily apparent to the person skilled in the art. Material screened off, may be redirected to other processes. For example, flake material is typically

of 20-50 mm in size and is readily screened off and sold as a stand alone product to customers who can use it in their processes. In a particularly preferred embodiment of the process of the present invention, multiple screens are used to screen off multiple stand alone products that can be used in other processes, including other drying processes. In a further embodiment, the larger materials are redirected in front of the present invention and are re-dug and passed through steps a and b

Slurrying and Fine Grinding

Step (c) of the process of the present invention may include multiple grinding steps. For example, water may be added to the lignite and the mixture ground once, before more water is added and further grinding carried out. This may be repeated as many times as necessary until the particle size is reduced to the desired specification. Typically a two stage fine wet grind is optimal. A single grinding device, or alternatively, multiple different grinding devices may be utilised.

The lignite particles may be ground by any suitable milling or grinding apparatus known to those skilled in the technology such as attrition mills, stirred ball mills, powder grinders, Dispax mill or fine grinders. The optimal particle size and size range will depend on the distance the slurry is to be transported to reach the locus at which it is to be dried, the manner in which it is transported (e.g. auguring or pumping), and also the desired characteristics of the dry end-product.

Slurry

Preferably the slurry comprises 15 to 40 wt% lignite, more preferably 20 to 30 wt% lignite, and 60 to 85 wt% water.

The slurry consistency will depend upon the amount of water added and any admixture.

Other additives or admixtures may include, for example, calcium hydroxide, ammonium hydroxide, coal ash, ash water or other alkaline material. Alkaline material is typically added to achieve better grinding performance including lower power consumption, a suitable slurry consistency and desirable final product characteristics.

Preferably the pH of the slurry is between 5.0 and 8.5 and alkaline material such as Ca(OH) ₂ or other hydroxides may be added to achieve the desired pH. Control of the slurry pH enables optimisation of the slurry milling, slurry rheology and lignite product. It has been found that at pH values below 5 the coal colloid spontaneously flocculates and the resultant lignite product tends to be soft, and of low density and high porosity.

Without wishing to be bound by theory it appears that the higher pH has a significant effect on the slurry product by altering the structure of the lignite. Lignite is essentially composed of a bimodal mixture of partially degraded plant matter; the larger particles (xylitic fragments, spores, waxes, resins and possibly rod like cell remains) are held together by a "humic acid" glue. When brown coal is extracted by a strong base the humic glue is removed leaving behind weak conglomerates of xylitic plant residues. These xylitic fragments are highly porous and have a large surface area. The humic acids contain many oxygen functional groups and in raw coal function as a binder in a similar fashion to that of a colloid which is flocculated by polyelectrolyte polymers. Accordingly, alkali digestion of porous raw coal significantly disrupts the structure and many of the pores and cavities which have developed during the coalification process are either destroyed or filled with small particles and humic acid macromolecules.

It will be appreciated that additives may be mixed with the particulate lignite or slurry to assist grinding, initiate digestion and aid slurry formation or pumping.

Slurry stability and consistency may be enhanced by the addition of vegetable gums or starches.

Transporting the slurry

The slurry is typically transported using an auger or a slurry pump, such as a centrifugal slurry pump, or positive displacement pump. While the particle size of the lignite is less than 10 mm, in many cases the particle sizes in the slurry will range across the entire 10mm. The particle size range is one factor that affects the pumpability of the slurry. When the slurry is pumped through a pipeline, optimally the average size of the lignite particles in step (c) is proportionally related to the distance the slurry is transported in step (d).

The slurry may require continuous agitation to achieve a flowable product for pumping. If the slurry is to be cast into a slot in the lignite created during digging, further agitation and possibly screeding may be required to create a uniform product.

Drying

The drying may be carried out by any convenient means such as spray drying, cyclone drying technologies, fluidised bed, application of waste heat or heat from other low grade heat sources including geothermal or hydrothermal sources, roller drying or solar energy. For example, suitable cyclone drying technology includes the Windhexe® device from Vortex Drying Technology in the USA or a cyclone destruction machine available from Australian company DevourX Pty. Ltd which claims 100t/hr performance.

In a particularly preferred embodiment the method of the present invention utilises solar power, and locus for drying is a slot in the lignite seam created when the lignite being processed was dug from the seam.

In the initial stages of drying the slurry, where bulk water is being evaporated, the drying rate is equal to that of free water. As more water is evaporated the drying rate decreases as moisture is progressively removed from the macropores, capillaries and micropores.

For high slurry moisture contents (~ 70%) practically 100% runoff is achieved when solar and air drying is used, however, below a slurry moisture content of about 35%, harvesting and further drying in windrows or covered areas may be required. Supplementary heating or airflow may assist drying.

