GB2055090A - Manufacturing a plastering material - Google Patents

Abstract

A plastering material is manufactured by incorporating fly-ash in the material to be used for plastering.

Description

SPECIFICATION A method of manufacturing a plastering material The present invention relates to a method of manufacturing a plastering material and particularly applies to a method of manufacturing a plastering material from fly-ash produced by conventional thermal power stations which burn fossil fuels.

Disposal of combustion products, such as ashes etc. which occupy large storage areas and for which the disposal costs are high, is a serious problem in such power stations. Some of these waste products can be of use, particularly in the building industry. Fly-ash, a very fine particulate solid residue which is drawn out of a combustion boiler with the combustion gases and may be collected in precipitators can be of use in, for example, the building industry.

In Spain, the main producers of fly ash are the fossil fuel power stations. Table 1 lists the main power stations in operation or under construction in Spain.

TABLE 1 Output power Fuel Station (MW) Location Rich Compostilla lyll 449 Ponferrada (Leon) coals Guardo 148 Velilla del Rio Carrion (Palencia) Narcea I 65 Soto de la Barca (Astu rias) Ujo 24.9 Santa Cruz de Mieres (Asturias) Poor Lada 255 La Feiguera (Asturias) coals Ponferrada 1 2.6 Ponferrada (Leon) Puertollano 50 Puertollano (C. Real) Soto de Ribera 321.5 Soto de Ribera (Asturias) Lignites Aliaga 45 Aliaga (Teruel) Escatron 172.5 Escatron (Zaragoza) Figols 14.4 Figols las Minas (Barcelona) Puentes de Garcia 321 Puentes de Garcia Rdriguez Rodriguez (Corurina) Puerto Alcudia 77.5 Alcudia (Mallorca) Mixed Almeria 71.5 Almeria Aviles 97.5 Aviles (Asturias) Badalona I y 11 457 Badalona (Barcelona) Besos 1 50 S.Adrian del Besos (Barcelona) Burcena 92 Burcena (Bizcaya) Cadiz 1 33 Cadiz Guadaira 92 Seville Malaga 1 22 Malaga Pasajes 214 Pasajes de S. Juan (Guipuzcoa) Puente Nuevo 79.8 Espiel-Villabarta (Cordoba) Liquid Cristobal Colon 218 Punta de Sebo (Huelva) Escombreras 569 Cartagena (Murcia) Huelva 8.5 Huelva Mata 1 26 Barcelona Pit Abono 31 5 Abono (Asturias) Coal Aviles 7.5 Aviles (Asturias) La Robla 270 La Robla (Leon) Puertollano 220 Puertollano (C.Real) Anthracite Compostilla I (5th) 25 Ponferrada (Leon) Narcea i 1 54 Soto de la Barca (Asturias) Anthracite Compostilla II (3rd) 315 Ponferrada (Leon) and pit coal Lignite Serchs 1 75 Serchs (Barcelona) Puentes I 1 50 Puentes de Garcia Rodrigues (corunna) Utrillas 1 75 Utrillas (Teruel) TABLE 1 continued Output power Fuel Station (MW) Location Liquid Aceca (1st y 2nd) 627.2 Aceca (Bledo) Algeciras 720 Algeciras (Cadiz) Besos (2nd y 3rd) 800 S.Adrian del Besos (Barcelona) Castellon (1st y 2nd) 1,082.3 Castellon Ebro 300 Zaragoza Sabon (1st y 2nd) 420 Corunna San Adrian (1st y 2nd) 680 San Adrian (Barcelona) Santurce (1st y 2nd) 918.6 Santurce (Biscay) Not all of these stations produce fly-ash in amounts sufficient for industrial exploitation since some of these power stations consume only liquid fuels and others a mixture of liquid and solid fuel.

Table 2 specifies the power stations producing most fly-ash and gives the amounts produced.

This table has been collated using details in some cases provided by the stations, and in other cases calculated taking into account the amount and quality of the coal consumed from which, knowing the percentage of fly-ash produced by the particular coal used, the amount of fly-ash produced can be determined.

TABLE 2 Total Ash Total Fly-ash Production Production Power Station in Tonnes in Tonnes Aliaga 80800 58000 Almeria 8260 5920 Aviles 63635 45800 Badalona 37150 26800 Burcena 25859 18600 Cadiz 25190 18200 Compostilla 1 92300 66700 Compostilla II 206000 148000 Escatron 196700 141800 Figols 17500 12600 Guadaira 23500 16900 Guardo 136000 98000 Lada 194000 140000 Narcea 47000 33850 Pasajes de San Juan 83000 60000 Ponferrada 41000 29500 Puente Nuevo 79100 57000 Puentes de Garcia Rodriguez 66300 47700 Puertollano 82200 59100 Soto De Ribera 1 95840 69000 However, such amounts, although produced, are not at present available for exploitation as this material is at present little used and is usually disposed of because of the substantial amount of storage space required.

Where fly-ash is not used, the ash may be thrown in the sea, retained in dumps or stored in silos to await transportation to one or other of the disposal places mentioned above.

For power stations located at the seashore disposal of the fly-ash by dumping in the sea is the most economical method. However, this method of disposal is becoming more and more expensive because, due to the laws against water polution, the ash has to be thrown into the sea at points increasingly further away from the coast.

The dumping method of disposal is used by inland stations and by some stations situated at the coast which have dumps. Such dumps, if well cared for, may be capable of providing a good saleable fly-ash product.

Storage of the ash in silos is the best method if the ash is intended for further transportation and then sale. Unfortunately, it is also the most expensive method, requiring a large initial expenditure.

Having regard to greater possibilities for the use of the ash, the best solution in all cases is to store the ash in silos and, exceptionally, in dumps exercising maximum possible control over the product in order to determine the variations in the product in time and to ensure that it is put to the most appropriate use.

There are several possible methods of transporting the fly-ash from the silos to the point of use.

The ash may be transported in either a wet or dry state; handling is substantially easier if the ash is wet and it is not then necessary to employ special means of transportation, lorries or wagons being sufficient allowing consequential savings.

