PROCESS FOR THE MANUFACTURE OF A POROUS SINTERED AGGREGATE

Abstract: A porous sintered aggregate consists of a sintered product of a raw mixture comprising 1) a material the main components of which are carbon and the oxides of Si, Al, Fe and Ca in proportions: SiO₂: 35-70 per cent; Al₂O₃: 15-35 per cent; Fe(calculated as Fe₂O₃): 3-30 per cent; CaO: 1-10 per cent; and C: 1-15 per cent; and 2) fine calcareous material. The composition of the raw mixture is such that the weight ratio of total CaO over total Al₂O₃ in the mixture lies within the range 0.35 - 1.00, the weight ratio of total SiO₂ over total CaO is greater than 1.5, and the weight ratio of total SiO₂ over total Al₂O₃ is greater than 1.0. A process for manufacture of such a product is described.

Description: SINTERED AGGREGATE AND PROCESS FOR ITS MANUFACTURE The present invention relates to a porous sintered aggregate made from clay-like material with a minor carbon content, and a process for the manufacture of such aggregates. In the present context, the term n clay-like material with a minor carbon content?? is hereinafter defined as a material the main components of which are carbon and the oxides of Si, Al, Fe and Ca, in the following proportions: SiO2: 35-70 per cent; A1,0 15-35 per cent; Fe (calculated as Fe203) : 3-30 per cent; CaO: 1-10 per cent; and C: 1-15 per cent. Examples of such material are both unburnt waste materials, such as coal shale, and burnt waste materials comprising coal ash and coal ash slag produced by combustion of non lignitic coal. During the combustion of coal, e.g. in power stations, the coal ash is distributed into two fractions, bottom ash collected from the bottom of the combustion chamber, and fly ash collected from the stack gases by air pollution control equipment. The older types of furnaces based on the stoker principle transform most of the ash content into coal ash slag. Coal ash as well as coal ash slag are composed largely of silica, alumina, iron oxides, lime and carbon. The average analysis varies considerably depending on the type of coal in question and the burning conditions, but usually falls within the above mentioned range. Since the components have been exposed to a high temperature, the oxides are in general fused into a glass. Typically from 10 to 85 per cent of the total coal ash production of a power station, is in the form of fly ash, and occurs as spherical particles, normally ranging in diameter from 0.5 to 100 microns. The colour depends on the carbon content and varies from light tan to black The particles of the bottom ash are coarser than those of the fly ash and its colour varies irom gray to black. Coal ash slags are ashes which have been sintered and air cooled. Typical dimensions of slag particles are 80-100 mm. and their colour varies from gray to black. These waste materials represent an increasing disposal problem, and many suggestions have been made for using these materials when manufacturing aggregates for various purposes in the building industry. British Patent Specification No. 972851 discloses a process for the production of aggregates from pulverised fuel ash, finely divided slag, or shale or any mixture of such materials, which includes the steps of mixing such materials with water and clay, dividing the mix into granules and preheating and sintering these granules in a rotary kiln. It is further known to use a travelling grate sintering machine for the abovementioned heat treatment. However, two serious problems present themselves both when using a rotary kiln and a travelling grate machine. Firstly the ash typically exhibits small variations in chemical composition, leading to varying sintering tcmperatures with a resultant tendency of the granules to fuse together. Secondly, it is difficult to produce a carbon-fre product because of diffusion problems. Proper carbon removal necessitates diffusion of air into the granules to such an extent that all the fine coke particles distributed throughout the granules can be burnt away. However, this diffusion is seriously obstructed by the rather compact structure of the granules. West German OS 24 23 096 concerns a corresponding process for the production of expanded aggregates from fly ash, including the steps of mixing the fly ash with water, dividing the mixture into granules, drying these granules and heating them