NL1039305C2 - Method for processing red mud into shaped products. - Google Patents

Description

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Title: Method for Processing Red Mud into Shaped Products

The invention relates to a method for production of shaped products such as, but not limited to bricks, building blocks (solid and hollow), pillars, panels, beams, boulders for coastal protection (solid and hollow) etc. by using an inventive combination of mechanical processes and chemical reactions.

5 It is known that shaped products can be produced by mixing gravel and sand together with calcinated materials, fired at temperatures above 800 °C, such as cement, lime or other materials. The chemical reactions for achieving cementation are based on hydraulic reactions which occur by adding water to the mixture.

EP 0636592 A2 describes a process for processing sulphur and lime containing residues into 10 blocks, however this method demands that first the residue should be processed into granules and the granules should be embedded in ash material and hardened before it is possible to process the granules into blocks. Due to the different chemical composition, high fineness and high moisture content of red mud, this method cannot process the sodium containing red mud.

CN 101020603A and CN 102276219A are referring to processes for production of bricks by 15 mixing red mud with fly ash and lime and curing by using steam. These processes are not using granular materials and are based on using lime as binding agent. The lack of granular materials and the use of lime make these processes technically and commercially impossible. No large scale utilisations of these processes have been reported so far.

The current invention differentiates from the above referred invention because there is no 20 (additional) binding agent, such as lime required, rice husk ash is used instead of fly ash, granular material with an adapted grain size ratio is used and heating is based on hot air drying instead of steam curing. Furthermore the innovative elements of the current invention are that a solution has been developed for processing aggressive sodium containing materials, such as red mud, and that a large variety of shaped materials with much larger volumes, than the referred granules, can be 25 produced and that there is no need to granulate and harden the granules and also there is no need for embedding the product in ash to facilitate curing of the green material. Another innovative element of the current invention is the inventive way of creating a cementitious mixture, without the use of calcinated materials, such as un-slaked lime, cement, meta-kaolin etc. and thus avoiding the need for high amounts of energy, while at the same time a solution is provided for large scale processing of red 30 mud into useful products for building, construction and civil works.

Large scale processing of red mud is a challenge for the global society since more than six decades. We all need aluminium and the resultant is that annually million tons of red mud is generated and stored in large red mud pounds. The current invention provides a method for processing red mud and ash, which increasingly are representing an environmental threat. High level 1 0 3 9 3 0 5 -2- of caustic soda makes red mud an extremely aggressive material. Therefore, bulk processing of red mud is not possible by means of existing technologies. Other obstacles in developing solutions for processing red mud so far are the rheological and tixotrophic properties. Red mud is produced as a slurry with a very high fineness which makes it difficult to dry the material and this has been another 5 major hurdle in the development of industrial applications for red mud. The current invention provides the opportunity to convert red mud into shaped products that processes well, are stable over a longer time and can be processed for a large variety of industrial applications such as but not limited to buildings, houses, coastal protection, civil works etc..

Another inventive character of the process is the fact that there is no need for calcinations or 10 baking (as is the case when manufacturing cement or calcinated lime or producing ceramic product). So no need for calcinated materials and no need of calcinations during the hardening process.

According to the invention in a first aspect there is provided a method for bulk processing of red mud, which is so far not possible because of the high caustic soda content of this material, the high fineness and the tixotrophic properties of this slurryfied material, into a durable stone-like 15 material that can be used for a variety of purposes and applications. The physical and chemical properties of red mud is presented in table 1.

Table 1: Chemical and physical properties of red mud

Si02 14.30 % Moisture Content 34.2 % A1203 22.72 % Bulk density 922 kg/m3

Ti02 2.86 % Mass density 2376 kg/m3

Fe203 34.17% Fines >45 mm 12,5%

MnO 0.02 % Blaine area 4250 cm2/g

CaO 6.18% plï 13 K^Ö 0.01 % Lol @ 1000 °C 14.17 %m/m

MgO < 0.3 %

Na20 2.07 % P205 0.08 % ”s 0.10% A second aspect is the novel idea to produce shaped products using an admixture of sodium-containing residue and silicium-containing residue as a reactant; two waste materials are combined to 20 create a primary product.

The amount of said sodium containing material, such as red mud is at least 40% by weight of the total mixture whereas the amount of silicium material, such as ash, is not more than 50%. The ash originates from combustion of crop materials, such as sugar cane plants, rice husk, grass etc.. In accordance with the invention the combustion of these organic materials should take place under 25 controlled conditions at a temperature between 750 - 850 °C, enabling to recover energy and the formation of amorphous silica in the residual material. The physical and chemical properties of rice husk ash used for the research of this project is presented in table 2.

