Massive Australian coal deposit has great CTL and gasification potential

A huge coal formation has been defined in central Australia which could contain 1 trillion tonnes of coal or more suitable for underground gasification or CTL development.

Author: John Chadwick
Posted: Thursday , 19 Feb 2009

LONDON -

A new report has identified an area of the Simpson Desert straddling the South Australian-Northern Territory border as sufficiently promising to be a serious contender in Australia's rapidly emerging underground coal gasification, coal bed methane and gas-to-liquids (GTL) industries. The report, the full text of which will be released shortly, found potential for well in excess of 1 trillion t of coal in the Purni Formation of the Pedirka Basin with estimates suggesting the coal seam potential is very well identified between 200 and 1,000 m depth.

The findings will now be used by Perth-based Central Petroleum Limited (CTP) to explore further the potential for various technology applications such as underground coal gasification (UCG), GTL and coal-to-liquids (CTL). CTP commissioned the independent report, from independent geological consulting firm, Al Maynard & Associates, in the wake of the discovery by the company late last year of significant coal thicknesses of well over 100 m of cumulative coal seams in the Purni Formation. The report's authors were asked to determine the total tonnage of Permian coal in the Basin that could possibly be amenable to UCG and/or coal bed methane (CBM) gas extraction as a possible feedstock for GTL processes. Central Petroleum has other joint ventures over the acreage but is pursuing in its own right, outside the current Joint Venture, the potential for UCG and GTL options in the Pedirka Basin.

"The findings are a solid outcome and whilst there has not yet been sufficient drilling to arrive at a JORC resource estimate, the report has defined a coal Exploration Target potential of between 0.6-1.3 trillion tonnes above 1,000 m with a total tonnage inclusive of deeper coal sections of between 1.5 to 2.1 trillion tonnes in CTP's combination of Mining and Petroleum Act permits and applications that covers most of the same ground," Central Petroleum's Managing Director, John Heugh, said.

"This is a significant conclusion, as the estimate is based on a realistic contribution of several factors, including a fresh interpretation of the geometry of the Basin, three dimensional data from seismic surveys, 2008 drillholes including cumulative coal intercepts of much greater than 100 m, and supporting geophysical downhole logging data. We were already aware from previous modelling that the Basin's coal footprint extends over more than 9,000 km² of the Purni Formation alone in EP 93, just one of the company's Pedirka Basin tenements, so this is a further step along the confidence path to vindicate substantive coal focused drilling throughout the Basin."

The tonnage estimates are contained in a combination of petroleum and mining permits dually or singularly held by Central Petroleum over the total footprint hosting the coal seams. The dual tenure enacted by the Company is a strategic move to ensure that clarification by State Authorities (NT & SA Governments) of how competing coal gas extraction technologies will be permitted will be covered under either ‘Mineral' or ‘Petroleum' legislation.

Industry experience to date suggests that the range of energy available per tonne of coal via UCG processes is 10-20 Gj/t. This equates to approximately 1 bbl - 2 bbl of liquid petroleum products such as diesel, jet fuel or naphtha per tonne of coal if the gas was put through a GTL plant.

"In addition to today's findings, we are also encouraged by the positive UCG trial results being obtained by sector leading operators such as Linc Energy and Carbon Energy," Heugh said. "These trial results also compare favourably with the long history of UCG applications fuelling a 400 MW power station in Uzbekistan. The operator there, Yerostigaz, is, so far, the only company in the world that has commercially produced Underground Coal Gas (UCG) for use as feedstock for the generation of power. Not only has it commercially produced gas for nearly 50 years commencing production in 1961 but it has done so continually and without interruption to gas supply during that period. The technology being favoured in Australia's developments in these sectors has also undergone significant innovative improvements in recent years compared to the old UCG and GTL technologies dating back to World War II. We are therefore encouraged and heartened at many levels about the near term coal technology options for the Pedirka Basin that CTP aims to commercialise.

