Slag Matrix Research Articles

05/00776 The influence of elevated pressure on the kinetics of evolution of selected gaseous products during coal pyrolysis

In this paper, we, for the first time report the properties of ash deposits collected from the combustion of lignite mixed with silica additive, in a 30 MWth pulverised-fuel boiler with sub-critical steam conditions. The combustion test was carried out continuously over ten months, and ash deposits were sampled from various locations in the boiler. As has been confirmed, the ash-related slagging and fouling have been controlled effectively. Apart from capturing volatile alkali and alkaline earth metals, silica added in an excess amount is beneficial in mixing with molten liquidus to promote their agglomeration, rather than adhering to the vicinity of coal burners for slagging. Compared to the formation of abundant Fe2+-slag that is prone to stick to coal burner vicinity in the raw coal case, the formation of friable calcium sulfate in fireside deposit is favored upon the mixing of silica with coal, which is even predominant in the fireside ash deposit. The calcium sulfate formed is highly layered in a plerospheric structure in which Ca-silicate is present as the core with calcium sulfate being the shell. The physical adsorption of free Ca on silicate surface is supposed to enhance its specific surface area for a quicker sulfation. The capture of Na by silica is also efficient, leading to the transformation of a remarkable portion of Na into coarse bottom ash with less being sulfated. This in turn alleviates the sulfate – relating fouling propensity on the water tube surface in the convection zone. A noticeable portion of Fe was noticed to remain as discrete grains outside the Ca-rich slag matrix. In contrast, the majority of Mg is highly crystallised as sub-micrometer Ca/Mg-silicate and oxide particles residing within slag matrix, which in turn increase the viscosity and decrease the sticking propensity of the molten slag.

Single-stage autotrophic nitrogen-removal process using a composite matrix immobilizing nitrifying and sulfur-denitrifying bacteria

We developed a novel single-stage autotrophic nitrogen-removal process comprised of two composite immobilized biomass layers-one of nitrifying bacteria and one of sulfur-denitrifying bacteria and elemental sulfur-in a Fe-Ni fibrous slag matrix. Nitrification and consumption of dissolved oxygen occurred in the outer part and sulfur denitrification in the anoxic inner part of the composite matrix, thus realizing autotrophic nitrogen removal in a single reactor. The complete conversion of ammonia into N2 in a single reactor was demonstrated in both batch-mode incubation and continuous-feed operation. The spatial profiles of the ammonia-oxidizing bacteria and denitrifying bacteria were evaluated by real-time PCR, targeting their functional genes, and stratification of these two types was observed in the matrix after several months of incubation. This process does not require any specific reactor type or conditions and thus has the potential to be applied to many different wastewater treatment processes due to its simplicity in both operation and construction.

Effect of Chlorine on the Vaporization Behavior of Zinc and Lead during High Temperature Treatment of Dust and Fly Ash

Smelting process is a practical method to treat incineration ash of solid wastes in terms of volume reduction and decomposition/immobilization of toxic materials. In this process, chlorine is a key element to determine the removal ratio of heavy metals such as zinc and lead. The present paper is aiming to give a general review of researches on the effect of chlorine on the vaporization behavior of zinc and lead in molten slag. First, the basic information such as solubility/activity of chlorine in the slag, vaporization mechanism and kinetics relating to chlorides during heat treatment of fly ash are summarized. Then, the vaporization behaviors of zinc, lead and their chlorides during heating are discussed together with important process factors such as atmosphere, temperature, chemical composition and slag matrix. Further, some information on the oxychlorides of Zn and Pb are introduced.

04/00483 Immobilization of mercury and zinc in an alkali-activated slag matrix: Qian, G. et al. Journal of Hazardous Materials, 2003, 101, (1), 65–77.

04/00483 Immobilization of mercury and zinc in an alkali-activated slag matrix: Qian, G. et al. Journal of Hazardous Materials, 2003, 101, (1), 65–77.

