Carbon-mineral adsorbents deposit

Abstract. A variety of pyrocarbon/silica gel adsorbents were prepared using commercial mesoporous silica gels Si-40, Si-60, and Si-100 as matrices modified by carbon deposits from pyrolysis of several organic precursors. The second type of hybrid carbon-mineral adsorbents was synthesized using spent natural palygorskite utilized in paraffin purification. The adsorbents were then heated, hydrothermally treated, or modified by additional deposition of carbon. Changes in the structural and adsorption characteristics of hybrid adsorbents before and after treatments were analyzed by microscopy, p-nitrophenol and nitrogen adsorption isotherms, and TG, TEM, XRD, and XRF methods. 

The process of carbon deposit formation in complex carbon-mineral adsorbents may be initiated and terminated in any stage or stages of scheme 1. This is dependent on the chemical nature of the carbonized substance, porous structure and chemical nature of adsorption and catalytic sites of the mineral matrix, etc. For this reason, the complex adsorbents prepared by the third and fourth methods have the carbon deposits consisting of the substances of different chemical and physical structure formed during the defined stages of Scheme 1. 

The formation of carbon-mineral adsorbents containing carbon deposits in the form of dendrites, whiskers or carbon black is not advantageous because of the poor mechanical properties of these deposits. The morphology of the coke depends on the mechanism and conditions of its formation on the mineral surface. Two main mechanisms of formation of carbon deposits can be distinguished consecutive reactions and carbide-forming. The latter mode consists in the thermal decomposition of hydrocarbons. 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

The zeolite-carbon adsorbents from mineral-carbon adsorbents group are novel and exhibit not quite well recognized properties with their unique, modified porous structure. The characteristic structures for zeolite, active carbon and intermediate structure exist in these materials. Such a structure results fi-om the modification of a surface of a mineral matrix by depositing carbon material. The efifectivity of enrichment of the structure of zeolite-carbon adsorbents (in relation to crystalline zeolite structure) in hydrophobic micropores (0.4 - 2 nm) and macropores (above 50 nm) is proportional to the fi action of carbon phase. Such combination of hydrophilic properties of mineral phase and hydrophobic properties of organic phase results in various sorptive properties of the material and the range of their application can be consequently extended. Additionally, the chemical resistance of these adsorbents for their exploitation in aggressive conditions takes place. 

An alternative method of preparation of zeolite-carbon adsorbents is the treatment of mixtures clay mineral with hard coal and waste carbon deposits. The treatment consists of several physicochemical processes i.e. formation, carbonization, activation and crystallization, presented in this paper. The adsorbents prepared with this procedure are not a simple mixture of two components but strongly dispersed material resulting fi om thermochemical transformation, thus fecilitating the surface structure. 

The adsorbents have been prepared fi-om the halloysite (H) - mineral fi-om kaolinite group with an admixture of carbonaceous materials refinery waste deposits (RSI), sediment communal sewage (CSew) and cellulose (Ce), and the fiaction of these mixtures were within 30 - 70 wt.%. The mbcture of raw material was thermally (carbonaceous materials carbonization, 973 K) and hydrothermally (crystallization of the amorphous metahalloysite in alkaline solution to zeolitic structure of NaA type, 373 K) pretreated in order to cilitate their specific structure [1,2]. 

The distribution of pores volume in carbonized coal deposits (Car), mineral-carbon and zeolite-carbon adsorbents (A) under examination, as well as those included in commercial standards is given in Table 1 and illustrated by adsorption isotherms for carbon dioxide and benzene in Figures 1 - 4. 

The admixture of clay mineral (halloysite) to carbonaceous deposit (as for waste materials) remarkably enriches the texture of mineral-carbon adsorbents in mesopores and leads to the decrease of magnitude of micropores volume with dimension of 0.4 - 2 nm. In the case of hard coal and kaolinite mixture this char contains the maximum sub- and micropores at the lowest content of mesopores. 

The fraction of carbon deposit introduced to the raw material mixture apparently influences the character of zeolite-carbon adsorbents. It mainly increases the specific volume of mesopores and, to a lesser extent, the pores with dimension of 0.4 - 2 nm. On the other hand, the increasing fiction of mineral phase resulted in increase of ultramicropores specific volume. It is a result of growing yield of forming zeolite phase in the adsorbents. The presence of transport pores leads to the higher reaction extent of amorphous aluminosilicate with sodium hydroxide and frtcilitates crystallization of zeolite. 

The specific macropores volume for mineral-carbon adsorbents decreases in the crystallization process. This can be attributed to the deposition of zeolite crystals inside the solid structure. 

It is obvious that chemical composition of carbon deposit depends not only on the conditions in which pyrolysis is carried out but also on the chemical composition of carbonized organic substances on the surface of the solid carrier. As a result, carbon matter deposited on the surface of mineral carrier, besides C and H, can contain heteroatoms (S,N,0). The problem was discussed in the paper [2]. Similarly, the morphological composition depends on the conditions in which carbonization reaction is carried out. Gierak and Leboda showed [22], that it was possible to obtain graphitized carbon deposit of good mechanical properties under very mild conditions. The obtained adsorbents are applicable in chromatography in separation of polar substance mixtures [22,23]. 

Thorium is generally associated with acid (and intermediate) rocks. It is very stable and will not dissolve in a solution. As a result of alteration, thorium is deposited only with detrital sediments, never with purely chemical sediments (carbonates, aragonite). Therefore, in carbonate reservoirs, thorium becomes a very important clay indicator. It is found in mudstones where it is adsorbed by clay minerals, and with heavy minerals that are often abundant in the silty fraction. Thorium is generally considered a marine element (Baker Atlas, 1985). 

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