Advanced Utilization of Serpentine

Preparation and Utilization of Amorphous Siliceous Materials
from Serpentine (Mg₂Si₂O₅(OH)₄) by Acid Treatment

SUMMARY

Serpentine is existent in a large amount on the earth's surface including Japan. Many chemical processes for the use of magnesium from serpentine had been widely investigated. However, serpentine has not used effectively in spite of those hitherto researches, and has been rarely noted as a raw material for silica production. This work mainly describes the preparation of amorphous silica and siliceous porous material by acid treatment of the ore, and to study the surface properties of the products. Furthermore, to evaluate the potential of the these products for industrial usage, we investigate synthesis of some kinds of siliceous materials obtained from serpentine. We further discussed the molecular adsorption properties of these porous products.

This work consists of seven chapters each of which is summarized below.

Chapter 1 is the introduction which describes the background and the purpose of this paper.

In Chapter 2, the preparation method of amorphous silica by acid treatment of serpentinite is presented. A high purity silica was obtained from the ore containing chrysotile as a main serpentine mineral by 6M-HCl treatment at 100^oC for 12h, and from the ore with major component of antigorite by 5M-H₂SO₄, at 100^oC over 18h. The microtexture of acid leached materials retained the morphology of the original serpentine; each fibrous aggregate consisted of fibers obtained from chrysotile, and masses consisted of platy crystals obtained from antigorite. Irrespective of original ore, Mg²⁺ dissolution finally led to formation of an amorphous silica with more than 99% SiO₂ content of which the specific surface area, pore volume and mean pore diameter were around 170m²/g, 0.08ml/g and 2.0nm, respectively.

In chapter 3, the preparation method of microporous materials from serpentine is presented. The microporous materials were prepared by H₂SO₄ treatment of antigorite and characterized by XRD, SEM observation, a gas adsorption method and ²⁹Si MAS NMR spectroscopy. Micropores 1nm in size were formed between the layers of SiO₄-tetrahedron by Mg²⁺⁺ dissolution. The micropore structure was changed by a condensation of silanol groups occurring in the pores. The maximum specific surface area was around 400m²/g, pore volume was 0.22ml/g and mean pore diameter was 1.2nm. Around 5% MgO was necessary to stabilize the micropore structure and achieve a specific surface area of over 300m²/g. A micropore formation mechanism was proposed based on the results obtained using various analytical methods, and it was shown that the strain occurring due to the misfit between two layers of the composite, the Si₂O₅ sheet and MgO₆ sheet, decreases with increasing degree of Mg²⁺ dissolution. Furthermore, it has been clarified that the microporous materials with the effective size of 1.2nm were effective to separate triaromatics from mono and diaromatics.

In Chapter 4, to evaluate the potential of the amorphous silica for industrial uses, syntheses of some siliceous materials were investigated by using this amorphous silica as a source of silica. As a result, ZSM-5 zeolite, silicon carbide etc. with well crystallinity were prepared for relatively short duration. Especially, several kinds of single-phase fluormicas were synthesized by solid reaction method using amorphous silica obtained from serpentine as a silica source, and their expandable properties and the formation mechanism were discussed. When amorphous silica obtained from serpentine was used as a silica source, the crystalline silicas were crystallized rapidly upon heating and single-phase mica was formed in a similar process as using crystalline silicas. However, in the case of most amorphous silicas, it crystallized slowly into cristobalite and -quarz, and single-phase mica was not obtained because intermediate materials would be existed in a definite temperature region. It was found that the amorphous silica obtained by acid leaching of serpentine, because of its high SiO₂ content and high reactivity, was a very useful material for industrial applications.

In chapter 5, the preparation method of kaolinite from serpentinite is presented and discuss the reaction mechanism. To prevent the formation of impurities, the mixture of AlCl₃ and HCl solutions were used. The kaolinization was achieved at lower temperature and for shorter duration than those from other twenty kinds of silicates. This was presumably because the crystal structures of serpentine and kaolinite fundamentally identical with each other and the reaction proceeds topotaxially.

Chapter 6 describes the neutralization of magnesium sulfate solution by MgO, and the preparation of magnesium hydrated sulfate by spray dry and synthesis of hydrotalcite-like compounds [Mg_1-xAl_x(OH)₂]^x+[SO_4x/2• mH₂O]^x- using the rifined magnesium sulfate solution. Hydrotalcite-like compounds with 0.22 <= x <= 0.33 were formed as a single phase, and each recovery of Mg and Al from the starting materials was above 90%. The ion-exchange properties of SO₄^2- for CO₃^2- in order to invesigate a useful application of another mainly component of MgO in serpentinite. The amount of CO₃^2- in the compound reached 90% at room temperature and 95% at 50^oC in 0.1mol/l Na₂CO₃ solution, respectively.

In chapter 7, the above results are summarized and the conclusions of this study is presented. Acid treatment of silicates is a classical and common technique to obtain new and more useful products for industrial applications, such as adsorbents, catalysts, and catalyst carriers. Dissolution of cations from a crystal framework generally leads to formation of an amorphous hydrated silica with complete breakdown of the original structure. However, acid treatment of some minerals are known to result in formation of a porous material with a texture similar to that of the original mineral. The porous materials and even amorphous silica prepared from serpentine was thought to have a microstructure of two-dimensional SiO₄ tetrahedra framework, which is different from many kinds of silica including amorphous silica with the three-dimensional framework. These siliceous microstructure products obtained from serpentine would be responsible for high reactivity and characteristic molecular sieving effect. The acid-treated materials are considered to be a new siliceous material and will be utilized in a wide variety of fields.