John Chadwick is editor/proprietor of International Mining magazine - www.im-mining.com

(www.mineweb.com)

Goldcorp reports record production in 4Q08, 7% increase in reserves

Goldcorp reported it has increased total gold and silver reserves by 2.9 million ounces and 182 million ounces respectively in 2008. Author: Dorothy Kosich
Posted: Friday , 20 Feb 2009

Goldcorp Thursday reported adjusted net earnings of $397 million or 56-cents per share for 2008, down from adjusted net earnings of $440.4 million (62-cents/sh) in 2007.

The 2008 adjusted income included a $1 billion loss on foreign exchange and other writedowns.

The company also announced that proven and probable reserves increased 7% to 46.3 million ounces as of the end of December 31, 2008, which was Goldcorp's fifth consecutive yearly reserve increase. Silver reserves also increased by 182 million ounces to 1.247 billion ounces.

Goldcorp reported it produced 2.32 million ounces of gold at total cash costs of $305/oz. Record gold production of 691,800 ounces was reported during the fourth-quarter 2008.

For the fourth-quarter 2008 Goldcorp reported adjusted net earnings of $84.4 million or 12-cents per year down substantially from the $178.5 million (25-cents/sh) in adjusted net earnings reported during the same period of 2007.

The company plans to invest $430 million in the Pueblo Viejo joint venture project in the Dominican Republic this year. During a conference call Thursday Jeannes told analysts that the existing historic mine site has been demolished with the focus this year on new construction.

"Goldcorp ended 2008 with strong performance at every mine in our portfolio," said Chuck Jeannes, Goldcorp president and CEO. "Decreasing cash costs enabled the company to capture greater margins in a rising gold price environment, contributing to 2008 margins of $563 per ounce of gold sold. Our performance in the fourth quarter underscores the substantial operational improvements underway throughout the company, and our priority in 2009 and beyond is building on this improved performance while delivering on our goals and objections."

(mineweb.com)

De Beers suspends mines by Brendan Ryan

Posted: Fri, 20 Feb 2009

[miningmx.com] -- DE BEERS MD Gareth Penny has refused to specify the amount by which the firm will cut production this year, but market speculation is that the group’s 2009 diamond output could be 35% to 40% down.

That would drop production to about 29m carats from the 48.1m carats the group has reported for 2008. The speculation is backed up by the fact that all the mines owned by Debswana - the 50/50 JV between De Beers and the Botswana government which controls the group’s main producing mines in Botswana - are currently on care and maintenance.

In reply to a question posed during the Anglo American results presentation in London, Penny said production would be “significantly reduced” but that the actual extent was still “work in progress”.
Diamond mining industry sources had indicated De Beers has already taken drastic action in Botswana and South Africa to chop production with output at the huge Jwaneng and Orapa mines halted as well as from the Venetia mine in South Africa. A spokesperson for Debswana confirmed that all Debswana mines were on care and maintenance until further notice.

The spokesperson commented, “Debswana is currently in consultation with various key stakeholders about mitigation actions resulting from the global downturn and the necessity to reduce production during 2009 to align with demand, to conserve cash, to protect employment and maintain readiness for an eventual upturn in the market.

“Once all these consultations have been concluded, the media will be informed accordingly.”
The De Beers results for 2008 were published on Friday, as part of the overall annual results presentation from Anglo American.

That is a sharp break from the previous tradition, in which De Beers always presented its numbers individually ahead of the Anglo presentation. De Beers spokesman Tom Tweedy said the change had been made at Anglo’s request, given that it is the largest shareholder with a 45% stake. The results showed De Beers is in a dire financial situation - but just how dire only emerged in response to questions from analysts.

The results statement noted that the three De Beers shareholders - Anglo American, the Oppenheimer family and the Botswana government - had agreed to provide US$500m in loans to the company this year in proportion to their shareholdings. It only emerged through follow-up questions that the shareholders had already kicked $300m in loans during 2008 and that the latest $500m loan would be interest-free for two years, after which it would revert to market pricing.

“If the shareholders are prepared to provide interest-free loans, you have to wonder why they did not simply put in more equity funding. The answer could well be that one of the three shareholders may not have been prepared to commit more equity to the business.
“The other point to consider is that perhaps De Beers’ financial situation is so bad that it cannot afford to pay any interest on the extra loans, given that it already has net interest-bearing debt amounting to $3.6bn,” said an industry source.

