HARDENING GRANULATED MATERIAL AT ELEVATED TEMPERATURES

Abstract: A mixture of inorganic binder, water and fly ash is granulated; the formed granules (7) are deposited upon a continuously supplied layer of fly ash embedding material (9) and said fly ash layer together with granules is fed into a hardening reactor (14). The granules are hardened by injecting steam (16) into the reactor so that the temperature of the granules is raised and maintained at a temperature comprised between 50 and 100°C. After curing the hardened granules are separated from the embedding material which is used for forming new granules. The embedding material has a water absorbing capacity of 15-25% and the pH of the moist embedding material is above 4.

Description: BACKGROUND OF THE INVENTION This invention relates to an improved method of producing hardened granulated material, obtained by mixing at least water, a binder and fly ash and forming granules by any of the usual granulation techniques, and curing at elevated temperatures. Many patents and publications describe the production of hardened material on the basis of fly ash and a binder. As a binder calcium hydroxide or a product that will produce calcium hydroxide in an aqueous system may be used ( e.g. US Patents 4419312:4399176:4490178 ) A particular embodiment of this reaction is disclosed in Dutch Patent Application 8200193, wherein uncured granules and hot fly ash are mixed in a mixer which is also fed with steam. The hot fly ash comes directly from a coal fired plant. The mixture from the mixer is fed to a curing or hardening silo in which a temperature of just below 100 °C is reached. The partially cured granules and fly ash leaving the silo are separated from each other and the fly ash is recycled for use as starting material for producing new granules. The partially cured granules are passed into an intermediate storage silo where the partially cured granules are further cured by means of steam. Another embodiment is disclosed in US Patent 4,490,178. Granules of better compressive strength are obtained by subsequently hardening the granules at temperatures between 85 and 212 °F (30 and 100 °C). In said method the granules are hardened under moist conditions by embedding said granules in a mixture of sand, water and calcium oxide. An advantage of this process is that the heat generated by the slaking of calciumoxide is used in direct contact with the granules for curing or hardening of the granules at elevated temperatures. A mixture of sand and calcium oxide as embedding material provides very high strength granules but is not always economically practical or required. For example, lower strength granules may satisfy some cement block manufacturing requirements. However, the use of spent embedding material is necessary for good process economics, but lower strength granules do not require the use of sand in the granule mixture, and thus spent sand embedding material is not recycled back for granule mixing. It would be economically advantageous to therefore use an embedding material which could be recycled back into the mixture for forming granules. Also, although all of the required heat for the hardening of the granules can be solely supplied by steam without embedding material, our experiments have shown that the sole use of steam as a heating source for curing the granules without embedding material has various disadvantages. (i) Handling and storage of non hardened or insufficiently hardened granules is difficult and causes degradation of the granules. (ii) The condensation of steam involves formation of water causing the granules to stick together which will hinder the separation of the hardened granules at the end of the hardening process (iii) Direct contact of the granules with steam can cause cracks in the granules which will lower their final strength considerably. Thus the use of embedding material is deemed to be advantageous. However, such embedding material must meet specific criteria for curing. SUMMARY OF THE INVENTION A primary object of the present invention is to provide an improved method to harden granulated material at temperatures between 85 and 212 °F (30 and 100 °C) This is attained in accordance with the present process of manufacturing hardened granules produced by mixing at least fly ash obtained from combustion of a material selected from a coal product and lignite, an inorganic binder and water, forming granules of said mixture, hardening said granules in an embedding material at a temperature between 30 and 100 °C by applying heat and separating the hardened granules from the embedding material, optionally said embedding material being used for forming new granules, comprising at