Immobilization of mercury and zinc in an alkali-activated slag matrix

The behavior of heavy metals mercury and zinc immobilized in an alkali-activated slag (AAS) matrix has been evaluated using physical property tests, pore structure analysis and XRD, TG-DTG, FTIR and TCLP analysis. Low concentrations (0.5%) of mercury and zinc ions had only a slight affect on compressive strength, pore structure and hydration of AAS matrixes. The addition of 2% Hg ions to the AAS matrix resulted in a reduction in early compressive strength but no negative effects were noticed after 28 days of hydration. Meanwhile, 2% Hg ions can be effectively immobilized in the AAS matrix with the leachate meeting the USEPA TCLP mercury limit. For a 2% Zn-doped AAS matrix, the hydration of the AAS paste was greatly retarded and the zinc concentration in the leachate from this matrix was higher than 5 mg/l even at 28 days. Based on these results, we conclude that the physical encapsulation and chemical fixation mechanisms were likely to be responsible for the immobilization of Hg ions in the AAS matrix while only chemical fixation mechanisms were responsible for the immobilization of Zn ions in the AAS matrix.

Ageing of chromium(III)-bearing slag and its relation to the atmospheric oxidation of solid chromium(III)-oxide in the presence of calcium oxide

Slag arising in ferrochromium and stainless steel production is known to contain residual levels of trivalent chromium. As the chromium is normally bound in the slag matrix in various silicate or spinel phases, and hence not easily mobilised, utilisation or controlled disposal of such slag is generally considered unproblematic. Experimental test work with a number of slag materials indicates, however, that very gradual oxidation of trivalent to hexavalent chromium does occur when the slag is exposed to atmospheric oxygen, rendering a quantifiable but small portion of chromium in this much more mobile and toxic form. Mechanisms and rates of the oxidation reaction were investigated in a number of long-term studies using both original slag materials and artificial mixes of chromium and calcium oxides. Powders of these materials, some of them rolled into balls, were left to age under different conditions for periods of up to 12 months. In the slag samples, which contained between 1 and 3 wt.% chromium, 1000–10 000 μg Cr(VI) were found per gram of chromium within 6–9 months of exposure to an ambient atmosphere. The rate of the oxidation reaction decreased exponentially, and the reaction could generally be said to have ceased within 12 months. In mixtures of calcium and chromium oxides the oxidation reaction is presumed to occur at the boundaries between chromium oxide and calcium oxide phases through diffusion of oxygen along the grain boundaries and of Cr 3+ across the boundaries, resulting in the formation of calcium chromate. In the slags, where calcium and chromium oxide can form a solid solution, the oxidation is likely to occur at the exposed surface of grains containing this solution.

Characterization of mercury- and zinc-doped alkali-activated slag matrix: Part II. Zinc

The compressive strength, setting time, pore structure and hydration product of Zn-doped, alkali-activated slag (AAS) matrix have been investigated by examination of physical properties, micropore analysis, thermal analysis, FTIR, SEM and TCLP methods. The results show that the effects of Zn 2+ on the AAS matrix depend on Zn 2+ ion concentrations. At low Zn 2+ ion concentrations, little negative influences on the compressive strength, setting time and distribution of pore structure were observed. Moreover, low concentrations of Zn 2+ ion could be effectively immobilized in the AAS matrix. For 2% Zn-doped AAS matrix, the hydration of AAS paste was greatly retarded and leaching from this matrix was higher than TCLP zinc limit even at 28 days. Based on the analyses of hydration products, the chemical fixation mechanisms are considered responsible for the immobilization of Zn 2+ ions in the AAS matrix.

Characterization of mercury- and zinc-doped alkali-activated slag matrix: Part I. Mercury

The physical properties, pore structure, hydration process and hydration products of mercury-doped (Hg-doped) alkali-activated slag (AAS) matrixes have been evaluated by examination of physical properties, pore structure analysis and XRD, TG-DTG, FTIR and TCLP methods. Low concentrations of Hg 2+ ions had little effect on the compressive strength, pore structure and degree of hydration of AAS matrixes. The addition of 2% Hg 2+ ions into the AAS matrix brought out an evident retardation on early hydration and reduction of early compressive strength, but no negative effects were noticed after hydration for 28 days. The results also show that up to 2% of Hg 2+ ions can be effectively immobilized in the AAS matrix, with the leaching meeting the TCLP mercury limit. Two mechanisms, physical encapsulation and chemical fixation, are assumed to be responsible for the immobilization of mercury in the AAS matrix.