As the fly-ash is similar to cement, it requires more attention when carried in the dry state. In particular, the small grain size and lightness of the ash means that the possibility of the ash being blown away by the wind is large and that ash in the dry state should therefore be carried in adequate container means.

Carriage by seafaring vessels can only occur when the power station and the place of use of the ash are at the coast. However, this means of transport should be used whenever possible, even for a part of the journey, as it is very economical.

The use of bags for carrying the ash is inconvenient because, although the price of transportation per ton and kilometre is cheaper, the total cost is large because of the price of the bags themselves, the labour used in loading and unloading and losses incurred during transportation which when transporting cement are about five percent and which will therefore, because of the small grain size of the ash, be greater when transporting fly ash.

The most suitable means of transport is by bulk in lorries or tankers. The tankers or lorries have a large area of use as they make handling of the fly-ash easier, reduce labour and allow carriage of substantial amounts of ash, then reducing costs.

Usually, the tankers or lorries are loaded by gravity but unloading is always done by compressed air which forces the product, for example, cement, fly-ash, etc., out of the tanker.

This system of course requires that the places where the material is to be unloaded possess an air compressor and a pump. This is not, however, a problem since the places most likely to use the fly-ash are those which already use cement and would usually have such installations.

The fly-ash may be put onto the market in two different forms according to whether the ash is to be used with inert material or for its pozzolanic properties.

Coal burnt in power stations is the main source of fly-ash and contains carbonus material (ash) in variable amounts from 5 to over 50 percent. This material, under certain conditions, gives rise to fly-ash.

When large coals are burnt in grates only a small fraction of the ash is released in a physically divided state, the amount of such particles produced representing merely 0.3 percent of the coal consumed. These particles are sufficiently small and light to be drawn up by the combustion gases whereas the remaining larger and heavier particles making up the furnace bottom ash are deposited into the ash pit.

If the same coal is finely ground and combustion takes place under the necessary conditions, the greater part of the residue is released together with the combustion gases as fly-ash.

Fly-ash is defined as the very fine particulate residue produced by the combustion of pulverized coal which is drawn out of the combustion furnace by the combustion gases. Such particles may also be found in suspension in regions of low temperature in the combustion furnace where they have solidified.

This material, whose grain size is in most cases smaller than that of Portland cement, is very complex, heterogeneous and may vary greatly in its physical and chemical properties depending on the source of the coal from which the ash is produced, the type of combustion equipment, the degree of pulverization of the coal and the degree and control of the combustion process.

Since the ash is picked up in hoppers of mechanical or electrostatic dust precipitators or, preferably, a combination of both, the size of the fly-ash particles depends on the type of collector used.

The composition of the material making up the ash is different from that of the coal before combustion. The main elements of the fly-ash are unburnt coal, shale or clays and magnetite.

The ash retained in the ash pit, furnace bottom ash, is a solid residue of the combustion of pulverized coal evacuated from the furnace grate. This residue comprises particles of a size greater than the fly-ash particles, their form depending on the type of coal used and the particular combustion process adopted.

When the fly-ash is initially treated in the laboratory, density, granulometric and chemical analysis tests, etc., performed on the collected samples produce widely varying results.

A study of different samples from several sources reveals that it is a mistake to consider fly ash as a chemical product having particular properties and a defined chemical composition. The nature of a particle of fly-ash is determined by the composition of the source fuel, but boiler operation procedure etc. also effect the fly-ash produced. Therefore, samples of ash coming from stations of different types and ages and fed by coal from different sources in fact have very different characteristics and properties.

The properties of the particles of fly-ash remain close to average values when the conditions of the combustion process are practically the same for the same type of coal. Under such conditions to ensure that all the fly-ash collected shall have close to average characteristics, it is necessary to eliminate the fly-ash produced during ignition and stoppage of the boilers, as the ash then produced has properties very different from those of the average fraction.

The characteristics of fly-ash produced by a power station which operates uninterrupted and is provided with fuel always from the same source are found to be very consistent. A great number of conventional thermal power stations usually fulfill these conditions especially those located in mining basins.

The different methods of production mentioned above, and the different ways of collecting flyash mean that the ash produced is a very complex and heterogeneous material capable of great variation in its physical and chemical properties.

However, the fly-ash will contain no more elements than the source coal and, like the material present in the coal, the material present in the ash comprises no more than five or six ores: one or two clays, pyrites, quartz and mixed calcium, iron and magnesium carbonates; the fly-ash therefore contains few classes of compounds. In addition, during the combustion process there is very little contact between the particles in the boiler and the composition of the fly-ash will therefore depend on the original components of the particles of coal, even when its composition is different from the composition of the original ore.

Also, during combustion, the ash particles only remain in the combustion chamber for a very short time; a time which is not sufficient to allow the ash particles' compositions to reach a condition of equilibrium.

Many of the fly-ash particles may melt while in the furnace depending on the type of mineral, temperature of the furnace and gas atmosphere. Those particles reaching melting point become vitreous when resolidified and have no perfectly defineable composition, the proportion of unburnt material changing from particle to particle.

The water of hydration of the minerals associated with the coal ore is lost during combustion, other components like hydrocarbons are vapourized, coals are broken up and pyrites are oxidized. The agglomeration of the particles of ash produced is very unlikely because of the limited opportunjity for physical contact and therefore from 75% to 85% of the ash passes through the combustion chamber to the chimney in the form of discrete particles which when examined under the microscope appear as small hollow spheres of partially melted silicates, or as small solid spheres of melted silicates, iron or silica oxides, associated with unburnt materials, mineral fragments and combustion gas bubbles.

The characterization of the composition of fly-ash is a complex problem that cannot be solved by studying the composition of fly-ash produced in the laboratory because the latter is exposed to temperatures lower than those obtained in the combustion chamber of a furnace and the time of exposure may be several hours, whereas in a power station the exposure is limited to fractions of a second.