Lignite product

The lignite product of the method has a moisture content of less than 40 wt% (as compared with 60 to 70 wt% moisture in lignite in situ.) Preferably the lignite product produced by the method of the present invention has a moisture content of 15 to 35 wt% water.

The lignite product typically has a Hardgrove Index values from 30 to 70, preferably between 40 and 60, more preferably 45. The Hardgrove Index gives an indication of the grindability of a material, the lower the index the harder and more difficult the material is to grind. (For comparison, the typical Hardgrove Index values for briquettes vary from 40 to 70 and for brown coal vary from 70 to 190.)

The lignite product also has a needle hardness greater than the typical values for brown coal. Typically the needle hardness for the lignite product is from 7.0 to 15.0 kg/mm ² and the specific gravity is from 0.900 to 1.3 g/cc. The needle hardness results are considerably lower than those of briquettes (25-50 kg/mm2).

The product naturally forms into hard dense lumpy lignite product by cracking formation during the drying process. The product lumps resist weathering and therefore assist in reducing nuisance dust and fire risk on the mine levels. The product may be harvested

directly from the casting pit on the mine levels, and may be placed in windrows for further air drying, or used directly in the power generation facility or taken off for use in other process plants. Harvesting can be carried out by the machine of the present invention or by auxiliary plant. Since the product has had much of the moisture removed from the lignite, an energy dense product (MJ/kg) is the available for transport from the mine in place of low energy density lignite.

The lignite seam for this processing would typically be located on a mine bench or mine floor. As previously mentioned, lignite mines are typically located adjacent electric power generation facilities. Accordingly, in a preferred embodiment the product of the method of the present invention would typically be fed directly to the fuel system of the combustion chamber of the power generation facility or it may be transported to act as a feedstock for other processing plants. Coal in a clean dry state can be turned into a range of other products.

While the number of harvests per year will be variable due to seasonal variations, it is expected that between three to five harvests will be achievable, based on previous experience, and the availability of the mine's coal level for the digging and slurrying the coal, casting, and drying the product .

Advantages

The method of the present invention has a number of significant advantages as compared with the prior art. For example, the method of the present invention eliminates the need for large size, specially prepared drying ponds as used by the prior art. Furthermore, the present invention avoids the need for a specialised fixed processing plant, steam or heat raising plant and virtually eliminates cartage and handling to the processing plant. Furthermore it creates an energy dense product for storage or transport directly from the

mine, and as the product lumps resist weathering and cover the coal surface on a level, they are expected to assist in reducing nuisance dust and fire risk on the mine levels.

If the product of the method of the present invention is fed directly to the fuel system of the combustion chamber of the power generation facility adjacent the location where the lignite has been mined, this would improve the efficiency/greenhouse intensity of the power generation.

Drawings

Various embodiments/aspects of the invention will now be described with reference to the following drawings in which,

Figure 1 is a drawing of a typical process of the prior art.

Figure 2 is a drawing of a preferred embodiment of the present invention.

Figure 1 is a flowchart setting out the typical stages in a process of the prior art for making a dry lignite product. The steps can be summarised as follows:

• Dig lignite from an open cut mine bench, the dug lignite having a maximum particle size of 250 mm.

• Transport the dug lignite from the mine bench to a plant in 20 tonne trucks

• Store until required for processing.

• Transport the lignite from storage to a grinding mill by conveyor belt.

• Crush the lignite in the grinding mill to a particle size of 10 mm. • Create slurry by adding water.

• Grind and screen the lignite in the slurry to a maximum particle size of 7.5 micron and pump the slurry into purpose built cement ponds.

• Sun-dry the slurry in the cement ponds.

• Harvest is typically by wheeled or tracked loader or excavator and loaded into truck or onto conveyors which ever is available from the operation. » Transport the harvested product to storage area.

• Store the product in windrows.

Figure 2 is a flowchart setting out the typical stages in one embodiment of a process according to the present invention for making a dry lignite product. The steps can be summarised as follows:

• Dig the lignite from an open cut mine bench, the dug lignite having a particle size diameter less than 100 mm.

• Grind and screen the dug lignite to less than 10 mm.

• Mix the lignite with water and any necessary admixtures to form a slurry and grind the particles in the slurry as required to provide the desired product characteristics.

• Transport the slurry into the slot formed when the lignite was dug and leave the slurry to dry under the sun, the resultant product having a typical moisture content of20 to 35 wt%.

• Harvest the lignite product.

• Transport the harvested product to storage area.

• Store the harvested product in windrows. • Or use directly from the slot if required.

The word 'comprising' and forms of the word 'comprising' as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions. Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.