2.6.1 COAL FLY-ASH
Coal fly-ash is a solid material extracted by electrostatic and mechanical means from the flue gases of furnaces fired with pulverised bituminous coal (Addis, 1994). Coal ash, a ceramic material, is essentially an aluminosilicate glass with inclusions of mullite, spinel, quartz and lime. The properties of this coal ash are determined mainly by its unique chemical and mineralogical composition. In turn, these are dependent upon the type of coal, as well as the thermodynamic environment prevalent during the combustion processes. In modern power stations, the coal is ground to a very fine powder before being injected into the boilers. In the boilers the combustibles burn giving off heat energy to produce steam. The non-combustibles form the ash. Due to very high flame temperatures the ash is in the liquid state in the flame and on cooling solidifies in the form of hollow spheres, as shown in Figure 2.3 (Mantel, 1991).
While the composition of coal fly-ash produced within any one particular South African power station is remarkably consistent, there are differences between the various power stations. The major source of coal fly-ash in South Africa is the Lethabo power station near Vereeniging. The exact composition of the coal fly- ash is also dependent on the particular particle size range (Kruger, 1990). The surface area of various ashes varies from 400 – 600 m2.kg-1 (Mantel, 1991).
The primary components of power station coal fly-ash are silica (SiO2), alumina
(Al2O3) and iron oxide (Fe2O3), with varying amounts of carbon, calcium (as lime
or gypsum), magnesium and sulphur (sulphides and sulphates) (Malisch, 1981).
2.6.1.1 World production of coal fly-ash
Worldwide, some authorities forecast coal fly-ash volumes of more than the current world output by as much as 800 x 106 ton by the year 2010, (Swanepoel
& Strydom, 2002).
In the United Kingdom, approximately 50 % of the coal fly-ash produced is used while in India only 6 % (Satapathy, 2000) despite various efforts in using coal fly- ash in traditional applications.
In India, thermal power plants generate more coal fly-ash than in other countries. It is estimated that currently about 90 megaton of coal fly-ash is generated every year in India alone. Only a small amount of the total coal fly-ash generated is utilised in making bricks or concrete building blocks, or blending with cement (Chandra et al., 2005).
According to Ilic et al. (2003), coal-fired power plants in Yugoslavia produce approximately 5 megatons of coal fly-ash per year. Of this only 20 kilotons are currently used in the cement industry for the production of paving slabs, building blocks and ready-mixed concrete. For this reason it is of utmost importance to develop new applications and uses for coal fly-ash.
The coal fly-ash used in this study was received from the company Ash Resources. Coal fly-ash is an inorganic waste material from the coal fired Lethabo electrical power station, situated near Vereeniging and Sasolburg in the Free State province of South Africa.
The oxides present in coal fly-ash make it an ideal raw material. Coal fly-ash will introduce to the mixture the necessary oxides needed to manufacture insulating refractory materials.
2.6.2 PHOSPHOGYPSUM
Phosphoric acid waste gypsum (phosphogypsum) (Smadi, Haddad & Akour, 1999) is a by-product resulting from the phosphoric acid process for manufacturing fertilizers. The phosphogypsum used in this study was obtained from AECI/Kynoch. This material originated from fertilizer production.
It consists mainly of CaSO4.2H2O and contains impurities such as P2O5, F- and
organic substances. The quantity of phosphogypsum is very large: for each one ton of phosphate (P2O5) produced, there is a co-production of five tons of calcium
sulphate (phosphogypsum). The annual world production of this material is 180 million tons. Only 15% of the phosphogypsum is utilised by cement and gypsum industries as a setting moderator for cement and for making gypsum plaster. The remaining 85% of phosphogypsum is not used, causing an environmental problem and creating a need for large areas for disposal. Therefore, attempts were made to use phosphogypsum in applications such as road and rail works fills, stabilisation of base course and building constructions. In addition many other applications of phosphogypsum are sought (Smadi, Haddad & Akour, 1999) as in some jurisdictions, phosphogypsum is considered a radio active waste due to the levels of radon and other radioisotopes present in it, which leads to disposal problems.
Phosphogypsum (Lutz, 1995) has substantially higher water content than other synthetic gypsums or natural gypsum, often as high as 30%. This is only true immediately after production. This gypsum can also contain varying amounts of residual phosphates, sodium and fluorine compounds, organic products and other impurities depending on how the preceding phosphoric acid process and preparation step are managed. The particle size of this gypsum is usually below 200 micrometer. Phosphogypsum has low strength and poor adhesive properties, but is added to bodies to assist in setting of concrete (Mantel, 1991).
2.6.3 IRON RICH WASTE
There is little, if any, literature on iron rich waste, as it is a waste product from the vanadium extraction process. Annual world production of vanadium pentoxide averaged 62 200 t between 1980 and 1993. South Africa’s share of this production has averaged at 42 %. South Africa’s reserves of vanadium-bearing titaniferous magnesites in the Bushveld complex are vast (Shürmann & Marsh, 1998). The titaniferous magnitude magnetite of the Bushveld complex is not amenable to physical beneficiation techniques, it contains sufficient vanadium to permit recovery by the salt-roast and leach process (Shürmann & Marsh, 1998).
The smelting stage generates a titanium bearing slag, containing about 15% TiO2 and 75 % Fe2O3. Currently it is stockpiled (Grohmann, 1995). TiO2 and
Fe2O3 act as a flux in ceramic materials. Iron-rich waste for this study was
obtained as a slag from the vanadium manufacturing company Vametco, situated near Brits in the North-West province of South Africa.
All of the above mentioned inorganic waste materials pose a problem to the South African industry with regard to waste disposal. Therefore new applications for these waste materials are continuously sought.