Metakaolin production

The 2eohtes are prepared as essentially bindedess preformed particles. The kaolin is shaped in the desired form of the finished product and is converted in situ in the pellet by treatment with suitable alkaU hydroxide solutions. Preformed pellets of 2eohte A are prepared by this method. These pellets may be converted by ion exchange to other forms such as molecular sieve Type 5A (1). ZeoHtes of higher Si02/Al202 ratios, eg, 2eohte Y, can be obtained by the same method, when sodium metasiUcate is incorporated in the preshaped pellets, or when acid-leached metakaolin is used. 

The first is metakaolin. This is a partially calcined product that forms above about 500 °C. Only about 10% of the original hydroxyl groups of the kaohnite are retained and much of the crystalline nature of the structure is destroyed. Metakaolin is considerably more reactive than the original kaolin and appears to have an especially reactive surface. It is generally used uncoated and finds most use in plasticised PVC cable insulation, where it is reported as giving uniquely useful electrical properties [86]. 

Geopolymers are another type of intermediate products that lie between cements and ceramics [7]. A geopolymer is made by pyroprocessing naturally occurring kaolin (alumina-rich clay) into metakaolin. This metakaolin is then reacted with an alkali hydroxide or sodium silicate to yield a rock-Uke hard mass. Thus, a chemical reaction, which is not fully understood, is employed to produce a hard ceramic-Uke product. Though this product is produced like cement, its properties are more like a sintered ceramic. It is dense and hard like a rock. 

Zeolites, particularly zeolite A, can be manufactured from kaolinitic clays, which as particularly found in Central Europe, Great Britain, Japan, China and USA. To transform kaolin into zeolite, it has to be thermally converted, e.g. by shock heating to > 550°C, to metakaolin. The metakaolin is then su.spended in sodium hydroxide solution and converted at 70 to 100°C into zeolite A. Some of the impurities contained in the natural raw material are retained in the final product. If amorphous silica is added, Si02-rich zeolites are produced. This process enables the transformation of preformed bodies into zeolite materials. 

Studies on pure metakaolin-Portland cement pastes with a metakaolin content of 15% revealed that at ambient temperature the pozzolanic reaction reaches a maximum rate at about 14 days, followed by a period of sigiuficant retardation attributable to the formation of an inhibiting layer of reaction products on the metakaolin particles (Wild and Khatib,... 

Calcination is used to produce value-added kaolin products. Commonly, there are two families of calcined kaolin. One is metakaolin, which is produced by heating... 

The hydrothermal conversion of modified clay mineral, for example, metakaolin, is another manufacturing route to zeolites. Although the raw material costs may be as much as 15% lower than that of the hydrogel route, the major shortcoming of the process is that iron impurities in the raw material may cause an undesirable coloring of the zeolite product [15,18,99-101,104,108,116,122,123,125,126]. 

Metakaolin. An intermediate product formed when kaoiinite is heated at temperatures between about 500 and 850°C the layer structure of the parent kaolinite persists in modified form but collapse of the layers destroys any periodicity normal to the layers. At higher temperatures (925 C) metakaolin transforms to a cubic phase with a spinel-type structure at 1050-1100 C mullite is formed. 

Above 500 °C kaolinite starts to lose its water of crystallisation and, by 650 °C, approximately 90% of this dehydroxylation is complete, leaving residual OH groups randomly distributed but isolated so that condensation will not occur readily. The product formed is known as metakaolin. Some crystalline structure is retained [22] but X-ray diffraction patterns are so diffuse and weak that no better, more recent, identification has been made. After these structural changes the aluminium, which was originally in six-fold octahedral sites, occupies four- and five-fold sites almost equally [23]. 

When kaolin is heated, it undergoes chemical and physical changes. Between approximately 450 °C and 700 °C, dehydroxylation occurs (often described as loss of water of crystallisation) to form a product called metakaolin. This is accompanied by approximately 13% loss of mass as water. At higher temperatures, between 950 °C and 1030 °C, metakaolin undergoes an internal rearrangement to form an amorphous product described as a defect spinel. For rubber applications, products are manufactured at temperatures just above these transitions. Metakaolin is produced primarily for use in flexible polyvinyl chloride wire and cable insulations, but is also used in EPDM cable insulations. 

Metakaolin (calcined clay) is an ultra fine pozzolana, produced from the mineral kaolin at temperature between 700°C and 900"C in that temperature kaolinite loses water. This thermal activation is also referred to as calcining. The obtained product is highly pozzolanic and in recent years there has been an increasing interest in the utilisation of metakaolin used as partial replacement of Portland cement (up to 30%), often as an alternative to the use of SF. 

It has also been reported that the second derivative DTA is more useful to examine the products formed during the autoclaving of cement-quartz-metakaolin mixtures. Klimesch and Rayt ] subjected a mixture of quartz (38.5%) and cement (61.5%) containing different amounts of metakaolin and autoclaved them for 8 hrs at 180°C. It was found that the second derivative differential thermal curve provided a more detailed information, particularly in temperatures of 800-1000°C. In Fig. 35, DTA and second derivative curves for cement-quartz-metakaolin pastes are compared. The exotherms occur at 840, 903, and 960°C due to the formation of wollastonite from C-S-H, aluminum-substituted tobermorite, and anorthite from the hydrogamet residue respectively. The small endot-herm at 828°C preceding the first exotherm is probably caused by well crystallized calcite. 

Soils and clays, in general, when calcined give off adsorbed, interlayer, and hydrated types of water. These effects produce endothermal peaks or loss of weight in DTA and TG, respectively. The endothermal peaks are followed by exothermal peaks that are caused by re-crystalliza-tion. Although many types of clay minerals such as montmorillonite, illite, and some shales show these effects, they are not suitable as pozzolans in concrete. Metakaolin, formed by heating kaolinite, seems to be the most suitable additive material for cement. Heating of kaolinite involves removal of adsorbed water at about 100°C and dehydroxylation at above 600°C, followed by the formation of metakaolinite, an almost amorphous product. The sequence of reactions is as follows ... 

Metakaolin reacts with lime to yield calcium silicate hydrate. Metakaolin may also be activated by other materials such as alkali metal hydroxides, water glass, etc. Activation leads to a polycondensation product with cementing properties. The t5q)e of MK, composition, temperature at which it is produced, surface area, etc., determine the strength development characteristics of the product. In conduction calorimetry, an exothermic peak results by the reaction of MK and the activator. A strong as5mimetric peak in calorimetry is associated with an amorphous inorganic... 

The effect of heating rate also depends on the manner of containing the specimen, i.e., on the type of sample holder. If only a change of state is involved, the properties of the sample holder are singularly important for heat transfer considerations ( ). Using rates of 6, 12, 18, and 2UC per minute, Arens found no shift of transition temperature for either a to j quartz or the kaolin to metakaolin decomposition. Weight-loss reactions showed peak shifts to be associated with the time-dependent nature of decomposition product effusion from the specimen. 

---