Persio de Souza Santos

Studies on the acid activation of Brazilian smectitic clays

Fullers earth and acid activated smectitic clays are largely used as bleaching earth for the industrial processing of vegetable, animal and mineral oils and waxes. The paper comments about the nomenclature used for these materials, the nature of the acid activation of smectitic clays (bentonites), activation laboratory procedures and presents a review of the acid activation of bentonites from 20 deposits from several regions of Brazil. The activated clays were tested and show good decolorizing power for soybean, castor, cottonseed, corn and sunflower oils.

Surface area, crystal morphology and characterization of transition alumina powders from a new gibbsite precursor

A new procedure was used to prepare a microcrystalline powder constituted by thin euhedral hexagonal gibbsite plates, 0.2 to 0.6 µm in diameter and 32 nm thick. The powder, fired between 200 and 1000 °C, produced chi and kappa transition aluminas. Alpha-alumina is formed from 1000 °C and recrystallized up to 1500 °C. At 1000 °C, kappa- and alpha-alumina coexisted, but kappa-alumina could only be characterized by SAED. The details of the internal organization of the transition alumina pseudomorphs were clearly observable in TEM due to the great thinness of the I-gibbsite plates. The specific surface area varied from pristine I-gibbsite (24.9 m2.g-1) to chi- and kappa transition aluminas (25.4 m2.g-1) at 1000 °C to alpha-alumina (4.0 m2.g-1) at 1500 °C. The maximum value of specific surface area is 347 m2.g-1 in chi-alumina powder at 300 °C, a difference from Bayer gibbsite, in which the chi-alumina highest surface area is 370 m2.g-1 at 400 °C.

Pseudomorphic formation of aluminas from fibrillar pseudoboehmite

Abstract Bundles of microfibrils of the aluminum monohydroxide (pseudoboehmite) prepared by the thermal aging of aluminum dihydroxide monoacetate sols change into pseudomorphs of microcrystals of γ-Al 2 O 3 after firing at 400°C; after firing at 700°C, the pseudomorphs are of δ-Al 2 O 3 ; firing at 900°C gives pseudomorphs of θ-Al 2 O 3 and at 1100°C of α-Al 2 O 3 . This sequence is similar to well crystallized boehmite.

ARGILAS ESPECIAIS: ARGILAS QUIMICAMENTE MODIFICADAS - UMA REVISÃO

The simultaneous use of the specific values of some structural and chemical properties of clay minerals, such as kaolinite, montmorillonite and talc, allows the development of new properties for these materials, especially in relation to the external and internal microcrystal surfaces. These developments are very diversified for montmorillonite, due to the high specific surface area, expansible basal spacings, easy intercalation inside the 2:1 structural layers and a reversible and high cation exchance capacity. The review presents examples of chemical modifications on kaolins, montmorillonites (bentonites) and talcs.

Characterization of the aluminum hydroxide microcrystals formed in some alcohol–water solutions

Abstract The crystallization in water of non-crystalline Al(OH)3 to form somatoids of aluminum trihydroxide bayerite is altered into other aluminum hydroxides when certain water-soluble alcohols are added. The crystalline and non-crystalline phases formed under the different environments were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray elemental microanalysis by using EDS/TEM and scanning electron microscopy (SEM). Ethylene glycol leads to nordstrandite and fibrillar pseudoboehmite. Methanol produces gibbsite and fibrillar pseudoboehmite. Ethanol has different effects according to its concentration: 20% produces bayerite+nordstrandite+fibrillarpseudoboehmite, 50% yields gibbsite+bayerite+pseudoboehmite, but 94% yields only fibrillar pseudoboehmite. Glycerol leads only to fibrillar pseudoboehmite. Isopropyl alcohol produces bayerite and nordstrandite. The presence of pseudoboehmite in the several systems suggests that the monohydroxide acts as an intermediate in the crystallization of the aluminum trihydroxides during aging in those alcohol–water solutions.

Thermal transformation of synthetic euhedral and fibrillar crystals of boehmite into aluminas

Abstract Fibrillar pseudoboehmite and euhedral lath-like boehmite aqueous sols were prepared from the reaction of aluminium secondary butoxide and acetic acid in order to have a solid content of 70g/1 of AlOOH. The dried solids were fired from 200 °C to 1500 °C and examined by TEM, SEM, SAED, and XRD to compare formation of aluminas and their crystal morphological changes at the same temperatures. It was observed that the sequence of transformations of fibrillar pseudoboehmite is closer to that for euhedral boehmite than to the “gelatinous boehmite” described in the literature.