at 1000-1450 C. According to the specification an expansion amounting to 20-500 volume per cent can be achieved by the burning step. This process presents the same fusion and carbon removal problems as referred to above. The object of the present invention is to provide a new sintered product made from clay-like material with a minor carbon content, exhibiting form stability during sintering, high compressive strength and a colour acceptable for use as building bricks as well as for structural fill, which material can be manufactured without addition of substantial amounts of correction materials. A further object of the invention is to provide a process for the manufacture of this product. It has now been found that the first object can be achieved by a new product which is characterized in consisting of a sintered product of a raw mixture comprising fine clay-like material with a minor carbon content, and fine calcareous material in proportions such that the weight ratio of total CaO over total A1203 in the mixture lies within the range 0.35-1.00, preferably equal to 0.55, the weight ratio of -total SiO2 over total CaO is greater than 1.5, and the weight ratio of total SiO2 over total Awl2 03 is greater than 1.0. As fine clay-like material with a minor carbon content, there may be used, for example, fine, pores ferably ground, coal ash, preferably of a fineness which is defined by the sieve .res$due n a 90 micron screen being smaller than 15 per cent. It is particularly advantageous to use fly ash, because it requires no grinding since it already has a fineness appropriate for a homogeneous sintering of the mixture. It is a further advantage of fly ash that the original iron compounds in the coal are separated from the other oxides when passing through the coal flame and occur in the fly ash as small spheres consisting of elementary Fe or FeO. The product according to the invention maintains this state which contributes to giving the product its characteristic properties. As fine calcareous material there may be used chemically precipitated CaCO3, e.g. waste products containing such CaCO3, ground limestone, or dolormitic limestone,preferably having a total carbonate content of more than 96 per cent, and preferably having a fineness;defined by the sieve residue. on a 90-micr-on screen being less than 15 per cent. Furthermore, dust precipitated in electrostatic kiln gas filters from cement factories, having up to now presented serious disposal problems, can be used as fine calcareous material. Total CaO comprises all Ca compounds (including CaCO3) in the raw mixture calculated as CaO. The volumetric change by the sintering of the raw mixture has proved to be so small that the raw mixture can be pressed to green bricks prior to the sintering, which bricks undergo no shrinkage or expansion during preheating and sintering. The carbon content of the clay-like material with a minor carbon content is not high, i.e. less than 15 per cent. The resultant product will have a whitish to light tan colour. The advantageous colour is due to two circumstances which are both dependent upon the presence of CaCO3 in the raw mixture. Firstly, efficient removal of the fine coke particles in the raw mixture is obtained, as removal does not depend solely on diffusion of oxygen through the pellets or bricks, but is effected by formation of CO by reaction between the fine coke particles and the C02 liberated when calcining the omnipresent CaCO3. Secondly, the composition of the raw mixture ensures formation of a sintering product containing a large amount of small anorthite crystals (CaO.A1203.2SiO2) which reflect the light and hence cause the light colour. It has further been found that the second object of the invention can be achieved by a process for the manufacture of the new product, which is characterized by preparing a raw mixture comprising fine clay-like material with a minor carbon content, and fine calcareous material in proportions such that the weight ratio of total CaO over total A1203 in the mixture lies within the range 0.35-1.00, preferably equal to 0.55, the weight ratio of total SiO2 over total CaO is greater than 1.5, and the weight ratio of total SiO2 over total A1203 is greater than 1.0; preheating the mixture at substantially 8500C until the calcareous materials are calcined and the carbon oxidized, and