It has also been verified that not all fly-ash is good for all uses. Use of each ash must be made according to its properties. With the present state of knowledge about fly-ash it is not possible to enunciate international standards, even for the most common use, but countries separately specify and set their own standards for the properties required of fly-ash.

Investigations carried out with fly-ash show that mineral composition is very variable because, as has been seen, it depends inter-alia on: the type of coal used, lignite, pit-coal or anthracite (within three such large groups the fly ash produced is not similar, since the beds from which the world's fuels come are very different); the subsequent treatment of the coal, that is, mechanical treatment in the mills, heat treatment in boilers and electrical treatment in the electrostatic precipitators; and the method of collection. Thus, every sample of fly-ash has unique properties.

Generally, fly-ash has the following major components: silicone dioxide (six2), aluminium oxide (Al203), iron oxides (Fe203), lime (CaO) and carbon (C), and in lower proportions (usually less than the 5% by weight): magnesium oxide (MgO), sulphur oxides (S03), alkalis (Na2O and K2O) and other components in even smaller amounts such as titanium, vanadium, manganese, phosphorus, germanium gallium, etc.

However, within the samples fly-ash produced from the same types of coal the composition varies between narrow limits and shows certain correlations even between different countries which do not occur when samples of ash produced from different kinds of coal are compared.

Coal is always present in fly-ash but, when combustion in the boiler is satisfactory, the amount of unburnt material is less than the 5%. In exceptional cases it can reach 20% or more, but an amount greater than 10% may prevent the fly-ash from being suitable for certain uses, for example, for use as a substitute for cement in preparation of concretes. However, in the manufacture of bricks and the preparation of synthesized aggregates the presence of the unburnt material in the ash might be useful since the coal will supply part of the fuel required in the production process. The principal crystalline components usually found in fly-ash are mullite (3AI2032Si02), quartz (SiO2) magnetite (Fe304) and haematite (Fe203).As can be observed, there is a great difference between the composition of the ash and the minerals in the coal ore from which the ash comes, which ore usually comprises: quartz, kaolin, montmorillonite, mica, pyrite, etc., in varying proportions.

By way of information, the compositions of the fly-ash produced by certain countries are shown in the accompanying tables. Thus table 3 shows the three types under which fly-ash is usually classified in Poland. In table 4 the composition of fly-ash produced in the United States (given in percentage by weight) is shown.

TABLE 3 Types of Polish fly-ash 11 Ill lignite ash Pit coal ash Turoszow Konin (% by Basin Region Components weight) % SiO2 43 -57 45 -52 19-34 CaO2 4 -10 2.5- 4.0 38-52 Fe203 7.5-16 18 6-11 Awl203 18 -28 28 -32 2-4 MgO 1 - 5.5 0.15 2-4 SO3 0.3- 3.3 0.5 9-16 Na2O + K20 1 - 3 - 0.2 Insoluble substance, according 76 -89 78 42 to AFNOR.

TABLE 4 U.S.A. fly-ash Fly-ash Components Silica (SiO2) 34 -38 Alumina (Al203) 17 -31 Iron oxides (Fe203) 6 -26 Calcium oxide (CaO) 1 -10 Magnesium oxide (MgO) 0.5- 2 Sulphur oxide 0.2- 4 Unburned materials 1.5-20 In table 5 the composition of fly-ash from several French stations is detailed and in table 6 in composition of the fly-ash produced by the various types of French coal is illustrated. In table 7 the average composition of the fly-ash produced by a great number of French power stations is compared with the composition of some of the materials for which fly-ash can be substituted.

In table 8 the composition of fly-ash produced by several English power stations is given.