least the step of embedding hardenable granules in a hardening reactor in a fine grained material being free from a compound generating heat by contacting water and supplying heat to the mixture of granules and embedding material in said hardening reactor, characterized in that the granules are supplied to the hardening reactor deposited upon a layer of embedding material, in the hardening reactor the granules are embedded in a fine grained material still having a water absorption capacity of at least 10%, the ratio volume between the quantity of embedding material and granules being comprised between 0.2 and 1, steam being introduced into the hardening reactor to raise and maintain the temperature at a value comprised between 30 and 100 °C, said steam condensing on said embedding material, the pH of the embedding material in the moist state being above 4. It should be noted that in the process of the invention heat must be supplied by introducing steam into the mixture of granules and embedding material, contacting the granules with hot fly ash of a temperature sufficient for raising the temperature to a value comprised between 85 °F and 212 °F (30 °C and 100 °C ) will result in granules not meeting the strength properties stipulated for practical use of such hardened granules. If the ratio between the quantity of embedding material and granules is smaller than 0.2 no sufficient embedding is possible resulting in granules of insufficient strength. At a ratio higher than 1.0 the process will produce a stream of embedding material which cannot be used as a whole for forming granules. This means waste of energy as the embedding material which cannot be used directly for forming granules will cool to ambient temperature during storage and cannot be used for forming granules of a somewhat higher temperature as in the process of the invention. The expression embedding material having a water absorption capacity of at least 10% means an embedding material still having this absorption capacity at the time of use of the material. E.g. embedding material having in a dry state an absorption capacity of 25% may be used in a moist state provided that when used the absorption capacity is still at least 10%. In an advantageous embodiment of the invention a fly ash material having a water adsorption capacity of 15-25% is distributed on a conveying system between a granulating station and a hardening reactor whereby the granules drop on to a layer of fly ash. Mixing of fly ash and granules is completed when the granules with fly ash enter the hardening reactor. Because of the presence of fly ash the uncured granules can be handled without breaking and can then be piled up in the curing reactor without deformation. Steam for increasing the temperature as required for hardening the granules particularly at a temperature comprised between 85 and 212 °F (30 °C and 100 °C) that condenses will then deposit on the embedding material which has a higher specific surface area than the granules. Heat released by condensation of steam will be transferred to the granules by conduction so that virtually no steam condensation will occur at the surface of the granules. After completion of the hardening, the granules are separated from the embedding fly ash and this fly ash is returned as a feedstock to a mixing station where fresh fly ash, lime and water are added. In the process the use of lime as a binder is preferred, said lime can be added either as calcium oxide or as calcium hydroxide. Other suitable binders could be latent hydraulic components which would produce calcium hydroxide if mixed with water. Very advantageously is the process of the invention wherein granules deposited upon a layer of embedding material are supplied to the hardening reactor. Advantageously the mixture of embedding material and granules is held free from steam injection before entering the hardening reactor. Preferably the weight ratio of embedding material to granules is from about 0.3 to 0.45, the volume ratio is about 0.3 to 0.5, more preferably about 0.4. The embedding material becoming moist by condensation of steam has a pH above 4 in the moist state. DESCRIPTION OF THE DRAWINGS The figure is a block flow diagram illustrating an embodiment of the invention The figure shows an installation for executing the process of the invention. This installation comprises a mixer for mixing lime as binder from a binder supply container 2, water from a water supply container 3, fly ash from a fly ash supply container 1 and optionally other components from supply container 4. The mixture is conveyed from mixer 5 to a granulator 6, the granules 7 from said granulator are deposited upon an endless belt 8 bearing