Penny said: “We have taken steps to significantly reduce production levels, costs and capital expenditure across all operations. These actions, together with the business restructuring initiatives already completed, have positioned De Beers to weather this tough economic environment.”

Last year’s production of 48.1m carats was 6% down on the 2007 production level of 51.1m carats. The main drop came from South Africa, where production fell to 12m carats (2007 – 15m carats) mainly as a result of the sale of the Cullinan mine and the closure of the Oaks mine.

Production from Debswana was marginally down at 32.3m carats (33.6m carats) while the only growth came from Canada, where the newly-opened Victor and Snap Lake mines produced 1.6m carats (81,000 carats). Rough diamond sales for 2008 were virtually unchanged at $6.9bn ($6.8bn), but that will be altered dramatically this year.

RBC Capital Markets analyst Des Kilalea said: “Rough diamond sales are likely to be 40% to 50% down in the first half of the year. There may be some recovery in the second half, but sales of $3.5bn to $4bn look a reasonable target for the full year.”

(miningmx.com)

Limestone

Limestone is the most abundant of all commercially used sedimentary rocks. It is formed in marine, and sometimes freshwater environments by the accumulation of CaCO3 shells and skeletons in layered beds. In some cases, limestone is formed by direct precipitation from seawater, partly triggered by biogenic activity. Direct precipitation by chemical changes is responsible for limestone formed as stalactites, stalagmites, oolitic limestones, and travertine.

Limestone commonly contains three minerals: calcite (CaCO3), its polymorph aragonite (CaCO3), and dolomite (CaMg(CO3)2, with calcite the most abundant.

As an industrial mineral, limestone probably has more uses than any other mineral commodity. The uses of limestone can be broadly subdivided into those based on its chemical composition and others based on its physical properties. The chemical uses are those where calcium carbonate is converted to other calcium compounds with silicates and aluminates to make cement, oxides to make quicklime and hydroxides to make hydrated (slaked) lime. Other chemical uses are where limestone is used as a reactant in metallurgical processes, glass manufacture, agricultural lime, water purification, and sewage treatment. Physical properties of limestone, such as colour, reflectivity, crystal size, particle size, density, absorption characteristics and hardness, all are significant factors for uses in various fillers.

Limestone and lime are generally abundant but the high quality materials required by industry standards are uncommon. In terms of total tonnage, limestone ranks among the top four of all mineral commodities, and lime is the second most used chemical after silica in most countries. Limestone is generally used for making portland cement and lime, as a flux in steel making, in copper and lead smelting, and as a raw material in glass.

Other uses are: aggregates; agriculture; as a filler in paper, paint, rubber; in plastics extrusion, and as stone dust in coal mines.

Calcined limestone (lime) is used in many industries: steel, alumina, sugar, building, water treatment, road stabilisation, and gold extraction.

Many manufactured objects require limestone or lime in some phase of their production, either as a primary or incidental processing material. Although the strategic importance of limestone is frequently overlooked, limestone is essential for future industrial development. The trend in limestone production indicates a steady increase as demonstrated over the last decade.

Since so much of the limestone is used for cement, future growth will depend on the development of major projects and the local building industry. However, potential limestone resources along the coastal regions are being threatened by possible development of land required for urban and industrial purposes and reserves for water catchment and storage reservoirs. Other threats include international discussions on reducing greenhouse gas emissions which are being monitored very closely by the lime and cement industries. Any program regulating carbon dioxide emissions would have a direct impact on these industries.
With the recent 4.6% rise in the total value of construction commencements in commercial and engineering projects the cement industry is set to improve in the near future. The industrial growth is expected to continue and limestone seems assured of an expanding market.

Bauksit

Bauksit merupakan bahan yang heterogen, yang mempunyai mineral dengan susunan terutama dari oksida aluminium, yaitu berupa mineral buhmit (Al2O3H2O) dan mineral gibsit (Al2O3 .3H2O). Secara umum bauksit mengandung Al2O3 sebanyak 45 – 65%, SiO2 1 – 12%, Fe2O3 2 – 25%, TiO2 >3%, dan H2O 14 – 36%.