sintering the preheated mixture in a neutral to slightly oxidizing atmosphere at a temperature within the range 1050-1250 C, preferably within the range l100-12000C. The only restriction on the components of the raw mixture is that the raw mixture has a composition falling within the above limits when the two components are mixed, i.e. that the clay-like material with a minor carbon content has a sufficiently high SiO2 content and a sufficiently low CaO content. In case of practically all relevant clay-like materials with a minor carbon content, these requirements will be met if limestone with a suitably high CaCO3 content is used as the calcareous material. If it is desired to manufacture aggregates in the form of pellets the raw mixture can, if desired after addition of 1-10, preferably 4-5 per cent water, be pressed into pellets or nodules prior to preheating and sintering which may be carried out in a grate heater. Preferably, a rotary kiln is used, which may be provided with a cyclone or grate preheater. It is also possible to omit pelletizing/nodulizing the raw mixture and introduce the raw mixture directly into the rotary kiln in which it will nodulize to clinker during the sintering step. If desired, the raw mixture may comprise small amounts, e.g. 5 per cent, diatomaceous earth or clay. By this is achieved improved stability of the green pellets or bricks. If desired, the sintering temperature may be lowered by adding small amounts, e.g. 1-5 per cent, fluxing agents to the raw mixture. As fluxing agents may be used alkali metal compounds, particularly carbonates and oxides of potassium and sodium, If it is desired to manufacture aggregates in the form of bricks, the raw mixture is pressed into bricks prior to the heat treatment which is then preferably carried out in a tunnel kiln. When manufacturing bricks it may be advantageous to operate with a raw mixture to which is added as much as 70 or even 80 per cent ground sintered aggregates according to the invention, i.e. by a production process analogeous to the one used when manufacturing firebricks. This product has the advantage as compared with e.g. chamotte, that it exhibits less reactivity towards CaO. The preheating is carried out within the temperature range 750-900 C preferably at 800-850 C. The period of preheating depends upon the shape and size of the pellet/brick. Pellets with dimensions not exceeding 2 cm are preheated in 30 to 60 minutes at the stated temperature. The pellets are gradually heated to the preheating temperatures in the course of an hour. The sintering which is carried out in an ordinary kiln gas atmosphere, i.e. with a content of 3-7 per cent 02, occurs within the temperature range 1050 to 1250 0C, preferably between 1100 and 12000C. The bodies are finished when the entire specimen has reached the required sintering temperature; for instance, a retention time of 6 minutes is sufficient for speciments of up to 2 g when fed directly to the burning zone. Longer sintering periods are not detrimental to the properties of the product. The cooling sequence plays no role with respect to properties of the product, and it is carried out in the most economic way. The aggregates according to the invention are suited for use as light filler in concrete, in cement bonded bricks, as filler in asphalt, as basic material when manufacturing ceramic stones, as artificial gravel, e.g. for tennis courts, and as absorption material, e.g. for oil. Further, in a ground.state, they. can be used as filler in cement. : The invention will now be illustrated by means -of a number of examples and comparative examples,in which per cent means per cent by weight. Example 1 (according to the invention). From afly ash, having the following metal oxide analysis: -sib2 49.0 per cent A1203 -. 19.8 - - Fe2O3 8.7 - CaO 5.5 - - MgO 2.8 - K2O 2.0 - Na2O 0.9 - ' Ignition Loss and a C content of 7.1 per cent by weight of the fly ash of the following fineness: micron +250 250-180 180-90 90-63 63-45 45-16 -16 per cent 1.8 2.7 9.2 5.5 5.8 22.2 52.8 was made a mixture with 13.6 per cent chemically precipitated CaCO3 (analytical reagent quality). The mixture was homogenized for 1 hour. The result was a raw mixture with total CaO/total A1203 = 0.66 total SiO2/total CaO = 3.74 total SiO2/total A1203 = 2°47 4 per cent water having been added, the mixture was pressed into tablets of approx. 