a layer of an embedding material in the form of fly ash 9 fed by line 10 from embedding material container 11. Preferably no steam is introduced in said mixture as carried by the endless belt 8 though not shown steam from supply 16 may be supplied to the mixture before entering the reactor 14. The mixture of embedding material 9 and granules 8 is passed into the hardening reactor 14 being in open connection with the ambient. By means of steam line 15 steam from steam supply 16 is introduced into the mixture of granules and embedding material in the reactor 14. After hardening the granules are separated from the embedding material by means of screen 17, the granules being conveyed by belt 18 to a storage, the embedding material being fly ash being conveyed to mixer 5 through conveyor 19. The embodiment of the invention offers the possibility to produce hardened granules according to the figure without producing any waste streams whatsoever. If the embedding material i.e. fly ash is separated from the granules after hardening and it is used directly as a feedstock for the process, the heat contained in this fly ash is also recuperated. The hardening is advantageously effected at a temperature above 85 °F (30 °C) and more preferably above 160 °F (71 °C). The ratio of granules to embedding material is such that for 100 lbs (45,4 kg) of granules 20-100 lbs (9,1-45,4 kg) of fly ash are used as embedding material. More preferably 30-45 lbs (13,6-20,4 kg) of fly ash are used for 100 lbs (45,4 kg) of granules. The volume ratio is preferably 0.3 tot 0.5, more preferably 0.4. These other components can be residues of combustion processes, such as bottom ash, ash from fluidized bed boilers (atmospheric, pressurized or circulating), ash from municipal waste burners, ash from Refuse Derived Fuel burners, ash from the combustion of sub-bituminous coals, ash from lignite, ash from peat, sand, gypsum, calcium sulfite or various inorganic waste products from industrial processes. The embedding material may consist of warm or cold dry fly ash, but wet fly ash from temporary disposal sites can also be used. Other embedding materials can be used, provided that such materials have sufficient adsorption capacity to absorb all the condensing steam and that such material will not interfere with the hardening reactions. In all instances the embedding material should be fine grained in order to be able to cover the soft granules and to fill voids between the granules. Fine grained means a particle size distribution with more than 80% passing a 1 mm screen and more than 50%, preferably more than 80%, passing a 250 mm screen. EXAMPLE I A quantity of 110 lbs (49,9 kg) of fly ash from a coal-fired power station (water absorption capacity 20%) are mixed with 6.2 lbs (2,8 kg) of powdered quick lime as inorganic binder and 26,9 lbs (12,2 kg) of water. After mixing and granulating, the granules are deposited upon a continuously supplied layer of dry fly ash such that in the hardening reactor a 10 lbs (4,5 kg) sample of granules is mixed with 1.8 kg (4 lbs) of dry fly ash 90% of said fly ash having a particle size of smaller than 100 µm. The granules embedded in the free fly ash are heated at 195°F 90 °C in a moist atmosphere by introducing steam of 265°F 129 °C, the pH of the moist fly ash formed by condensation of steam has a pH above 4. Another 10 lb (4,5 kg) sample of granules is heated in the same way without embedding material. After 16 hours the compressive strength of the granules is determined. From each 10 lbs (4,5 kg) batch, 20 granules ranging in sizes from 7,9-9,9 mm (0.31-0.39 inch) diameter are selected and compressed between parallel metal plates. Granules embedded in fly ash show an average compressive strength of 55 lbs (25 kg), while granules without embedding material have an average compressive strength of 4 lbs (1,8 kg). EXAMPLE II A quantity of 110 lbs (49.9 kg) of fly ash from a coal fired power station are mixed with 7.5 lbs (3.4 kg) of calcium hydroxide and 20 lbs (9 kg) of water. The mixture is granulated. A 10 lbs (4,5 kg) batch of granules is embedded in 4 lbs (1,8 kg) of dry fly ash. A 10 lbs (4,5 kg) batch of granules is embedded in 4 lbs (1,8 kg) of moist fly ash containing 6% water. A 10 lbs (4,5 kg) batch of granules is not embedded. After curing at 195°F (91 °C) for 16 hours in a moist atmosphere by introducing steam of 265°F (129 °C) in the mixture of fly ash and granules, the compressive strength for the granules is respectively: 27,7kg (61 lbs) for granules embedded in dry fly ash 22,7kg (50 lbs) for granules embedded in moist fly ash 1,0kg (20 lbs) for granules without embedding. EXAMPLE III A 15,000 lbs (6810 kg) batch of granules is prepared by