Bijih bauksit terjadi di daerah tropika dan subtropika dengan memungkinkan pelapukan sangat kuat. Bauksit terbentuk dari batuan sedimen yang mempunyai kadar Al nisbi tinggi, kadar Fe rendah dan kadar kuarsa (SiO2) bebasnya sedikit atau bahkan tidak mengandung sama sekali. Batuan tersebut (misalnya sienit dan nefelin yang berasal dari batuan beku, batu lempung, lempung dan serpih. Batuan-batuan tersebut akan mengalami proses lateritisasi, yang kemudian oleh proses dehidrasi akan mengeras menjadi bauksit.

Bauksit dapat ditemukan dalam lapisan mendatar tetapi kedudukannya di kedalaman tertentu.
Potensi dan cadangan endapan bauksit terdapat di Pulau Bintan, Kepulauan Riau, Pulau Bangka, dan Pulau Kalimantan.

SILICA

What is Silica?

Silica is the name given to a group of minerals composed of silicon and oxygen, the two most abundant elements in the earth’s crust. Silica is found commonly in the crystalline state and rarely in an amorphous state. It is composed of one atom of silicon and two atoms of oxygen resulting in the chemical formula SiO2.

The first industrial uses of crystalline silica were probably related to metallurgical and glass making activities in three to five thousand years BC. It has continued to support human progress throughout history, being a key raw material in the industrial development of the world especially in the glass, foundry and ceramics industries. Silica contributes to today’s information technology revolution being used in the plastics of computer mouses and providing the raw material for silicon chips.

Geology and occurrence of industrial silica

Silica exists in nine different crystalline forms or polymorphs with the three main forms being quartz, which is by far the most common, tridymite and cristobalite. It also occurs in a number of cryptocrystalline forms. Fibrous forms have the general name chalcedony and include semi-precious stone versions such as agate, onyx and carnelian. Granular varieties include jasper and flint. There are also anhydrous forms - diatomite and opal.

Quartz is the second most common mineral in the earth’s crust. It is found in all three of the earths rock types - igneous, metamorphic and sedimentary. It is particularly prevalent in sedimentary rocks since it is extremely resistant to physical and chemical breakdown by the weathering process. Since it is so abundant, quartz is present in nearly all mining operations. It is present in the host rock, in the ore being mined, as well as in the soil and surface materials above the bedrock, which are called the overburden.

Most of the products sold for industrial use are termed silica sand. The word “sand” denotes a material whose grain size distribution falls within the range 0.06-2.00 millimetres. The silica in the sand will normally be in the crystalline form of quartz. For industrial use, pure deposits of silica capable of yielding products of at least 95% SiO2 are required. Often much higher purity values are needed. Silica sand may be produced from sandstones, quartzite and loosely cemented or unconsolidated sand deposits. High grade silica is normally found in unconsolidated deposits below thin layers of overburden. It is also found as “veins” of quartz within other rocks and these veins can be many metres thick. On occasions, extremely high purity quartz in lump form is required and this is produced from quartzite rock. Silica is usually exploited by quarrying and it is rare for it to be extracted by underground mining.

Physical and chemical properties

The three major forms of crystalline silica -quartz, tridymite and cristobalite- are stable at different temperatures and have subdivisions. For instance, geologists distinguish between alpha and beta quartz. When low temperature alpha quartz is heated at atmospheric pressure it changes to beta quartz at 573oC. At 870oC tridymite is formed and cristobalite is formed at 1470oC. The melting point of silica is 1610oC, which is higher than iron, copper and aluminium, and is one reason why it is used to produce moulds and cores for the production of metal castings.

The crystalline structure of quartz is based on four oxygen atoms linked together to form a three-dimensional shape called a tetrahedron with one silicon atom at its centre. Myriads of these tetrahedrons are joined together by sharing one another’s corner oxygen atoms to form a quartz crystal.

Quartz is usually colourless or white but is frequently coloured by impurities, such as iron, and may then be any colour. Quartz may be transparent to translucent, hence its use in glassmaking, and have a vitreous lustre.

Quartz is a hard mineral owing to the strength of the bonds between the atoms and it will scratch glass. It is also relatively inert and does not react with dilute acid. These are prized qualities in various industrial uses.