TABLE 5 Composition of fly-ash produced by French Power stations (%) C L B B V L D D T h a e u e i o e o b n i n a r c e c o v I d r n h r v q u r y i n g y s e i y n e e s s s s i e r Components e SiO2 46.8 47.7 47.3 47.2 42.3 47.7 46.6 46.9 46.0 Al2O3 34.6 33.4 28.6 27.0 27.0 27.9 29.1 29.6 29.1 Fe203 7.4 6.9 8.6 11.0 7.4 8.1 4.5 5.9 4.6 CaO 2.8 3.3 4.0 2.9 2.1 2.7 2.1 1.5 3.1 MgO 2.3 2.2 1.4 2.0 1.8 2.6 1.4 1.6 1.1 SO3 0.6 0.6 0.5 0.5 0.5 0.6 0.1 0.5 0.7 Alkalis 2.8 4.1 6.4 5.2 2.7 3.0 6.6 3.5 5.8 Notdosificated 1.9 0.8 0.1 0.2 1.8 1.6 0.1 0.8 0.1 C 0.8 1.0 3.6 4.0 14.4 5.8 0.5 0.7 9.5 II Ill IV V VI VII SiO2 50.7 47.5 47.8 46.1 23.3 42.0 17.8 Al203 30.2 30.0 27.8 36.4 10.4 34.0 12.9 Fe203 7.6 7.2 17.0 9.0 9.1 8.7 6.4 CaO 2.8 3.3 2.8 4.5 5.9 3.7 45.9 MgO 2.0 1.9 4.0 2.0 3.7 2.3 1.9 SO3 0.2 0.4 0.8 1.1 0.6 0.4 5.9 Alkalis 5.6 5.2 2.6 2.3 1.7 4.1 8.3 9.4 1.1 0.6 - - C 1.5 4.9 2.1 18.1 44.7 3.7 1.3 TABLE 6 Composition of fly-ash produced by different types of French coal (%) lignite lignite pit from from Anthra Composition coal provence Landes cite SiO2 47 -53 18 -24 75 -88 42 Al203 28 -35 13 2 -15 19 Fe203 3.6-11.5 6.3- 7.2 3.6 17 CaO 1.3- 4 46 -48 3 10 MgO 1.4-25 1.9- 2.8 0.7- 1.9 6.6 SO3 0.1- 0.9 6 - 6.4 0.6- 2.1 1 Na2O + K2O 0.9- 7.3 4.5- 8.3 0.4- 0.8 3.2 Notdosificated. 0.1 0.5 0.1 1.2 TABLE 7 Comparison of the average composition of French fly-ash with the average composition of some materials for which fly-ash can be substituted (%) Components Ashes Scums Pit coal Cement glass SiO2 49.0 29.6 50.20 22 73.00 Al203 31.0 14.4 17.00 6 1.00 Fe203 7.0 - 7.60 3 CaO 2.5 45.0 5.13 65 8.13 MgO 1.8 - 6.49 - - SO3 0.5 - 0.40 - - Alkalis 4.8 - 1.37 1 17.13 Loss under fire 2.6 - 11.28 - Not dosificated 0.8 - - - - TABLE 8 Composition of fly-ash produced by different English Power stations (%) Fly-ash SiO3 Awl203 Fe203 CaO MgO Na2O K2O Carmarthen Bay 41.4 23.9 12.9 2.5 1.8 0.8 3.7 Castle Donington 45.9 24.4 12.3 3.6 2.5 1.0 3.2 Cliffquay 45.5 25.5 13.5 3.2 2.2 0.8 2.6 Croydon B 42.8 26.1 9.3 2.4 1.4 0.6 3.6 Dunston 1 47.7 28.6 8.3 2.1 1.9 0.4 2.6 Dunston 2 50.7 34.1 6.4 1.7 1.7 0.3 1.8 Ferrybridge 1 48.7 27.9 9.5 2.4 1.6 1.5 4.2 Ferrybridge 2 47.7 27.5 10.3 2.1 2.0 1.7 4.0 Hams Hall 48.6 28.0 8.1 3.4 1.9 1.9 3.1 Rye House 43.6 24.6 11.3 7.7 2.9 0.7 2.2 Skelton Grange 1 47.2 26.7 11.9 3.4 1.9 0.9 3.8 Stella South 46.1 27.5 11.8 3.7 2.4 0.2 2.2 Skelton Grange 2 46.5 26.6 12.0 2.7 1.7 1.2 3.8 Uskmouth 44.2 26.5 8.6 1.9 1.6 0.6 3.8 Fly-ash TiO2 Mn304 SO3 P203 C H2O Soluble in water Carmarthen Bay 0.7 0.1 0.7 0.2 10.0 0.2 1.8 Castle Donington 0.9 0.1 0.9 0.6 4.1 0.1 1.8 Cliffquay 1.0 0.1 1.2 0.3 3.7 0.3 3.0 Croydon B 0.8 0.1 0.6 0.3 11.7 0.3 2.0 Dunston 1 1.1 0.1 1.4 0.2 3.1 0.6 3.6 Dunston 2 1.2 traces 0.6 0.2 2.0 ND 1.7 Ferrybridge 1 0.9 traces 1.2 0.2 1.5 0.3 3.3 Ferrybridge 2 0.9 0.1 1.8 0.3 0.9 ND 4.1 Hams Hall 1.0 0.1 1.3 0.6 0.6 0.2 2.7 Rye House 1.0 0.2 1.2 0.6 2.4 0.1 4.0 Skelton Grange 1 0.8 0.1 1.0 0.2 2.1 0.1 2.8 Skelton Grange 2 0.9 0.1 1.1 0.4 2.3 ND 2.6 Stella South 1.0 0.1 2.5 traces 0.8 0.3 4.7 Uskmouth 0.9 0.1 0.8 0.5 8.9 0.2 2.2 In table 9 the composition of fly-ash produced in different Spanish power stations is given.

Generally, the composition of fly-ash is very similar to the composition of the sterile mixtures of the coal layers from which the ash comes. In the North of France these layers are shales of similar composition to the clays and they do not differ much from one area to the next.

From the above tables it can be seen that the amounts of iron and coal very substantially in samples of fly-ash produced from the same type of coal. The variation in the amount of unburnt materials is determined by the construction of the boiler and the amount of volatile material in the feed coal. Generally, with the same boilers a greater amount of unburnt material is produced when using dry pit-coals and anthracites rather than oily pit-coals.

TABLE 9 Composition of various samples of Spanish fly-ash (%) Fly-ash SiO2 Al2O3 Fe203 CaO MgO SO3 Almeria 43.7 31.6 13.9 4.6 1.0 1.6 Badalona 44.2 18.9 11.5 - 1.1 3.4 Badalona 50.2 28.8 9.4 3.4 1.7 2.2 Burcena 39.4 26.7 11.7 5.3 2.1 0.4 Cadiz 43.7 31.6 13.9 4.6 1.0 1.6 Compostilla II 44.4 24.2 8.8 7.3 2.4 2.6 Ensidesa 37.6 25.5 27.9 5.9 1.1 0.9 Ensidesa 51.4 30.3 9.9 2.2 1.6 0.3 Ensidesa 47.1 29.6 9.1 0.9 1.4 0.7 Escatron 39.0 28.7 24.9 5.3 0.6 0.9 Figols 46.4 11.6 19.7 - 1.4 2.3 Figols 46.0 21.1 8.6 - 1.4 2.6 Guardo 44.8 24.2 8.8 7.3 2.4 2.6 Malaga 43.7 31.6 13.9 4.6 1.0 1.6 Puertollano 46.9 34.8 11.0 4.1 1.5 Soto de Ribera 50.3 31.7 7.7 - 2.1 0.6 Soto de Ribera 49.6 31.0 7.6 - 2.0 Ujo 36.4 24.8 18.1 11.3 2.9 3.0 Not do K2O sifica Fly-ash Na2O ted Mn304 P304 Mn304 P.P.C.