mixing batches of 175 lbs (79,5 kg) of fly ash, 3.0 lbs (1,4 kg) of calciumsulphate dihydrate, 7.3 lbs (3,3 kg) of lime inorganic binder and 44 lbs (20,0 kg ) of water. The granulated mixture is mixed with moist fly ash containing 5% water. The embedded granules are introduced in a hardening reactor. By introducing steam of 275°F (135°C) the temperature is raised to 204°F (113 °C) After 18 hours at that temperature the reactor is discharged. Granules are separated from the embedding material by screening. The average compressive strength for the 0.31-0.39 inch (7.9 mm-9.9 mm) diameter granules is 23,6 kg (58 lbs). The separated embedding material is used for forming granules as in example I. The compressive strength of granules obtained from fly as that has been used as embedding material does not differ from the values found in example I. EXAMPLE IV Example III is repeated but instead of 3.0 lbs (1,4 kg) of calciumsulphate dihydrate, 3.0 lbs (1,4 kg) of a mixture of calciumsulphate dihydrate and calcium sulphite as obtained from flue gas desulfuration or 3.0 lbs (1,4 kg) of fly ash from the combustion of sub bituminous coals or 3.0 lbs (1,4 kg) of ash from municipal waste burner is added. Similar results as in Example III are obtained. In this example and the preceding examples the bulk density of the fly ash and of the granules is about 1.0 EXAMPLE V Example II is repeated with 100 lbs (45 kg) of bottom ash having a bulk density of 1.6, the bulk density of the instead of 110 lbs (50 kg) of fly ash granules is 1.1. The volume ratio between the granules and embedding material is 0.4 as in all the preceding examples. However, the weight ratio is in this example about 0.6.

Claims: A process of manufacturing hardened granules produced by mixing at least fly ash obtained from combustion of a material selected from a coal product and lignite, an inorganic binder and water, forming granules of said mixture, hardening said granules in an embedding material at a temperature between 30 and 100 °C by applying heat and separating the hardened granules from the embedding material, optionally said embedding material being used for forming new granules, comprising at least the step of embedding hardenable granules in a hardening reactor in a fine grained material being free from a compound generating heat by contacting water and supplying heat to the mixture of granules and embedding material in said hardening reactor, characterized in that the granules are supplied to the hardening reactor deposited upon a layer of embedding material, in the hardening reactor the granules are embedded in a fine grained material still having a water absorption capacity of at least 10%, the ratio volume between the quantity of embedding material and granules being comprised between 0.2 and 1, steam being introduced into the hardening reactor to raise and maintain the temperature at a value comprised between 30 and 100 °C, said steam condensing on said embedding material, the pH of the embedding material in the moist state being above 4. A process according to claim 1, characterized in that the mixture of fine grained embedding material and hardenable granules being fed into the hardening reactor is held free from steam injection before entering the hardening reactor. A process according to any one of the preceding claims, characterized in that the granules are cured at a temperature between 70 and 100 °C. A process according to claim 1, characterized in that the ratio between the volume of embedding material and said granules is 0.3 to 0.5, preferably 0.4. A process of any one of the preceding claims, characterized in that fly ash, fly ash at an elevated temperature serving as an additional heat source for the hardening reaction and/or moist fly ash still having a water absorption capacity of at least 10% is fed as embedding material into the hardening reactor. A process according to claim 1, characterized in that embedding material having a water absorption capacity of 15-25% is introduced as embedding material into the hardening reactor. A process according to any one of the preceding claims, characterized in that, the binder is selected from the group of calcium oxide plus water, calcium hydroxide, milk of lime, and cement. A process according to any one of the preceding claims, characterized in that the granules are produced from a composition chosen from a mixture of at least lime, fly ash, water and other combustion products; or a mixture of lime, fly ash, calcium sulphate dihydrate and water.

Inventors: RADEMAKER, PIETER DIRK, SPANJER, JACOBUS JOHANNES
Applicants: AARDELITE-HOLDING B.V.
Application year: 1988
Main classification: C04B18
IPC codes: C04B18, C04B28, C04B7, C04B40