1 cm3, which after 1 hour of preheating at 850 0C were sintered for 15 min. at 11500C. The resulting product was light cream in colour. The product compression strength measured on small specimens (cylindrical cross-sectional area of 1 cm2) was 200 kp/cmê. Microscopic examination of porosity showed Volume per cent closed pores 5.5 per cent Volume per cent open pores 34.3 - Volume per cent total porosity 39.8 per cent Weight per unit of volume determined by measurement and weighing 1.2 g/cm3. Shrinkage due to heat treatment: 1.8 per cent. Example 2 (according to the invention). Briquet ash having the following metal oxide analysis: SiO2 53.5 per cent A12 3 32.1 - - Fe2O3 4.1 - - CaO 0.9 - MgO 4.7 ¯ ¯ Na20 0.6 - ignition loss 11.2 - and a C content of 8.7 per cent by weight of ash was mixed with 29 per cent limestone having a total carbonate content of 96 per cent. The mixture was interground to a fineness of 11 per cent sieve residue on a 90 micron screen. The resulting raw mixture composition was total CaO/total A1203 = 0.52 total 5L02/total CaO = 3.23 total Sing /total A1203 = 1.67 32 23 Tablets of 1.5 cm were pressed after addition of 4 per cent water; the tablets were preheated at 850 0C for 45 min. and then sintered at 1150 0C for 15 min. The resultant product was cream-coloured and had a compression strength of 75 kp/cm2 and a weight per unit of volume of 1.15 g/cm3. Shrinkage due to heat treatment was -0.5 per cent (i.e. slight expansion). Example 3 (according to the invention). From a fly ash having the following metal oxide analysis: SiO2 46.4 per cent A1203 22.0 - - Fe203 9.6 - - CaO 7.6 - MgO 5.4 - 2.3 ¯ ¯ Na20 0.6 ignition loss 4.7 - and a C content of 4.3 per cent by weight of ash having the following fineness Micron +250 250-180 180-90 90-63 63-45 45-16 -16 per cent 4.2 4.7 14.8 6.0 6.1 26.6 36.7 was made a mixture with 11.3 per cent chemically precipitated CACAO3 (analytical reagent quality). The mixture was homogenized for 1 hour. The resultant raw mixture composition was total CaO/total A03 = 0.63 total SiO2/total CaO = 3.34 total SiO2/total A1203 = 2.10 The mixture was pressed into tablets of approx. 1 cm3 after addition of 4 per cent water; after 1 hourts preheating at 8500 the tablets were sintered for 6 min. at 1050, 1150, 12500C respectively. For purposes of comparison sintering at 13000C was also carried out. The product properties appear from the following table 1: TABLE 1 Sintering Open Closed Compress Total Shrinkage Colour tempera- pores pores strength inner ture volume volume surface C per per kp/cm2 mm/mm per cent cent cent 1050 22.0 3.7 95 210 1.6 light tan 1150 25.4 4.1 190 166 1.6 tan 1250 41.0 3.0 180 26 2.4 dark tan Microscopic examinations of the products sintered according to the invention at 1050, 1150 and 125OOC show a structure similar to the structure of expanded clay with small balls of Fe and iron oxides. X-ray diffraction was used for examination of the mineral composition of the specimens: Product sintered at 10500 C: Quartz, Anorthite, Gehlenite, MulliteJHaematite, Anhydrite. Product sintered at 11500 C: Anorthite, Haematite, Quartz. Product sintered at 12500 C: Anorthite, Quartz. Quartz and Haematite derive from unmolten particles in the fly ash. The specimens melt at 13000C, Example 4 (according to the invention). Coal shale having the following metal oxide analysis: Si02 41.9 per cent A1203 21.8 - Fe203 5.7 - CaO 1.5 - MgO 1.2 - K20 3.4 - - TiO2 0.8 - ignition loss 21.5 - and a C content of 12.0 per cent by weight of shale was interground with 23.3 per cent chemically precipitated CaCO3 (analytic reagent quality) to a 10 per cent sieve residue on a 90 micron screen. The resultant raw mixture composition was given by total CaO/total A403 = 0.66 total Si02/total CaQ = 2.91 total SiO2/total Al2% = 1.92 Tablets of approx. 1 cm3 were pressed after addition of 4 per cent water; the tablets were preheated at 850 0C for 1 hour and then sintered for 15 minx at the temperature below. The results of measurement of compression strength and carbon residue appear from Table 2. TABLE 2. Sintering Compress.. Residue Carbon temperature Strength content c kp/cmê per cent 1100 230 0.004 1150 360 0.09 1200 310 0.08 The sintered products were light coloured and completely resembled the sintered products based on fly ash. Microscopic examinations show a finished sintered structure which very much resembles the structure described for sintered fly ash. The product sintered at 11000C has a fine net-like structure and contains spherical iron oxide particles. With increasing temperature, the network gets coarser and at 1200 C resulting in closed pores. The iron remains as spherical iron oxide particles. 