Almeria - 1.6 - - - 3.0 Badalona 0.5 - - - - 0.8 Badalona - 4.2 - - - - Burcena - 4.8 - - - 9.5 Cadi - 1.6 - - - 3.0 Compostilla II 3.3 - - - - 9.0 Ensidesa - - - - - 1.1 Ensidesa - - 0.3 0.2 - 3.0 Ensidesa 2.6 - - 0.1 0.1 7.3 Escatron 0.4 - - 0.4 - 0.2 Figols 0.1 1 - - - 0.1 Figols 0.2 - - - - 0.7 Guardo 3.3 - - - - 9.0 Malaga - 1.6 - - - 3.0 Puertollano 0.8 0.4 - - - - Soto de Ribera 3.5 - - - - 4.6 Soto de Ribera 3.1 - - - - 2.2 Ujo 0.5 - - - - 33.8 The differences in concentration of calcium and iron for the same source coal are due to the fact that the coal basins may or may not be rich calcareous rocks and iron ores.

The unburnt materials can be eliminated by reinjection into the boiler of all or part of the trapped ash together with more pulverized coal in which manner a greater proportion of granulated fly-ash is collected.

Generally, iron, usually in the form of Fe203, comprises between 5% and 20% of the fly-ash.

Iron usually does not have any detrimental effects on the fly-ash apart from causing the density of the ash to be increased. However, in certain cases, it is desirable to remove the portions rich in iron in order to lighten the product.

The iron concentration is always specified in terms of ferric oxide, but this is not the only iron compound in the fly-ash. By use of magnetic and analytic techniques several compounds have been detected, for example, the result of an analysis made by Simmons and Jeffery, of ash of the following composition: 35-51% SiO2, 21-30% Awl203, 5-27% Fe203 and 2-9% CaO, plus further components in amounts not exceeding 5%, was as follows: Soluble iron (FeSO4)5% Non-magnetic iron (2CaO Fe203)-5% Magnetic iron (magnetite and haematite)-20% Iron silicate (glass)4% Thus, the iron minerals found in the fly-ash can be classified as soluble, non-magnetic, magnetic and silicate as described above.

Other elements of interest from the point of view of their recovery from fly-ash are germanium, gallium, vanadium and titanium. However, the known methods of recovery of these elements are barely profitable even though the extraction of germanium and titanium could become economically feasible.

Due to the small proportions in which these elements are found in fly-ash, no properties of the ash which would be attributed to these elements are found to be detrimental to the incorperation of the ash in, for example, cement, ceramic materials, etc.

As already mentioned, fly-ash comes mainly from power stations. In Spain, at the end of the year 1967, the maximum total power output capacity of stations amounted to 1 2.889 megawatts (MW), of which 8.222 MW was accounted for by hydro-electric power stations and 4.667 MX by conventional thermal power stations, that is, 36.2% of the total maximum power output was provided by the conventional thermal power stations. However, the annual production for 1 967 of 4.077 x 10'0 KWh (Kilo-watt hours) consisted of 2.264 x 10'0 KWh produced by hydro-electric means and 1.813 x 10'0 KWh produced by conventional thermal means. Thus, the conventional thermal power stations produced 44.4% of the total number of KWh produced.Such figures are explained by the characteristics of the national electricity system of Spain, where the electricity output is related to the amount of rain fall. Thus, in a rainy year hydroelectric power output dominates and in a dry year the output of conventional thermal power station dominates. This obviously influences the output of fly-ash and it is therefore difficult to forecast the amount of fly-ash that will be produced in a given year.

However, the proposals of the national electric plan include estimates of the coal consumed in conventional thermal stations in certain years and the corresponding outputs of fly-ash. These estimates are shown in Tables 10 and 11.

TABLE 10 Amount of coal required for the years 1975, 1978 and 1981 (in 103T.) 1975 1978 1981 Types of coal Dry Average Humid Average Average Pit coal 4501 4501 4501 8461 8256 Anthracite 2113 2113 2113 2736 2736 Lignite 2601 2601 2601 2601 3298 T. 9215 9215 9215 13789 14290 TABLE Ii Estimated output of ashes and fly-ash for 1975, 1978 and 1981 (In 103 T.) Year Ashes Fly-ash 1975 2293.7 1758.7 1978 3449.5 2483.6 1981 3572.5 2572.5 Up to now, there are few world statistics on fly-ash and there is little technical documentation on the tonnages produced, number of people employed in this industry etc. The Working Party on the use of ash of the Solid Combustible Employment of the United Nations Coal Committee of the Economic Commission for Europe has been looking into fly-ash and its uses, collecting details and information.

As an indication of the present output of ash, Table 1 3 shows the fly-ash output of several countries during the period 1964-1968.

TABLE 13 Output of ash (in thousands of tonnes) Country 1964 1965 1966 1967 Western Germany 1100 10400 11000 11600 Australia 800 - 600 512 Bulgaria - - 3200 Czechoslovakia 5310 - 10200 10500 Denmark - - 594 501 United States 18000 20000 25189 27900 Finland - - 277 France 4500 4022 3779 4114 Greece 320 250 375 604 Netherlands 400 391 366 Hungary - - 3300 Poland 4365 4990 5840 5840 United Kingdom 9150 9440 9256 9514 Romania 1500 1500 800 Turkey 210 - 430 338 Yugoslavia - - 1000 Estimates have also been made of the future production of ash in some of the Countries; giving the results listed in the following table: Estimate of future output of ash (In thousands of tonnes) Country 1970 1975 Western Germany 1 2500 1 3800 Czechoslovakia 1 2000 1 4000 France 4980 4280 Poland 6900 8000 Romania 4200 8000 It is convenient to note that all the output figures, either evaluated or anticipated, refer to ash coming from coal, without differentiating between the types of coal: anthracite, pit coal and lignite.

Fly-ash can be used wet, the water content of the ash varying between 10% and 20%, which makes transportation and storage easier since the ash can then be carried as if it were sand. The composition of the fly-ash is not of decisive importance since the really important property is its fineness. Such a property causes the ash to increase plasticity in concretes and mechanical strength in poor mixtures of Portland cement while also decreasing the weight and porosity of concretes and preventing segregation. Generally the fine size of fly-ash particles means that the ash may be used in agregates in order to complement the grain size of the concrete and thus make it more continuous, having the advantage of also substantially decreasing the concrete permeability, especially in the plastic state.