-3- I Table 2: Chemical and physical properties of rice husk ash 1

SiC>2 93.15% Moisture Content <0.1% AI2O3 0.21% Bulk density 195 kg/m3

Ti02 0,02% Mass density 2008 kg/m3

Fe203 0.21% Fines >45 mm 12.5%

MnO 0,015% Blaine area 16196 cm2/g

CaO 0.41% “pH 7 K20 " 2.31% Lol @ 1000 °C 2,91% m/m

MgO 0.45%

Na20 2.36% P205 0,03% ~S 0,03%

The grit material used, is ideally a combination of gravel and sand, in such proportions that 5 the resultant mixture is for 50% smaller than 5 mm but at least 90% > 1 mm and the other 50 % of the grit material has a diameter in the range of 5 - 8 mm. These specifications are required to obtain the right mechanical structure of the mixture, which is key in obtaining a processable mixture with the right intensity of chemical reactions and physical properties. The amount of grit material is at least 10% by volume of the total mixture.

10 The pH of the mixture is controlled by adjusting the alkaline (residual NaOH in the red mud)/silica ratio.

Suitably, the amount of water used is not more than 16% by weight of the total composition, to obtain an easily compressible composition, with the right density and mechanical structure and provide sufficient amount of water for the targeted chemical reactions.

15 The inventive concept further consists in that, unlike with mixing methods in accordance with prior art, a phased mixing process is followed to accommodate for the mechanical and chemical properties (see table 1 and table 2). To this end, in accordance with die invention, there is provided a method for the manufacture of a shaped product comprising the steps of: step 1) gravel (1) and sand (2) (also referred to as grit materials) are dosed in the mixer (6) to obtain a 20 mixture with a specified mechanical structure and defined physical properties; step 2) ash (silica containing material) (3) is fed to the mixture while rotating, followed by adding red mud (4) while rotating the mixture, followed by adding water (5) while rotating to obtain a shapeble mixture (green mixture); -4- step 3) feeding the green mixture into a hopper (7) for conditioning and controlled feed into the press machine; step 4) feeding the green mixture into a press machine equipped with moulds (8) with the targeted dimensions, vibrating the mixture in the mould, refilling the mould, compacting the vibrated mixture 5 through pressing, while vibrating, demoulding the shaped product and feeding of the pressed shaped products (9) towards the curing section; step 5) warming up of the pressed, demoulded shaped mixture from step (4) through hot air curing (10) for a period in the range 8 to 12 hours at a temperature in the range 48 to 80 deg °C; step 6) removing the cured products (11) from the curing reactor and feeding them to the storage yard.

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During the process the material is heated with hot air and the maximum temperature throughout the process is 80 °C (degrees Celsius), making the process a so-called cold-bonded process. The presence of hot air provides an environment in which sodium hydroxide (NaOH) from the red mud becomes activated and can be used for the formation of cementitious minerals. These minerals IS are responsible for the desired cement-like matrix. Given the high pH, the elevated temperature, a siliceous gel is formed and subsequently NaOH can react with silicium (SiÜ2) from the ash to form Na2Si03-nH20. In addition to these reactions NaOH reacts with alumina (A1203) to form NaO.Al2O3.nH2O. The minerals thus formed (sodium-silicate minerals) arc stable and durable under a variety of chemical and mechanical circumstances. Moreover, the process stimulates continuation of 20 the sodium-silicate reactions resulting into more chemical bounds, chemical stability and chemical durability. The heating should be for at least 6 hours, but no longer than 12 hours (depending on the dimensions of the products), in an enclosed environment at a temperature in the range 48-80 deg °C.

To stimulate the desired chemical reactions in the compacted and shaped green mixture the mixture composition must be such that the pH value is sustained at a level of 12.0 -13.0 throughout the curing 25 process.

The described process has been tested by using red mud and ash material from different locations and the development of the process will be illustrated below by using samples as specified in table 1 and table 2.

Test 1. Red mud was mixed in a conventional manner with gravel, sand, (in equal 30 proportions), cement (ratio 250 kg/m3 concrete) and water, subjected to a vibration treatment, pressed into building blocks and stored for a week in a conditioned room (23 °C) for curing. After 1 week the blocks were tested on compressive strength and were found to have practically no bonding strength. After a few handling the blocks were disintegrated.

