Mercury Adsorption by Sulfurized Fibrous Silicates

-To eliminate mercury vapor from gas streams, three major methods are used: condensation, absorption, and adsorption. This work deals with adsorption, using elemental sulfur as an active phase supported on sepiolite and palygorskite fibrous clays. Sulfur loads of 5-30% were deposited by catalytic oxidation of hydrogen sulfide at temperatures of < 200~ the clays acting first as a catalyst of the reaction and then as a carder of the obtained sulfur. The ability of the sulfurized clay adsorbents to retain mercury was studied at 45"C and 1 mrn Hg pressure. The high values found, about 4 g Hg/g S supported on clays, compared with 1.69 g Hg/g S on activated carbon under the same conditions are related to a more appropriate pore size distribution, with more of the pore widths > 6 nm for the sulfurized silicates. Also, the allotropic state of the deposited sulfur, where S~r (octo-catena) is better than SX (octo-cycle), may also be a contributing factor. Key Words--Activated carbon, Adsorption, Mercury, Palygorskite, Pore size distribution, Sepiolite, Sulfurized silicates. I N T R O D U C T I O N Mercury is a powerful poison, not only for human beings but also for industrial gas streams. In the former, mercury reacts with thiol-containing enzymes, thereby blocking their activity; in the latter, the presence of mercury shortens, by amalgamation, the life of the mechanical parts of equipment and degrades the metallic catalysts involved in the chemical processes. Significant amounts of mercury can be found in hydrogen streams emanating from chloro-alkali manufacturing plants; the quantities depend on the temperature of the gases. Mercury is also present in effluents from roasting sulfide ores in amounts ranging from 40 to 200 ppm, depending on the origin and purity of the feed involved. Fossil fuels such as petroleum, natural gas, or coal, contain some mercury (~0.2 ppm), which can be inhaled by human beings or transformed via a complicated biological cycle into alkyl-mercury derivatives, much more dangerous than the original mercury itself. Mercury must therefore be removed or, at least, its concentration must be decreased to the levels below those stipulated by regulatory agencies (threshold limit value (TLV) set up by the American Conference of G o v e r n m e n t a l Indus t r ia l Hygienist in 1987-1988 (ACGI~I, 1987-1988) 0.1 mg/m 3) before such industrial streams can be safely used. The various methods that have been developed to reduce the level of Hg in industrial streams can be grouped in physical (condensation), chemical (absorption), and physiochemical (adsorption) processes. The actual method adopted depends on the available facilities, the initial mercury content in the gaseous streams, and the final level to be reached; its cost will be inversely related to the desired final level. In general, Copyright 9 1991, The Clay Minerals Society adsorption is the most suitable method if small concentrations (<500 ppm) and final levels of <0.001 ppm are the challenge. Sulfurized active carbon has been extensively studied as an adsorbent (Sinha and Walker, 1972; Lovett and Cunniff, 1974; Steijns and Mars, 1974) and developed by companies, such as Norit and Montedison, but drawbacks are its high cost and low possibilities of mercury recovery once the adsorbent is saturated. In the development of a less expensive and more recoverable adsorbent, some natural or slightly treated silicates appear to be promising materials, because they possess a developed network of pores and a large surface area. Moreover, the clays have the advantage of a much lower cost and higher thermal stability than activated carbon, which enables mercury recovery by dry distillation (~560~ or roasting. Daza et al. (1987) described a general mechanism of sulfurization and demercurization using granular diatomite, a texturally and structurally uncomplicated noncrystalline silica having a significant macropore content and low surface reactivity. In the present work, the study has been extended to the fibrous silicates sepiolite and palygorskite, which possess interand intra-particle porosity and high surface reactivity. The work deals first with the preparation of sulfurized adsorbents and second with the influence of textural and structural parameters of the sulfurized samples in their mercury-scavenging capacity.