Gibbsite formation

Barnhisel,R.l. and Rich, C.I., 1963. Gibbsite formation from aluminum-interlayers in montmoril-lonite. Soil Sci. Soc. Am., Proc., 27 632-635. 

Bates, T.F., 1962. Halloysite and gibbsite formation in Hawaii. Clays Clay Miner. Proc.,... 

Bayerite formation is favoured at room temperature while gibbsite formation is favoured at 70°C. 

It was easier to prepare clays using cations that formed a brucite structure rather than those forming a gibbsite structure. This is, in part, due to the difficulty of forming octahedrally coordinated Al3+ compounds in the presence of silicon at other than low pH conditions. Henin and co-workers believed that hydroxide sheets are formed on which silica is subsequently fixed. The presence of silica tends to cause the hydroxides to precipitate at lower pH than they normally do. Most of these experiments were carried on at 100°C, although crystalline products were obtained at temperatures of 20°C. The rate of formation varies in the ratio 1 560 between 0°C and 100°C. 

In a later study (DeKimpe et al., 1964) they demonstrated that it was necessary that the Al be in six-fold coordination before it would combine with Si to form kaolinite. Starting with the Al in six-fold coordination presumably decreased the free-energy barrier necessary for the formation of kaolinite. Gibbsite, Al(OH)3, was used as a starting material but was too stable to be attacked by depolymerized silica it was necessary for the gibbsite to be in the process of reorganizing ( dynamic stage) into boehmite, AIO(OH). 

In addition to gibbsite there are other routes to manufacture Al(OH>3 and the consecutive transition oxides. One is the precipitation of Al(OH)3 from aluminum salts by adjusting the pH between 7 and 12 by adding bases. Precipitation at elevated temperatures and high pH leads to formation of bayerite, whereas at lower pH pseudoboehmite and subsequently boehmite are formed. By heating, these materials can be converted to the active transition aluminas. 

Thermochemical cycles (Figure 2) involving the formation of compounds (a-quartz, gibbsite, amines, tetrapropylammonium bromide, etc.), the dissolution of these compounds and of the HFI-type samples in 25% HF, and the dilution of aqueous HF allow the calculation of the standard enthalpy of formation of the samples. 

In the 1950s, de Boer and his co-workers (de Boer et al., 1954, 1956 de Boer, 1957) used a variety of techniques in their studies of the thermal dehydration of gibbsite and bayerite and a more detailed picture was obtained of the conditions under which the two decomposition routes were manifested. For example, it was shown that relatively well-crystallized boehmite could be produced by the treatment of gibbsite or bayerite in saturated steam at temperatures of c. 165°C. These and other findings provided qualitative confirmation that the formation of boehmite involved an intragranular hydrothermal transformation. 

In the conversion reaction, CAH,q and C2AH8 redissolve and C,AH is formed, together with additional AH the structure of which then approxi-mates to that of gibbsite. At 5°C, this process may take many years, but at 50°C or above, the formation of C,AHg and gibbsite is virtually immediate. The only phases in the pure Ca0-Al203-H20 system stable in contact with aqueous solutions up to well above 100°C are gibbsite, CjAH and CH. 

In natural systems, the role of water as a transporting agent is important. Geochemical conditions promoting removal of silicic acid from the solid-water interface e.g. water flow) favour the formation of gibbsite over kaolinite. 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

Kano L, Yamashita H., Saito F. Effect of heat-assisted grinding of a calcium hydroxide - gibbsite mixture on formation of hydrated calcium alummate and its hydration behavior. Powder Technology 1998, 98 279-80. 

Cordierite synthesis method based on mechanical activation of mixtures of hydrated oxides of calcium, aluminium and silicon, as well as natural hydrated compounds (talc, kaolinite and gibbsite), has been developed in [2, 3]. Mechanical activation of these mixtures does not lead to the formation of new phases but provides good mixing at the cluster level giving aggregates that form cordierite during the subsequent thermal treatment. 

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