Depending on how the silica deposit was formed, quartz grains may be sharp and angular, sub-angular, sub-rounded or rounded. Foundry and filtration applications require sub-rounded or rounded grains for best performance.

Processing Technologies

Silica deposits are normally exploited by quarrying and the material extracted may undergo considerable processing before sale. The objectives of processing are to clean the quartz grains and increase the percentage of silica present, to produce the optimum size distribution of product depending upon end use and to reduce the amount of impurities, especially iron and chromium, which colour glass.

Cleaning the quartz grains and increasing silica content is achieved by washing to remove clay minerals and scrubbing by attrition between particles. Production of the optimum size distribution is achieved by screening to remove unwanted coarse particles and classification in an upward current of water to remove unwanted fine material. Quartz grains are often iron stained and the staining may be removed or reduced by chemical reaction involving sulphuric acid at different temperatures. Impurities present as separate mineral particles may be removed by various processes including gravity separation, froth flotation and magnetic separation. For the highest purity, for electronics applications, extra cleaning with aggressive acids such as hydrofluoric acid combined with thermal shock may be necessary.

After processing, the sand may be dried and some applications require it to be ground in ball mills to produce a very fine material, called silica flour. Also, quartz may be converted to cristobalite in a rotary kiln at high temperature, with the assistance of a catalyst. Some specialist applications require the quartz to be melted in electric arc furnaces followed by cooling and grinding to produce fused silica.

Silica has played a continuous part in man’s development and been one of the basic raw materials supporting the industrial revolution (as refractory, flux, and moulding sand) and today’s information technology revolution (providing the raw material for silicon chips).


Industrial silica is used in a vast array of industries, the main ones being the glass, foundries, construction, ceramics, and the chemical industry.


Silica in its finest form is also used as functional filler for paints, plastics, rubber, and silica sand is used in water filtration and agriculture.

Other examples of everyday uses include the construction and maintenance of an extensive range of sports and leisure facilities.

Crystalline silica is also irreplaceable in a series of high-tech applications, for example in optical data transmission fibres and precision casting. It is also used in the metallurgical industry as the raw material for silicon metal and ferrosilicon production. Another specialized application is in the oil production.

Altogether there are several hundreds of applications of industrial silica in our daily life. Silica products have become so obvious to us that we don’t even know they are being applied. Reading this page, you will be surprised to find out how many times per day you see, touch and use products containing crystalline silica.

For more information on the socio-economic aspects related to industrial silica uses, please have a look into the Socio - Economic Review of Crystalline Silica Usage, Brian Coope, September 1997, whose conclusion is that if man wishes to live in silica free environment he must move to another planet.

The glass products containing silica include containers (bottles, jars, drinking vessels), flat glass (for windows, automotive glass, mirrors, etc.), decorative glass (glasses, decanters, bowls, figurines), fibreglass (reinforcing and insulating), technical glass (screens), and optical glass (spectacles and binoculars).
Quartz sand is a basic material for the production of moulds and cores in metal casting. It is also used for precision casting, dental applications and jewellery casting.

The construction industry is by far the largest volume consumer of silica minerals. Industrial silica is used in construction aggregates, in concrete, dimension stone, masonry mortars, tile glues, floor screeds, cement manufacture, road line markings, asphalt, in bridge and sewer refurbishment, in decorative bricks, not to mention in glass and steel structures.

Industrial silica is a structural ingredient of clay bodies and a major constitutent of ceramic glazes, ranging from refractory bricks to wall bricks, and from sanitaryware to tableware and tiles.

Quartz derivatives are used in many areas, such as pesticides, fertilisers and pharmaceuticals preparations. Another derivative from industrial silica is silicon carbide, which is the raw materials for abrasives, anti-slip and polishing products.

Silica in its finest forms find important usage as reinforcing filler for use in paint, plastics, rubber, and sealants.
In paints, silica is used to render the paint more resistant to chemicals and for enhancing hardness and wear resistance.

Ultrafine silica displays strong reinforcing properties in rubberplastics to impart flexural and compressive strenght. formulations and is thus a major ingredient in car tyres. Silica is also used in
Silica sand is the principal filtration medium used by the water industry to extract solid impurities from waste water.

