PROCESS FOR PRODUCING HEAT-ACCUMULATOR CERAMIC BLOCKS FROM RED CLAYS OF THE BAYER PROCESS

Abstract: Red mud is fired, ground and subsequently shaped and refired in high temperature furnaces. This mud appears when the bauxite undergoes the Bayer process. These high density bricks shew improved mechanical properties and thermal capacity and are chemically inert, being suitable for electric off peak heating storage. The goal of this procedure is to give an answer to the aluminia industry environmental and waste disposal problem (red mud), giving rise to bricks of similar characteristics of other commercial ETS bricks.

Description: Manufacturing procedure to obtain high density ceramic bricks for electric thermal storage heaters (ETS) through red muds processing. A high concentration soda (NaOH) solution added when alumina undergoes the Bayer process, in high temperature and pressure conditions. This process originates huge amounts of metallic hydroxides and other insoluble minerals. These products are known as red mud and their average composition is shown in tables 1 and 2. Millions of Tons of these residues are produced every year. This environmental hazard makes them desirable to be used in whichever industrial process. Mineralogical composition of red muds Hematite 30-35% Rutile 20% Aluminium Oxidroxides 30-40% On the other hand, raw materials used in commercial ETS bricks are Fe oxides richclays, prereduced Fe oxides and magnesites. Chemical analysis of red muds SiO2=6,1% TiO2=22,6% Na2O=4,70% Al2O3=20,1% CaO=4,8% K2O=0,10% Fe2O3=31,8% MgO=0,2% PO=9,50% Both raw materials and fabrication procedure determine the properties of the ETS bricks (Table 3). High thermal capacity, good thermal shock behaviour, high density and reasonable price are desired. Properties of other commonly used ETS bricks Composition Density gr/cc Porosity Specific Heat Dense Clay 2.0 20% 0.19Kcal/Kg.°C Magnesite 2.8 22% 0.25Kcal/Kg.°C Ferric Oxide I 3.8 15-25% 0.22Kcal/Kg.°C Ferric Oxide II 3.7 20% 0.22Kcal/Kg.°C EP 0 318 305 discloses a process for producing a refractory material, in which a binding agent and/or an additive (silica, alumina, sodium salts, cryolite, aluminate, aluminium) is added to red mud. The obtained material is useful as corrosion-resistant refractories in refractory - linings and similar applications. This innovative method allows to obtain materials with appropriate characteristics (see table 3). This process fulfills two purposes a) use and elimination of the red mud, for an industrial application. b) substitution of more expensive raw materials which are applicable to other cases. This procedure can be separated in three stages : 1- Red mud with low residual humidity is extruded or pelletized to obtain particles with adequate size distribution. Then they are dried and prefired at high temperature (900°C-1300°C). This firing causes chemical and structural transformations that leads mechanical and chemical stability in the grog. The main phases present in the fired material are hematite, aluminum titanate, iron and a variable composition glass. If firing is carried out in reducing conditions, changing amounts of magnetite are formed. 2- The grog and the granules are mixed with little amounts of binder (, red mud, ) this mixture can be shaped by pressing, extrusion, or any other process. Some water can be added to obtain the required plasticity. 3- The obtained blocks are dried and fired at 900°C to 1300°C Example 1 Red mud from Alumina Aluminio S.A. (San Ciprián, Lugo) with 5% humidity is shaped into particles of sizes between 0 - 2 mm., dried at 120°C and then fired at 1100°C far an hour to obtain the grog. Then this grog is mixed with 10% of raw red muds, to achieve a homogeneous bulk. This bulk is pressed at 550Kg/cm2 in a steel matrix of 200x100x 50mm., and then released from the mold. The blocks are dried and fired at 1100°C for four hours. The features of the resulting material are shown in table 4. Properties of red mud bricks. Density 3.4gr/cc Porosity 18% Compression strength 350Kg/cm2 Thermal shock behaviour Good Specific Heat 0.22 Kcal/Kg.°C

Claims: Process for obtaining high density ceramic bricks for electric thermal storage heaters from red muds of the Bayer process, characterised in that it comprises the stages of precalcinating at 900 to 1300° C and granulating said red muds; mixing said calcinated muds with raw red muds and water as sole plasticizer or additive; shaping the mixture into bricks; and firing the obtained bricks at 900°C to 1300°C. Process according to claim 1, characterised in that the step of granulating is carried out by pelletization, extrusion, grinding, or any other industrial process.

Inventors: CONDE-PUMPIDO TOURON ROGELIO, DE AZA PENDAS SALVADOR, GUITIAN RIVERA FRANCISCO, MOYA CORRAL JOSE SERAFIN, CONDE-PUMPIDO TOURON, ROGELIO, DE AZA PENDAS, SALVADOR, GUITIAN RIVERA, FRANCISCO, MOYA CORRAL, JOSE SERAFIN, CONDE-PUMPIDO TOURON, Rogelio Universidade de, de AZA PENDAS, Salvador Universidade de Santiago, GUITIAN RIVERA, Francisco Universidade de, MOYA CORRAL, José Serafin Universidade de
Applicants: UNIV SANTIAGO COMPOSTELA, UNIVERSIDADE DE SANTIAGO DE COMPOSTELA
Application year: 1996
Main classification: C04B35
IPC codes: C04B35, C04B33