Natural sands from the beds of the rivers are scarce in particles smaller than 0.3 mm. The role performed by such particles in the properties of mortars and fresh and hardened concretes is of much importance.

The effect of fine granules on the maleability of mortars and fresh concretes is well-known. In particular, exudation is reduced for a given spreading at a shaking table.

The effect of fine particles in mortars and hardened concretes is also known, the durability of such products increasing on compaction.

In a first experiment the fly-ash used came from the Compostilla II power station. Its original granulometry is shown in the following Table 14. The cement used is P-350.

TABLE 14 Granulometry of the fly-ash Meshes/cm2 900 2500 4900 10000 16900 27225 % Residue retained 0.1 1.5 3.2 10.1 19.0 32.0 by the sieve Two siliceous sands of the Mino river were used whose granulometries are given in Table 1 5.

TABLE 15 Granulometry of the Mino River sands Size of mesh in mm Sample 4.86 2.35 1.16 0.61 0.30 0.13 Caldelas (% held) 0.0 11.0 24.0 47.5 90.0 98.5 Teanes (% held) 0.0 13.0 25.5 54.5 93.0 99.5 The effects of substituting fly-ash for part of the sand in fresh and hardened mortars were studied.

a) The effect on the properties of fresh mortars All the mortars were prepared with a cement-sand ratio of 1:3.

In each case the ratio of water to cement was adjusted so that with every mixture two spreadings at the shaking table were possible: one of 70% and the other of 100%.

An exudation test was performed using cylinders of 5 cm diameter and 20 cm length. These were filled to within 90% of the total cylinder height with three layers of mortar, the remainder of the cylinder being left empty to allow collection of the exudation water. The cylinders were tightly sealed and kept at 20 + 1 C for two hours, at the end of which the liquid at the top of the cylinders was pipetted off and the amount of liquid collected from each cylinder measured in grammes.

The mortars using the sand "Caldelas" and "Teanes" contained differing amounts of fly-ash; 5% and 8% fly-ash respectively, because of their different granulometries. In the Table 16, the results achieved are given, from which it can be determined that the addition of fly-ash to the mortar reduces the relation on account but allows the mortar to achieve the same spreading at the shaking table and increases the maleability of the mortar so that under the same spreading conditions exudation decreases.

TABLE 16 Effect on fresh mortars of the substitution of fly-ash for a proportion of the sand.

Sample Caldelas Teanes Spreading 70% 70% 100% 100% 70% 70% 100% 100% On account 0.56 0.52 0.58 0.55 0.54 0.51 0.57 0.53 % fly ash 0 5 0 5 0 8 0 8 Exudation 0.4 0.4 0.9 0.8 0.4 0.3 1.0 0.9 b) Effect on the properties of hardened mortars From the values obtained by two tests carried out on each mixture and each age, it is deduced that, for similar spreading of the mortar, the substitution of fly-ash for sand increases in every case the resistance of cured test pieces to compression and flexion. Moreover the water: cement ratio is decreased.

The present output of fly-ash in Spain and the likely future output of fly-ash can be evaluated from the basis of the National Electric Plan, in which the development of conventional thermal power stations is detailed.

With regard to consumption of fly-ash, the maximum amount of ash which each particular use could consume can be evaluated. However, at this time such maximum use is impossible, since the National output of ash would be insufficient.

It is unrealistic to think of reaching such values at this time, since countries with many years of investigation and experience in this field use amounts which only about 30% of their total output of fly-ash. However, we have seen that the percentages of fly-ash used varies substantially from country to country.

The countries which have been producing electrical energy by conventional thermo-electric means the longest, logically initially started investigations into the use of fly-ash and are consequently using more fly-ash. It is also a fact that the countries which utilise the largest amounts of fly-ash are those with the highest standards of living, for example, Denmark used 85% and France 50% of their respective outputs of fly ash in 1 966. However, this does not appear, at the moment, to be a general law since the United States used only 12% of its fly-ash output in the same year and the standard of living there is very high.

The average amount used annually was placed in 1966 at about 21% and can today be estimated at about 25%. Such averages are, however, of little representative value due to the wide variation in the values already obtained.

In the case of Spain it would seem logical that, if the use of fly-ash were highly promoted, the average amount of ash used could reach 25% of the total output and, perhaps, Spain being a developing country, the percentage used could be as much as 30%.

An average output of 1000000 T of fly-ash for a given year may be assumed. The division of such an amount of fly-ash among several uses cannot be done by taking into account the maximum possible amount of ash which each use could consume because, as has already been stated, this situation does not occur in countries using large amounts of fly-ash and, in such countries one field of use may use much more fly-ash than another.

For many reasons concrete may be the market using most fly-ash, particularly because it was the first product to incorporate fly-ash and to retain the use thereof. However, due to the expansion and automization of the industry in manufactured concrete, it would take a long time for such concrete to acquire a large proportion of the concrete market.

Taking the above into account, if the expected amount of fly-ash is divided among the various uses in proportion to the known importance of the uses, values such as those laid out in Table 1 7 can be determined. These values reflect the possible division of the amount of fly-ash that is estimated will be used in Spain in a year and a half.

Obviously, both the amounts shown in Table 1 7 and the distribution thereof depend on many variables that cannot be accurately evaluated in this publication.

TABLE 17 Likely distribution of produced amount Flying ash % of Use in T. the total Cement 20000 2.0 Concrete 1 30000 13.0 Manufactured concrete 50000 5.0 Roads 100000 10.0 TOTAL 300000 30.0 From the above, it can be deduced that use of fly-ash is made within one or more of the following industries.

a) Traditional manufacturing industries In such industries small amounts of fly-ash are added to the products in order to improve the technical features of the product and for economic reasons.

b) New techniques in manufacturing of materials A large amount of fly-ash may be used, for example in: cellular concrete; siliceous-calcareous bricks; synthesis of light aggregates and manufacture of concrete, etc.