Silica sand is used in farming, market gardening, horticulture, aquaculture, and forestry, in applications ranging from soil additive, surfacing material, and animal feed material.

Silica sand with soil is used in the manufacture of football and other sports pitches and golf courses. It is also used, often with polypropylene fibre or with rubber, for all-weather horse racing tracks, show jumping rings, dog racing tracks and equestrian training areas.

Quartz and the carbon reducing agents (wood, coal, coke, charcoal, electrodes) are put in an oven heated at a very high temperature (> 2000°C) thanks to an electric arc created through the electrodes. The metal is then cast, cooled and adapted according to the refining, granulometric and packaging specifications required by the customer.

In our daily lives, Silicon is the raw material for the following applications:

Silicones and silanes used for their waterproofness, for glues and mastics’ adhesion, for their insulating properties and for moulds’ production.

Iron and steel metallurgy: silicon is used to produce special up-market steels.

High-performance concrete: reinforcing concrete’s mechanical characteristics (e.g. resistance to compression).

Electronics: highly purified silicon gives birth to micro chips through high-tech and ultra automated processes.

Aluminium alloys: silicon increases the cast flow and the mechanical properties of aluminium alloys.

Closely sized grades of silica sand, with rounded to sub rounded particles, are used to stimulate oil well production. The sand is pumped into the oil bearing strata and increases its permeability thereby promoting the flow of oil into the well.

Mining as a Basic Industry

Mining and Agriculture are 2 basic industries which led to the development of modern civilization. Other basic industries include farming and fishing and more recently manufacturing.

Agriculture gives us chiefly our food and the materials from which clothes and some of our buildings are made.

Mining supplies us with :
structural material, such as stone, glass sand, clays and cement fuels, natural gas, coal and petroleum abrasives, such as garnet and corundum fertilizers, potash, phosphates and nitrates various industrial uses, such as sulfur, graphite, borax, and asbestos metallic minerals, gold, silver, copper, lead, zinc, iron and aluminum precious stones, diamonds, rubies and sapphires fissionable materials rare metals.
From these substances come the materials which are of such vital importance in time of war and which, in times of peace, are so necessary for the growth of our arts, sciences and industry.

Mineral Resources and International Relations
The importance of international negotiations to a country whose reserves of necessary minerals has been depleted is obvious. Mines are WASTING ASSETS and a country once rich in minerals may later be compelled to import these essential materials. For any nation, a well balanced supply of minerals is better than a supply of some and a lack of others. Of these resources, the mineral fuels and iron are of primary importance. Copper, lead, and zinc come next. With them should be ranked the fertilizer group of phosphates, potash and nitrates together with sulfur, of so much importance in the chemical industries. Gold and silver are of little importance in building up industrial development.

Nickel, manganese, fluorspar, vanadium, tungsten and other mineral products - asbestos, mica, mercury, graphite, antimony and tin - are needed in industry. However, the quantities required are small and can be transported long distances to industrial centers. Industrial nations must have secure access to such resources and their control is a matter of international concern.
Most industrial nations lack sufficient resources to be self-sufficient and normally STOCKPILE the necessary mineral raw resources. Under the urge of economic nationalism, there has been a multiplication of production in the mineral industry throughout the world, not because of a shortage of world supply but through fear of being at the mercy of another nation in times of emergency.

Fly Ash

Fly ash is the finely divided mineral residue resulting from the combustion of ground or powdered coal in electric generating plant (ASTM C 618). Fly ash consists of inorganic matter present in the coal that has been fused during coal combustion. This material is solidified while suspended in the exhaust gases and is collected from the exhaust gases by electrostatic precipitators. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape (Ferguson et. al., 1999). Fly ash particles those are collected in electrostatic precipitators are usually silt size (0.074 - 0.005 mm).
Fly Ash Classification

Fly ash is a pozzolanic material and has been classified into two classes, F and C, based on the chemical composition of the fly ash. According to ASTM C 618, the chemical requirements to classify any fly ash are shown in Table 3.1.