Improvement of metakaolin on radioactive Sr and Cs immobilization of alkali-activated slag matrix

Effects of metakaolin on simulated radioactive Sr or Cs immobilizing behavior of alkali-activated slag (AAS) matrix were evaluated by cation exchange capacity (CEC), distribution ratio of selective adsorption, leaching test and porosity analyses. The results revealed that the additions of metakaolin into the AAS matrixes largely enhanced their distribution ratios K d of selective adsorption on Sr and Cs ions. Meanwhile, this new immobilizing matrix M-AAS showed the lowest leaching rate. Hydration product analyzes by XRD, DTA, FTIR and SEM demonstrated that (Na+Al)-substituted calcium silicate hydrate (CSH) and self-generated zeolite were major hydration products in the M-AAS matrix. These products have better Sr and Cs selective adsorption as compared to C-S-H without any substitution and natural zeolite minerals.

Carbon adsorbents from brown coal tar

In this paper, we, for the first time report the properties of ash deposits collected from the combustion of lignite mixed with silica additive, in a 30 MWth pulverised-fuel boiler with sub-critical steam conditions. The combustion test was carried out continuously over ten months, and ash deposits were sampled from various locations in the boiler. As has been confirmed, the ash-related slagging and fouling have been controlled effectively. Apart from capturing volatile alkali and alkaline earth metals, silica added in an excess amount is beneficial in mixing with molten liquidus to promote their agglomeration, rather than adhering to the vicinity of coal burners for slagging. Compared to the formation of abundant Fe2+-slag that is prone to stick to coal burner vicinity in the raw coal case, the formation of friable calcium sulfate in fireside deposit is favored upon the mixing of silica with coal, which is even predominant in the fireside ash deposit. The calcium sulfate formed is highly layered in a plerospheric structure in which Ca-silicate is present as the core with calcium sulfate being the shell. The physical adsorption of free Ca on silicate surface is supposed to enhance its specific surface area for a quicker sulfation. The capture of Na by silica is also efficient, leading to the transformation of a remarkable portion of Na into coarse bottom ash with less being sulfated. This in turn alleviates the sulfate – relating fouling propensity on the water tube surface in the convection zone. A noticeable portion of Fe was noticed to remain as discrete grains outside the Ca-rich slag matrix. In contrast, the majority of Mg is highly crystallised as sub-micrometer Ca/Mg-silicate and oxide particles residing within slag matrix, which in turn increase the viscosity and decrease the sticking propensity of the molten slag.

Coal ash utilization technology for civil industry

In this paper, we, for the first time report the properties of ash deposits collected from the combustion of lignite mixed with silica additive, in a 30 MWth pulverised-fuel boiler with sub-critical steam conditions. The combustion test was carried out continuously over ten months, and ash deposits were sampled from various locations in the boiler. As has been confirmed, the ash-related slagging and fouling have been controlled effectively. Apart from capturing volatile alkali and alkaline earth metals, silica added in an excess amount is beneficial in mixing with molten liquidus to promote their agglomeration, rather than adhering to the vicinity of coal burners for slagging. Compared to the formation of abundant Fe2+-slag that is prone to stick to coal burner vicinity in the raw coal case, the formation of friable calcium sulfate in fireside deposit is favored upon the mixing of silica with coal, which is even predominant in the fireside ash deposit. The calcium sulfate formed is highly layered in a plerospheric structure in which Ca-silicate is present as the core with calcium sulfate being the shell. The physical adsorption of free Ca on silicate surface is supposed to enhance its specific surface area for a quicker sulfation. The capture of Na by silica is also efficient, leading to the transformation of a remarkable portion of Na into coarse bottom ash with less being sulfated. This in turn alleviates the sulfate – relating fouling propensity on the water tube surface in the convection zone. A noticeable portion of Fe was noticed to remain as discrete grains outside the Ca-rich slag matrix. In contrast, the majority of Mg is highly crystallised as sub-micrometer Ca/Mg-silicate and oxide particles residing within slag matrix, which in turn increase the viscosity and decrease the sticking propensity of the molten slag.