Pseudomorphic transformations of euhedral crystals of γ-AlOOH into aluminas

Abstract The thermal transformation of synthetic well-shaped hexagonal laths of boehmite-AlOOH was studied between 200°C and 1500°C by transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD), to characterize the transition aluminas that appear before the formation of α-Al 2 O 3 . It was observed in the single crystals that dehydroxylation produces, between 400°C and 500°C, pores regularly organized and parallel to the larger diameter cf the hexagons which have the same direction (100) as the original boehmite crystal; this gives a striated appearance to the pseudomorphs, which is maintained until after heating at 900°C, when the transformation from the γ phase coexist; the pores have lost their orderly disposition and larger pores or openings are now irregularly distributed in the mixed single crystals, whose crystalline structure can be observed in the lattice image by the use of high-resolution electron microscopy. The α-alumina pseudomorphs exists as platy elongated single crystals at 1100°C and coalesce at 1200°C; from 1300°C to 1500°C the pseudomorphs coalesce into round polygonal particles of α-alumina, whose crystalline structure could be demonstrated by SAED, XRD or lattice images.

Caulins brasileiros: alguns aspectos da geologia e da mineralogia

The size of Brazil and the diversity of geology is reflected in varying environments in which kaolinite assemblages have been formed. The geological environments of the Brazilian kaolin may be divided into the following groups - sedimentary kaolin, kaolin derived from pegmatite, from granitic rocks, from volcanic rocks and kaolin derived from anorthosite. The sedimentary clays are mainly found in the Amazon basin and those adjacent to the Jari River are being exploited commercially for export as a paper coating clay. Amazon kaolin is characterised by high iron and titania (lattice-held) with low levels of alkali and exhibiting euhedral kaolinite crystals. The South-eastern pegmatite, when not iron stained, are extremely low in iron and titania and a mixture of kaolinite 7A/10A-halloysite occurs in all deposits. The North-eastern pegmatite produces kaolin constituted only by euhedral kaolinites with an absence of halloysite. Kaolin from granites generally has higher iron levels when compared with pegmatite and deposits constituted only by kaolinite are rare, a mixture of kaolinite/7A-halloysite being common. Both the pegmatite and granite derived kaolin are utilised as a paper filler and in general ceramics. Volcanic rocks on alteration produce a fine siliceous clay with titania levels higher than other types and are generally mixtures of kaolinite-7A. Volcanic derived clays are utilized locally in ceramics. Kaolins derived from anorthosite are similar in iron and titania levels to those from granitic kaolins. Assemblages of kaolinite and small quantities of 7A-halloysite are found. These clays are used in both whiteware ceramics and paper filler.

Estudo por microscopia eletrônica das transformações durante a queima de argilas altamente aluminosas brasileiras

Two high alumina gibbsitic / kaolinite clays, extensively used in Brazil for refractory products, were studied to characterize the phase sequences formed during thermal transformation employing electron optical methods. The clays as powders were fired on platinum foils between 200 oC and 1500 oC and program cooled. After heating at 300 oC, it is possible to distinguish in the TEM, between gibbsite and kaolinite crystals of the same hexagonal size and shape. The phase changes of gibbsite and the low-defect kaolinite crystals follow independent series up to 1100 oC / 1200 oC. Morphologically it is easy to distinguish at 400 oC - 800 oC interval -chi and kappa-alumina pseudomorphs from kaolinite crystals and metakaolin pseudomorphs at 900 oC. Mullite content increases in both clays from 900 oC to 1550 oC, while alpha-alumina increases up to 1300 oC and decreases to 1550 oC, indicating interactions between the several phases, in special between silica and alpha-alumina.

Comparative study between yeasts immobilized on alumina beads and on membranes prepared by two routes

Alumina channeled beads and rough surface membranes prepared from aqueous sols of fibrillar pseudoboehmite are able to immobilize yeasts for ethanol fermentation of sugar solutions. This paper describes comparative results of assays carried out with yeasts immobilized onto alpha-alumina beads and membranes prepared under two different conditions of processing and firing. The fermentation tests evaluated by the decrease of fermentable sugars, referred as Brix degrees per hour, indicated that the yeasts immobilized on beads had similar performance, probably because their surfaces, even being morphologically different, presented the same value of open porosity. One type of membrane (asymmetrical; precursor: pseudoboehmite; firing temperature 1,150oC; crystal structure; alpha-alumina) had better performance than the other type (asymmetrical; precursor: fibrillar pseudoboehmite plus aluminum hydroxiacetate mixture; 1,150oC; alpha-alumina) because the yeast cells entered into their porous interior through the surface slits, were immobilized and their growth was easier than on the external surface.