Table 3.1. Chemical Requirements for Fly Ash Classification





Class F fly ash is produced from burning anthracite and bituminous coals. This fly ash has siliceous or siliceous and aluminous material, which itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds (Chu et. al., 1993). Class C fly ash is produced normally from lignite and sub-bituminous coals and usually contains significant amount of Calcium Hydroxide (CaO) or lime (Cockrell et. al., 1970). This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties (ASTM C 618-99).

Color is one of the important physical properties of fly ash in terms of estimating the lime content qualitatively. It is suggested that lighter color indicate the presence of high calcium oxide and darker colors suggest high organic content (Cockrell et. al., 1970).
Fly Ash Chemistry

Chemical constituents of fly ash mainly depend on the chemical composition of the coal. However, fly ash that are produced from the same source and which have very similar chemical composition, can have significantly different ash mineralogies depending on the coal combustion technology used. Because of this, the ash hydration properties as well as the leaching characteristic can vary significantly between generating facilities.

The amount of crystalline material versus glassy phase material depends largely on the combustion and glassification process used at a particular power plant. When the maximum temperature of the combustion process is above approximately 12000 C and the cooling time is short, the ash produced is mostly glassy phase material (McCarthy et. al., 1987). Where boiler design or operation allows a more gradual cooling of the ash particles, crystalline phase calcium compounds are formed.

The relative proportion of the spherical glassy phase and crystalline materials, the size distribution of the ash, the chemical nature of glass phase, the type of crystalline material, and the nature and the percentage of unburned carbon are the factors that can affect the hydration and leaching properties of fly ash (Roy et. al., 1985). The primary factors that influence the mineralogy of a coal fly ash are (Baker, 1987):

1. Chemical composition of the coal

2. Coal combustion process including coal pulvarization, combustion, flue gas clean up, and fly ash collection operations

3. Additives used, including oil additives for flame stabilization and corrosion control additives.

The minerals present in the coal dictates the elemental composition of the fly ash. But the mineralogy and crystallinity of the ash is dictated by the boiler design and operation.
Hydration of Fly Ash

Formation of cementitious material by the reaction of free lime (CaO) with the pozzolans (AlO3, SiO2, Fe2O3) in the presence of water is known as hydration. The hydrated calcium silicate gel or calcium aluminate gel (cementitious material) can bind inert material together. For class C fly ash, the calcium oxide (lime) of the fly ash can react with the siliceous and aluminous materials (pozzolans) of the fly ash itself. Since the lime content of class F fly ash is relatively low, addition of lime is necessary for hydration reaction with the pozzolans of the fly ash. For lime stabilization of soils, pozzolanic reactions depend on the siliceous and aluminous materials provided by the soil. The pozzolanic reactions are as follows:

Ca(OH)2 => Ca++ + 2[OH]-

Ca++ + 2[OH]- + SiO2 => CSH

(silica) (gel)

Ca++ + 2[OH]- + Al2O3 => CAH

(alumina) (gel)

Hydration of tricalcium aluminate in the ash provides one of the primary cementitious products in many ashes. The rapid rate at which hydration of the tricalcium aluminate occurs results in the rapid set of these materials, and is the reason why delays in compaction result in lower strengths of the stabilized materials.

The hydration chemistry of fly ash is very complex in nature. So the stabilization application must be based on the physical properties of the ash treated stabilized soil and cannot be predicted based on the chemical composition of the fly ash.
Leaching from Fly Ash

The total metals content for a specific ash source depends on the composition of the coal. The potential for leaching of these metals not only depends on the total metals content but also influenced by the crystallinity of the fly ash, as this would dictate whether the metals are incorporated within the glasseous phase or within crystalline compounds, which will hydrate (ACAA). The metals in the glasseous phase are expected to leach at much lower rate than that from the crystalline phase.

Since the degree of crystallinity is a function of boiler design and remains relatively constant for a given source, leachable materials remain relatively constant for a given ash source. A number of state regulatory agencies have issued source approval for specific generating facilities after the consistency of these materials had been demonstrated.

For stabilized soil, the leachability of metals not only depends on the property of the fly ash but also the soil that are used for stabilized soil. Some part of these metals leached from the fly ash will be adsorbed on the clay minerals of the soil.