New aluminium-rich alkali slag matrix with clay minerals for immobilizing simulated radioactive Sr and Cs waste

A new aluminium-rich alkali-activated slag matrix (M-AAS) with clay absorbents has been developed for immobilization of simulated radioactive Sr or Cs waste by introducing metakaolin, natural zeolite and NaOH-treated attapulgite clay minerals into alkali-activated slag matrix (AAS). The results revealed that the additions of metakaolin and clay absorbents into the cementitious matrixes would greatly enhance the distribution ratio, R d, of selective adsorption whether the matrix was OPC matrix or AAS matrix. The new immobilizing matrix M-AAS not only exhibited the strongest selective adsorption for both Sr and Cs ions, but also was characterized by lower porosity and small pore diameter so that it exhibited the lowest leaching rate. Hydration product analyses also demonstrated that (Na+Al)-substituted C–S–H(I) and self-generated zeolite were major hydration products in the M-AAS matrix, which provided this new immobilizing matrix with better selective adsorption on Sr and Cs and lower leaching rate.

Luminescence dating of archaeometallurgical slag: use of the SAR technique for determination of the burial dose

The potential of using luminescence techniques to date metallurgical slag of archaeological origin has been investigated. Slag is an important residue from the metal smelting process and there is no technique currently available to directly date it. An attempt has been made to apply an optically stimulated luminescence (OSL) single-aliquot technique using quartz extracted from the slag matrix. A single-aliquot regeneration technique was used and preliminary results are encouraging in spite of problems encountered in the determination of the equivalent dose ( D e) and apparent underestimation of the preliminary dates obtained. The results presented here were obtained from measurements on copper and iron slag from archaeological sites in Britain and Greece.

Characterization of vitrified slag from mixed medical waste surrogates treated by a thermal plasma system

Various mixed medical waste surrogates have been vitrified by a plasma system built at Institute of Nuclear Energy Research (INER) in Taiwan, Republic of China. Characterizations of vitrified slag were carried out in order to evaluate the effectiveness of an indirect plasma heating used in the INER system. After vitrification, a monolithic metal nugget was separated from the vitrified slag as a result of the gravity effect. The vitrified slag consisted of mostly amorphous state of SiO 2, which in turn dissolved other minor constituents from waste feeds and crucible container. In the slag, dispersive metal-bearing second phases in different shapes were observed. Due to their insoluble nature, these phases were embedded in the slag matrix, and no macroscopic segregation was detected. This observation indicates the presence of mixing state during the vitrification treatment. The formation of second phases was closely related to the metallic waste treated, as evidenced by the increased slag densities for the high metallic feed samples. Leachability analysis results revealed that the encapsulation of these second phases by the slag matrix was very effective. Therefore, with optimal feed compositions, the indirect plasma heating condition used in this study has been shown to be satisfactory for the thermal vitrification of mixed medical wastes.

Emissions Characterization and Off-Gas System Development for Processing Simulated Mixed Waste in a Plasma Centrifugal Furnace

Plasma arc technology is a high temperature process that completely oxidizes organic waste fractions: inorganic hazardous and radionuculide waste fractions are oxidized and encapsulated in a highly durable slag. The robust nature of the technology lends itself to application of diverse mixed and hazardous wastestreams. Over 500 hours of testing have been completed at the Department of Energy's Western Environmental Technology Office with a pilot-scale system. This testing was designed to demonstrate operability over a wide range of wastes and provide the data required to evaluate potential applications of the technology on both a technical and economic basis. In addition to characterization of the off gas for typical combustion products, the fate of radionuculide surrogates and hazardous elements within the Plasma Arc Centrifugal Treatment (PACT) system has been investigated extensively. Test results to date demonstrate that cerium, a plutonium surrogate, remains almost exclusively in the slag matrix. Hazardous elements such as chromium and lead volatilize to a greater extent and are captured by the off-gas system. Preliminary design work is underway to develop a minimum emissions off-gas system for demonstration on a engineering-scale plasma unit. The proposed system will filter particulate matter from the hot gas stream and treat them in an electric ceramic oxidizer, which replaces the conventional afterburner, prior to quenching and acid gas removal. Potential future work would use a liquid monoethanolamine system and membrane filter to separate carbon dioxide and nitrogen from the off-gas stream. The remainder of the gas stream, primarily oxygen, would be recycled back to the combustion chamber. This paper will provide: 1) a summary of the design, development, and performance of the PACT off-gas system and 2) results of radionuculide surrogate and hazardous element portioning.