Combined examination of these uses shows that the great majority of the fly-ash tonnage used corresponds with the amount of volcanic fly-ash produced (volcanic fly-ash being the original cause of use of the ash) showing the future of the residue. For example, in the building industry techniques are fast evolving and it is possible that further uses for fly-ash will be found in the future as new processes are initiated.

It should however be emphasized that there are some difficulties in making possible increased use of fly-ash. The main difficulties are: transportation expenses which substantially increase the price of the fly-ash and, in certain cases, make the cost prohibitive; lack of homogeneity in the material; and the demands of many customers that the producing stations help fund the facilities to be set up, something which in many cases is of no interest and is away from the main aims of the industries producing the fly-ash.

In a full study of a particular use of fly-ash it is necessary to keep in mind: the properties of the ash; the development of an industry able to use the ash in the vicinity of the producing centre; economical considerations; the ability of the price of the materials produced to become competitive; transportation costs; labour costs; amount of capital investment required; and favourable occasional occurences, such as a sharp increase in the demand for the material or a large building programme. Prior to the industrial development of fly-ash it is necessary that technical studies on the use of the ash should be well-known among future buyers, and that the experts should have verified the properties of fly-ash.

The use of fly-ash should be a problem with which the industries producing the ash are concerned. The future of fiy-ash appears good for the following reasons: international cooperation will allow the exchange of information; the number of users of substantial amounts of fly-ash such as the building and road building industries will increase; the ease of obtaining an abundant and cheap product that has many different uses; the improvement in the quality of the final products produced, in many cases, by adding fly-ash; and increase in transportation, allowing the fly-ash to be transported to places far away from the production centres, without substantial increase in prices.

From the above, it can already be said that the scientific and technical investigation into the use of fly-ash in the world is already channeled into the phase of industrial investigation and marketing. It is advisable that producers of fly-ash know the present state of the possible applications of fly-ash and equip their stations with means of collection and distribution suitable for the most appropriate use.

Some standardisation of the properties required of fly-ash to be put to a particular use are thought necessary. In all application fields it is convenient to have standards, but it is specially more urgent that standards are brought into being for industries which require large amounts of fly-ash, such as, the industries which manufacture pozzolanic cements from fly-ash and use flyash as a replacement for Portland cement in concretes; the manufacturers of blocks or similar products from light concrete, and the road construction industry.

With such standards it will be possible for laboratories at the power stations to test, either from time to time or systematically, the fly-ash produced in order to verify whether or not it complies with the standards and so to keep a rational control of output, elimination and storage.

Thus, the consumers may have a means of control of the product.

A very interesting step would be to set up a pilot plant to manufacture light aggregates from synthesized fly-ash which for similar or less cost could replace the gravels presently used in, for example, prefabricated and light concretes.

It should not be forgotten that, as has already been mentioned above, in many of the uses of fly-ash either a product of special qualities is produced at a price equal to or lower than the usual price or a product of similar characteristics to the original product is is produced at lower price.

Now in Spain fly-ash is being used more and more, for example, in cements, concrete etc.

and in building dams, roads, etc. and it is thought possible that the market for the product will increase provided certain minimum requirements are met and interest in the product increases.

Thus, the generalization that fly-ash is a useless residue is far from the truth. - Rather, considering the various uses of the ash and the results of experimental tests, it should be considered to be an interesting material worthy of consideration.

From the tables showing the composition of fly-ash produced by stations in various different countries, it can be seen that there is a great variation in the compositions specifically in the amount of lime (CaO) in the ash.

For example, fly-ash from Polish coals taken from the Turoszow basin comprises 2.5% to 4.0% CaO whereas coal taken from the Konin basin comprises 38% to 52% CaO.

The amount of CaO in American fly-ash varies between 1 % and 10% In France, depending on the type of coal used, the percentage of CaO varies between 1.3% to 4.0% (Pit-coal) and 46% to 48% (lignite from Provence).

In England, the variation in the proportion of CaO is less, between 1.7% and 7.7%.

In Spain the variation is largely between 3% and 4% and 43% (Alcudia Thermic Station).

From the above, the possibility of substituting fly-ash low in CaO concentration for sand arises.

According to the present invention there is provided a method of manufacturing a plastering material comprising incorporating fly-ash, a combustion waste product produced in conventional thermal power stations, in the material to be used for plastering.

The fly-ash may be extracted by a dry process from the combustion waste by air blowing means. Alternatively a wet process can be used, the fly-ash being extracted by washing the waste and collecting the liquid formed which will contain fly-ash.

Conveniently, the ash is dried to eliminate excess water and sieved to remove any large particles, which particles are then crushed and re-sieved.

in a preferred method in accordance with the invention, the extracted ash is hydrated or totally or partially slaked prior to being dried depending upon the amount of calcium oxide in the fly-ash.

Usually the sieved fly-ash is screened to eliminate unburnt elements and sulphur compounds.

Preferably a binding substance is added to the plastering material, the amount of binding substance added being variable and dependant on the type of fly-ash or the final product required. Generally the binding substance used is plaster or stucco. Alternatively the binding substance may be a hydraulic or air binder having characteristics similar to those of cement, magnesium oxide, calcium oxides or aluminium oxides.

Sand may be incorporated into the plastering material. The sand, gravel or similar substance is added in an amount which varies according to the quality or chemical composition of the flyash or the required properties of the final product. For example, in certain cases no sand need be added.

Preferably an agent which alters the colour, presentation or setting time of the plastering material, for example, such agents as sodium chloride, caustic soda, hydrophobic agents and pastefying agents may be incorporated in the plastering material.

The fly-ash may be slaked whilst the binding substance which is a base the amount of which is preferably regulated, is dehydrated by heat blowing means or addition of quick lime and the ash is slaked by adding wet sand or water thereto.

Conveniently, the plastering material may either be bagged and transported by conventional means or carried in bulk in special vehicles.

In order that the invention may be readily understood, a method in accordance with the invention will now be described by way of example with reference to the accompanying drawings in which the single figure diagrammatically illustrates the method embodying the invention.