Test 2. The procedure of test 1 was used to produce blocks using the same mixture as for test 35 1, except for that now rice husk ash was added to the mixture in a ratio of equal proportions of red mud / ash / gravel / sand. The blocks were stored in the same conditioned room and were tested after a week. Now it was found that there was a bonding in the blocks and the compressive strength was 2.8 -5- N/mm2. Subsequently the blocks were stored under water and were found to be disintegrated after 24 hours.

Test 3. Test 2 was repeated except for now the blocks were heated with hot air with a temperature of 80 °C under atmospheric pressure. After 12 hours of hot air exposure the blocks were 5 removed and tested. The cementing characteristics were much better and the compressive strength was 7.8 N/mm2. After storing the blocks for 1 week under water the compressive strength was measured and found to be unchanged

Test 4, The process (test 3) was optimised by chemical calculation of the mixture (ensuring availability of the reactant elements in the right proportions), chemical optimisation by monitoring the 10 pH, moisture content and temperature of the mixture, mechanical optimisation by changing the mixing process to the process as described in this application (see figure 1). The optimum conditions and parameters are already described in this document. The resultants are blocks and other products had a compressive strength of 26 N/mm2, durable and complying with the relevant standards for building materials.

15 1039305

Claims (8)

1. Methode voor het verwerken van red mud of andere natrirunbevattende residuen tot vormgegeven producten, door: (a) korrelig materiaal (zand en grind) te mengen met red mud en as (bestaande uit amorfe silica), gevolgd door toevoeging van water voor het verkrijgen van een vochtig en 5 vervormbaar mengsel, (b) voeden van het natte mengsel in de mallen van een machine voor verdichting, gevolgd door trillen en persen, (c) de gevormde producten op te warmen voor de vorming van cementerende mineralen opdat een harde steenachtige materiaal wordt verkregen.1. Method for processing red mud or other sodium-containing residues into shaped products, by: (a) mixing granular material (sand and gravel) with red mud and ash (consisting of amorphous silica), followed by the addition of water for the obtaining a moist and malleable mixture, (b) feeding the wet mixture into the molds of a compaction machine, followed by vibrating and pressing, (c) heating the molded products to form cementing minerals so that a hard stony material is obtained. 2. Methode volgens conclusie 1, met het kenmerk dat het mengsel volgens conclusie 1, voor minimaal 40% uit red mud bestaat.Method according to claim 1, characterized in that the mixture according to claim 1 consists of at least 40% red mud. 3. Methode volgens conclusie 1, met het kenmerk, dat het aandeel van as in het totale mengsel maximaal 50% bedraagt.Method according to claim 1, characterized in that the proportion of ash in the total mixture is at most 50%. 4. Methode volgens conclusie 1, met het kenmerk, dat de grootteverdeling van het gebruikte korrelig materiaal een grootteverdeling heeft van 1,0 tot 8,0 ram, waarbij ten minste 50% van het materiaal (in gewicht) een grootte heeft van 1-5 mm en de hoeveelheid korrelig materiaal ten minste 10% 15 van het volume van de totale mengsel bedraagt.Method according to claim 1, characterized in that the size distribution of the granular material used has a size distribution of 1.0 to 8.0 ram, wherein at least 50% of the material (by weight) has a size of 1- 5 mm and the amount of granular material is at least 10% of the volume of the total mixture. 5. Methode volgens een van de voorgaande conclusies, met het kenmerk, dat het vochtgehalte van het mengsel maximaal 16% bedraagt.Method according to one of the preceding claims, characterized in that the moisture content of the mixture is at most 16%. 6. Methode volgens conclusies 1 - 5, met het kenmerk, dat de verharding van de groene producten plaatsvindt door warme lucht bij een temperatuur tussen 48-80 °C, gedurende 8 tot 14 uur.Method according to claims 1 to 5, characterized in that the hardening of the green products is carried out by hot air at a temperature between 48-80 ° C for 8 to 14 hours. 7. Methode volgens conclusie 6, met het kenmerk, dat de pH van het mengsel moet worden gehandhaafd op een niveau van 12-13 om de vorming van de gewenste chemische reacties te optimaliseren.Method according to claim 6, characterized in that the pH of the mixture must be maintained at a level of 12-13 to optimize the formation of the desired chemical reactions. 8. Methode volgens de bovenstaande conclusies, met het kenmerk, dat het te gebruiken as afkomstig kan zijn van agrarische gewassen zoals rijstkaf, suikerrietplanten etc.. 25 10393058. Method according to the above claims, characterized in that the ash to be used can originate from agricultural crops such as rice chaff, sugar cane plants, etc. 1039305