Chromium Stabilization Chemistry of Paint Removal Wastes in Portland Cement and Blast Furnace Slag

The use of cement based systems for solidification and stabilization of hazardous wastes has been proposed. The stabilization of Cr contaminated paint removal wastes in ordinary Portland cement and in a Portland cement and blast furnace slag matrix was investigated. A loading by volume of 75% waste and 25% cement (or cement + slag) was used. The expression of pore solution was utilized to determine the chemical environment encountered by the waste species in the cement matrix. The highly alkaline conditions of ordinary Portland cement determined the stability of the metal species, with Cr being highly soluble. The replacement of 25% of the Portland cement by blast furnace slag was found to decrease the [OH-] of the pore solution resulting in a decrease of the Cr concentration. For cement wastes forms hydrated for 28 days, the Cr concentration decreased in the expressed pore solution. During the TCLP tests the cement waste form and extraction solution were found to react, changing the chemistry of the extraction solution. The expression of pore solution was found to give a direct measure of the chemistry of the waste species in the cement matrix. This avoids the reaction of the TCLP extraction solution with the cement matrix which changes the solubility of the hazardous metals.

Disposal of arsenic bearing copper smelter flue dust

A study of arsenic fixation was conducted to investigate the stabilization of arsenic in copper smelter flue dust by dissolution in slag matrices, and by encapsulation in cements, mortars, clays, and concrete. The success or failure of the stabilization was determined by immersion leach tests. Results of the study demonstrated that arsenic can be effectively stabilized by first converting it to an arsenate form, then dissolving it in a slag matrix. Further kinetic limitations can be imposed on arsenic release by encapsulating the arsenic bearing slag in cements, concretes, and mortars. Aqueous extraction results indicate minimal release.

MAGNESIA REFRACTORIES IN BASIC OPEN‐HEARTH STEEL FURNACES*

ABSTRACTBy microscopic examination of new and old magnesia bottoms of open‐hearth furnaces it has been shown that the periclase (MgO) and the slag matrix in the bottoms undergo considerable changes during the campaign of a furnace. The periclase absorbs large amounts of iron oxide. Thus examples described show that the iron oxide (contained in solid solution as MgO.Fe2O3) in periclase may be 3% in dead‐burned magnesia, 9% in a new bottom made of a 3:l magnesia‐slag mixture, and as much as 50% in the upper layers of an old bottom. At the same time the microstructure of the bottom changes materially. The periclase in a new bottom occurs as separate, rounded grains of diameters up to 0.2 millimeter, each grain entirely surrounded by a silicate matrix which is liquid at furnace temperatures. The periclase grains in the upper layers of an old bottom may have diameters up to 0.5 millimeter; the grains are no longer separated, but are joined to a more or less continuous aggregate while the silicates (together with some calcium ferrite) now may be gathered in scattered aggregates often surrounded by periclase. The changes have been produced by the influence of slags high in iron oxide and lime. Sometimes these slags have penetrated through cracks or pores to the deeper layers of the bottom where they have changed parts of the bottom to a dark aggregate containing a periclase high in iron oxide. Reducing gases given off by the molten metal may also have produced changes in the bottom especially by reducing the iron oxide in the periclase from the ferric to the ferrous state and thus irnparting a light color to this dominating constituent and to the aggregates in which it occurs. These light aggregates are often irregularly mixed with the dark aggregates formed through the influence of slags.Studies have been made of the reactions between magnesia brick used in end walls of open‐hearth furnaces and the furnace dust, rich in iron oxides, which accumulates on the inside surfaces of the walls. The reactions result in a gradual melting of the magnesia brick. Mag‐nesioferrite‐magnetite solid solutions and periclase‐magnesioferrite solid solutions partake, together with silicates, in this process of melting. The solid solutions must be formed before any appreciable melting of the magnesia in the brick can take place. The rate of melting will depend largely upon the rate of formation and the ultimate properties of the solid solutions. Low porosity of the brick and an oxidizing atmosphere are likely to prolong the life of the brick.