3 The unit weight was determined at 1.3 g/cm and no sample shrinkage was noted (i.e. less than 1 per cent). Example 5 (according to the invention). Chemically precipitated CaCO3 (analytic reagent quality) in amounts as stated below was added to fly ash with properties and composition as stated in example 3. 4 per cent water was added to the mixture, and tablets of approx. 1 cm3 were pressed. The tablets were preheated at 8500C for 1 hour, and subsequently sintered for 15 minutes at 12000C. The compression strength of the tablets was tested and the weight per unit of volume determined by measuring and weighing. The mineral composition was examined by X-ray diffraction. The test results appear from table 3 below. TABLE 3. Chemical composition of raw mixture. per cent w.pr. compression Si02/ CaO/ Six2/ CaCO3 u.v strength A1203 A1203 CaO addition g/cm3 kp/cm2 4.3 1.46 180 2.1 0.46 4.58 5.9 1.47 210 2.1 0.51 4.26 6.9 1.45 182 2.1 0.54 3.93 8.7 1.45 150 2.1 0.59 3.59 11.1 1.42 145 2.1 0.66 3.19 13.8 1.42 150 2.1 0.75 2.82 17.4 1.40 130 2.1 0.88 2.39 The only minerals found in the specimens were Anorthite, Quartz and Haematite. Quartz and Haematite derive from unmolten particles in fly ash. Shrinkage in respect of all the specimens was 1.8 per cent. Example 6 (according to the invention). From a fly ash having the following metal oxide analysis: SiO2 45.2 per cent A1203 26.6 - Fe203 6.8 - CaO 7.1 - MgO 3.0 - - 1.7 - 1 Na20 0.5 - - an ignition loss of 6.0 per cent and a C content of 4.6 per cent was made a mixture with varying amounts of dust precipitated in an electrostatic filter filtering exhaust gas from a cement production plant. The dust had the following metal oxide analysis: SiO2 15.5 per cent A1203 4.4 - Fe203 1.8 - CaO 41.0 - MgO 1.1 - - K20 4.9 - Na20 1.0 an ignition loss of 23.5 per cent,a Cl content of 0.7 per cent and a S content of 2.0 per cent. The mixture had a fineness of 4 per cent sieve residue on a 90 micron screen. Tablets of 1 cm3 were pressed after addition of 4 per cent water; the tablets were preheated at 850 0C for 45 min. and then sintered at various temperatures for 15 min. The chemical composition and the product properties appear from the following table 4: TABLE 4 (part 1). Chemical Composition. per cent filter CaO/A1203 Si02/Ca0 SiO2/Al2O3 dust addition 13.6 0.47 3.7 1.74 16.3 0.50 3.5 1.75 19.0 0.54 3.2 1.76 22 0.58 3.0 1.76 25 0.63 2.8 1.77 TABLE 4 (part 2). Per cent Weight per Compress. filter Sinter- unit vol. Shrinkage Strength SO3 addition temp. C g/m per cent kp/cm2 per cent 13.6 1150 1.39 0 210 0.63 13.6 1175 1.40 0 240 0.25 13.6 1200 1.46 1.8 360 0.05 16.3 1150 1.39 o 190 0.60 16.3 1175 1.39 0 270 0.30 16.3 1200 1+47 1.8 355 0.05 19.0 1150 1.38 0 185 0.70 19.0 1175 1.41 0.9 215 0.28 19.0 1200 1.44 1.8 370 0.03 22.0 1150 1.37 0 170 0.75 22.0 1175 1.41 0.9 205 0.22 22.0 1200 1.48 2.7 320 0.05 25.0 1150 1.38 0 225 0.82 25.0 1175 1.41 0.9 280 0.33 25.0 1200 1.50 2.7 485 0.05 The impurity content of alkali metal oxides and sulphur is characteristic of the filter dust used. These impurities however do not influence the product properties in a negative way as they will be insolubly integrated in the product. This is because preheating at 850 0C is a reducing burning reaction . The alkali sulphates are decomposed and the alkalis are integrated into low -melt silicate glasses. As the zone for calcining and burning of carbon advances towards the core of the tablets, a diffusion of S03 and CO/CO2 takes place in the opposite direction. Part of the S03 will be absorbed as anhydrite which starts decomposing from 1200 0C and upwards. The alkalis remain as silicates, Na is built into the anorthite and K is integrated in aluminasilicate; i.e., neither S03 nor the alkalis are water-soluble any longer.- Example A (comparative example). 