Referring now to the drawings, ash obtained as by-product of the combustion in a power station of any type of coal, anthracites or lignites, comes in two different types, which are collected in different parts of the power station. A first volatile type of ash, fly ash, is collected precipitators 1 disposed near or at the outlet of the furnace 2 whereas the second type of ash, furnace bottom ash collects in the furnace.

Removal of fly-ash from a station may occur by two principal processes, a dry process or a wet process. When the dry process is adopted the fly-ash is moved to silos 3 by blowers or conveyor-belts. At the silos the fly-ash is stored and prepared for transportation by lorry or rail to places of use.

The wet collection process requires the addition of water 4 to the fly-ash containing material.

The liquid from the resulting paste is carried to decanter tanks 5 where the greater part of the water is extracted.

The furnace bottom ash may include a large amount of ash. The contaminated furnace bottom ash may be washed 6 by the addition of water 7 and the resulting liquid containing fly-ash withdrawn and allowed to settle. Obviously the clean furnace bottom ash is rejected 8.

Thus, there are three types of raw material. Two of them, the liquid and byproduct of washing the furnace bottom ash, are already paste and slaked ash, and partially slaked ash respectively.

The fly-ash whether wet or dry can be carried by appropriate means to a place of use.

Fly-ash collected by dry extraction must be totally or partially slaked with water 9 on reaching the factory during which operation the fly-ash substantially increases in volume by 2 to 3 times the original volume because of the total or partial hydration 10 which occurs.

The advantage of the wet process over the dry process is that although slaked or partially slaked ash occupies more volume when transported, the hydration operation at the factory is not necessary if the wet process is used.

Whatever the source, the fly-ash must then be dried 11 in order to eliminate excess water 1 2 and to regulate degree of hydration.

Once the fly-ash is dry, it can, if necessary, be passed through a sieve 1 3. The material which cannot pass through the sieve may be crushed in a crusher 14 and then resieved if required.

The fly-ash may be further screened 1 5 to eliminate unburnt remains and sulphur compounds 16.

The fly-ash thus obtained is ready to be mixed 1 9 with other raw materials.

A binding substance 1 7 which is a base is added to the fly-ash. The binding substance may be plaster or stucco or some other type of similar material, for example, cement, lime, magnesium oxide, a byproduct obtained in the manufacture of phosphates or a combination thereof depending upon the type of fly-ash.

Preferably the binding substance is dehydrated before being added to the product. The dehydration process may be carried out while the fly-ash is being slaked or hydrated. The binding substance may be dehydrated by heat blowing means or by quick lime while the ash may be slaked either by water or the addition of wet sand.

The amount of binding substance added should be regulated according to the amount of calcium oxide contained in the fly-ash to be slaked The heat released during the slaking process may assist in the dehydration of the binding substance such that the plaster or calcium sulphate dihydrate combines with calcium oxide and under the action of heat calcium hydroxide and calcium sulphate are produced: CaSO42H2O + CaO-Ca(OH)2 + CaSO4H20; the product being useful for plastering.

Simultaneously, or successively, a variable amount 1 8 of river or sea sand, gravel, etc. is introduced into the mixture to balance the composition, the type of sand used being dependant on the type of fly-ash or the qualities required of the final product. Further ingredients which alter the setting tine, colour or final presentation of the plastering material, such as sodium chloride, caustic soda, hydrophobic agents and plastefying agents etc. may be incorporated in the mixture.

The mixture when homogenized can be sacked 20 or carried in bulk to the place of use and may be used either indoors or outdoors as a plastering material.

The incorporation of fly-ash in a plastering material helps solve the problem of disposal of the wastes from coal fired power stations and the method in accordance with the present invention is economical as there is no boiling of the materials involved.

The product obtained has great toughness, compactness and compression strength. Moreover, it will not rust iron and thus oxide stains are avoided. The product can also have a long setting period to allow better use of the material. Further, the product can be carried in bulk or packeted and does not deteriorate for at least six months.

Claims (17)

1. A method of manufacturing a plastering material comprising incorporating fly-ash, a waste combustion product produced in conventional thermal power stations, in the material to be used for plastering.

2. A method according to claim f, wherein the fly-ash is extracted by a dry process from the combustion waste by blowing means.

3. A method according to claim 1, wherein the fly-ash is extracted by a wet process fron the combustion waste by washing the waste and collecting the liquid formed which will contain flyash.

4. A method according to claim 2 or 3, wherein the extracted fly-ash is dried to eliminate excess water and sieved to remove any large particles, which particles are then crushed and resieved.

5. A method according to claim 4, wherein the extracted fly-ash is hydrated or totally or partially slaked prior to being dried.

6. A method according to claim 4 or 5, wherein, the sieved fly-ash is screened to eliminate unburnt elements and sulphur compounds.

7. A method according to any proceding claim, wherein a binding substance is added to the plastering material.

8. A method according to claim 7, wherein the binding substance is used plaster or stucco.

9. The method according to claim 7, wherein the binding substance is a hydraulic or air binder having characteristics similar to those of cement, magnesium oxide, calcium oxide or aluminium oxide.

10. A method according to any preceding claim, wherein sand is incorporated into the plastering material.

11. A method according to any preceding claim, wherein an agent which alters the colour, presentation or setting time of the plastering material is incorporated in the plastering material.

12. A method according to any one of claims 7 to 11, wherein the ash is slaked or hydrated while the binding substance which is a base is dehydrated before being added to the plastering material.

13. A method according to claim 12, wherein the amount of binding substance added is regulated.

14. A method according to claim 1 2 or 13, wherein the binding substance is dehydrated by heat blowing means or by addition of quick lime and the fly-ash is slaked by adding wet sand or water thereto

15. A method according to any preceding claim, wherein the plastering material is either bagged and transported by conventional means or carried in bulk in special vehicles.

1 6. A method of manufacturing a plastering material substantially as hereinbefore described with reference to the accompanying drawing.

17. A plastering material produced by the method of any of the preceding claims.

1 8. Any novel feature or combination of features herein described.