51 per cent clay of the following metal oxide analysis was added to the briquet ash described in example 2: SiO2 49.5 per cent Al2O3 17.7 - Fe203 9.1 - - CaO 4.4 - - MgO 2.7 - k o 2.8 - - Na20 0.7 - TiO2 0.9 - ignition loss 10.9- The mixture was ground and homogenized as described in example 2. The resulting mixture composition was total Ca0/total A1203 = 0.10 total SiO2/total CaO = 19.8 total SiO2/total A1203 = 2.06 The mixture was pressed into tablets which were heat treated as described in example 2. The resultant product was dark brown, expanded and of very uneven shape. During the sintering the product exhibited a strongly adhesive surface which makes it difficult to handle. Example B (comparative example). CaCO3 in the amounts specified below was added to the fly ash described in example 3. CaCO5 addition as chemically preciptated analytic reagent quality. 4 per cent water was added to the mixture and tablets of approx. 1 cm3 were pressed. The tablets were preheated at 850 0C for 1 hour and subsequently sintered at 12000C for 15 minutes. The compression strength of the tablets was tested and the weight per unit of volume determined by measuring and weighing. The mineral content was determined by means of X-ray diffraction. The test results appear from table 5 below. TABLE 5 (part 1) Chemical Composition per cent CaCO3 CaO/A1203 SiO2/CaO SiO2/A1203 3.0 0.43 4.9 2.1 11.3 0.67 3.2 2.1 40 2.05 1.03 2.1 70 2.99 0.33 2.1 TABLE 5 (part 2) per cent w.per per cent compression colour mineral CaCO3 u.v 3 shrinkage strengfih content g/cm kp/cmê 3.0 1.45 1.7 205 Whittish Anorthite Quartz Haematite 11.3 1.45 1.7 150 - Anorthite Quartz Haematite 40.0 molten - - Black Gehlenite Anorthite 70 1.34 5.6 90 Mottled Gehlenite Quartz derives from unreacted coarse particles in fly ash. The first two of the examples in Table 5, i.e. those with 3 and 11.3 per cent CaCO3 were in accordance with the invention.

Claims: CLAIMS 1. A porous sintered aggregate characterized in consisting of a sintered product of a raw mixture comprising fine clay-like material with a minor carbon content (as hereinbefore defined), and fine calcareous material in proportions such that the weight ratio of total CaO over total A1203 in the mixture lies within the range 0.35-1.00, preferably equal to 0.55, the weight ratio of total SiO2 over total CaO is greater than 1.5, and the weight ratio of total SiO2 over total A403 is greater than 1.0. 2. A porous sintered aggregate according to claim 1, characterized in that the fine clay-like material with a minor carbon content is fine coal ash. 3. A porous sintered aggregate according to claim 1, characterized in that the fine clay-like material with a minor carbon content is fly ash. 4. A porous sintered aggregate according to any of the preceding claims, characterized in that the -fine calareous material is chemically precipitated CaCO3. 5. A porous sintered aggregate formed from a raw mixture comprising up to 80 weight per cent ground sintered aggregate according to any of claims 1 to 4. 6. A process for the manufacture of a porous sintered aggregate according to any of the preceding claims, characterized by preparing a raw mixture comprising fine clay-like material with a minor carbon content (as hereinbefore defined), and fine calcareous material in proportions such that the weight ratio of total CaO over total A1203 in the mixture lies. within the range 0.35 -1.00, preferably equal to 0.55, the weight ratio of total SiO2 over total CaO is greater than 1.5, and the weight ratio of total SiO2 over total Ak03 is greater than 1.0; preheating the mixture at substantially 850 0C until the calcareous materials are calcined and the carbon oxidized, and sintering the preheated mixture in a neutral to slightly oxidizing atmosphere at a temperature within the range 1050-1250 C, preferably within the range 1100 - 1200 0C. 7. A process according to claim 6, characterized in that the raw mixture is nodulized or pelletized before preheating and sintering. 8. A process according to claim 6, characterized in that the raw mixture is pressed to bricks before preheating and sintering. 9. A process according to any of claims 6 to 8, characterized in that the preheating and sintering steps are carried out in a rotary kiln. 10. A process according to any of claims 6 to 8, characterized in that the preheating and sintering steps are carried out in a grate heater.

Inventors: FUNDAL, ERLING
Applicants: F.L. SMIDTH & CO. A/S
Application year: 1980
Main classification: C04B31
IPC codes: C04B38, C04B18, C09K3, C04B33, C04B35